Residual stress reducing method for welded joint and residual stress reducing apparatus for same

Abstract

The present disclosure relates to a method and an apparatus for reducing welding residual stress in a steel structure having a weld line. Specifically, the method for reducing welding residual stress comprises the steps of: plastically deforming a deformation target region, which is a base metal region adjacent to an interface between a weld bead of the weld line and a base metal, by bringing a circular disc into contact with the deformation target region and applying a compressive load to the deformation target region by means of the circular disc which is rotating; and transferring the steel structure or the circular disk such that an indentation caused by the plastic deformation extends in a continuous linear shape along the weld line in the deformation target region.

Description

Device and method for reducing residual stress in welded joints

[0001] The present disclosure relates to a device and method for reducing residual stress in a weld.

[0002] The residual stress that inevitably occurs after welding is caused by differences in local heating and cooling, and this residual stress affects the destructive performance of the weld, including fatigue failure. To reduce welding residual stress, post-heat treatment is commonly used. This involves heating the weld in a furnace for an extended period of time to reduce the relative deformation between the base metal and the weld material. However, the application of post-heat treatment is significantly limited due to the size limitations of the furnace.

[0003] For example, the tank of a liquefied carbon dioxide carrier (Type-C LCO2 Tank), which has recently been attracting attention, is designed to be significantly larger and uses high-strength steel (YS690 grade) to increase transport capacity. However, the size expansion is limited, and even if the size is expanded, there is a problem that stress-reducing heat treatment requires high costs, and there is a limitation that it is difficult to satisfy the heat treatment standards stipulated by the classification society.

[0004] One aspect of the present disclosure is to provide a method and device capable of relieving welding residual stress.

[0005] Another aspect of the present disclosure is to provide a method and device capable of relieving welding residual stress regardless of the size of a steel structure.

[0006] Another aspect of the present disclosure is to provide a method and device capable of quickly and precisely relieving welding residual stress in a weld.

[0007] Another aspect of the present disclosure is to provide a method and device capable of relieving welding residual stress in a weld joint for each base material even when the properties of the base materials to be welded are different.

[0008] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0009] A method according to the disclosure is a method for reducing welding residual stress in a steel structure having a weld line, comprising the steps of: contacting a circular disk with a deformation target area, which is a base material area adjacent to an interface between a weld bead of the weld line and the base material; applying a compressive load to the deformation target area with the rotating circular disk to plastically deform the deformation target area; and moving the steel structure or the circular disk so that an indentation due to the plastic deformation extends in a continuous linear form along the weld line in the deformation target area.

[0010] In one specific example, the deformation target region may be heated at least by frictional heat generated by rotation of the circular disk during the plastic deformation.

[0011] In one specific example, the depth of the indentation can be controlled by the heating temperature of the deformation target area and the compressive load applied by the circular disk.

[0012] In one specific example, the diameter of the circular disc may be 100 mm or greater.

[0013] In one specific example, the edge of the circular disc may be of a tapered type, a round type, or an eccentric type.

[0014] In one specific example, the circular disk may be perpendicular to the surface of the deformation target area where the indentation is formed, or may be tilted at an angle of 65° or less.

[0015] In one specific example, n indentations can be formed in the deformation target area by n (n is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line.

[0016] In one specific example, two circular disks are spaced apart from each other with the welding line in between, so that an indentation can be formed in each of the deformation target areas on both sides of the welding line.

[0017] In one specific example, a first disk set including j (j is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line, and a second disk set including k (k is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line are arranged spaced apart from each other with the weld line interposed therebetween, so that j indentations can be formed in one deformation target area among deformation target areas on both sides of the weld line, and k indentations can be formed in another deformation target area.

[0018] In one specific example, the welding line may include l circular disks (l is a natural number from 2 to 4) spaced apart from each other in the longitudinal direction.

[0019] In one specific example, a third disk set including the l circular disks and a fourth disk set including m (m is a natural number from 2 to 4) circular disks arranged at intervals along the length of the welding line are arranged at intervals with the welding line interposed therebetween, so that at least two or more indentations can be formed in each of the deformation target areas on both sides of the welding line.

[0020] In one specific embodiment, the circular disks forming the disk set may be the same or different in at least one factor selected from the diameter of the disk, the thickness of the disk, the shape of the disk edge, the tilting angle of the disk, the compressive load applied to the disk, and the rotational speed of the disk.

[0021] In one specific example, the deformation target area may be an area within 0.1 cm to 20 cm from the interface toward the parent material.

[0022] In one specific example, the frictional heat may be added to the deformation target area having the welding residual heat.

[0023] In one specific example, after the formation of a continuous line-shaped indentation is completed, the method may further include a step of annealing the steel structure in which the indentation is formed; and after the annealing, a surface treatment step of removing surface irregularities caused by the indentation.

[0024] In one specific example, the steel structure may be a fluid storage vessel, a ship structure, an offshore structure, an offshore wind monopile, or an automobile body.

[0025] A device according to the present invention is a device for reducing welding residual stress of a steel structure having a weld line, comprising: a deformation unit including a circular disk that is provided to be rotatable while in contact with a base material in a deformation target area, which is a base material area adjacent to an interface between a weld bead of the weld line and the base material; a load applying unit that applies an axial load in the direction of the base material to the circular disk of the deformation unit; and a transport unit that transports the steel structure in the longitudinal direction of the weld line so that the circular disk is positioned within the deformation target area.

[0026] Another device according to the disclosure is a device for reducing welding residual stress of a steel structure having a weld line, comprising: a deformation unit including a circular disk that is provided to be rotatable while in contact with a base material in a deformation target area, which is a base material area adjacent to an interface between a weld bead of the weld line and the base material; a load applying unit that applies an axial load in the direction of the base material to the circular disk of the deformation unit; and a transport unit that transports the deformation unit along the weld line in the longitudinal direction of the weld line so that the circular disk is positioned within the deformation target area.

[0027] In one specific example, the deformation unit may include two circular disks arranged spaced apart from each other and contacting deformation target areas on both sides of the welding line with the welding line interposed therebetween.

[0028] In one specific example, the deformation unit includes a first set of disks and a second set of disks that are arranged spaced apart from each other and contact deformation target areas on both sides of the welding line with the welding line interposed therebetween, the first set of disks including j (j is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the welding line, and the second set of disks including k (k is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the welding line.

[0029] In one specific example, the deformation unit includes a third disk set and a fourth disk set that are arranged spaced apart from each other and contact deformation target areas on both sides of the welding line with the welding line interposed therebetween, the third disk set may include l (l is a natural number from 2 to 4) circular disks arranged spaced apart from each other in the longitudinal direction of the welding line, and the fourth disk set may include m (m is a natural number from 2 to 4) circular disks arranged spaced apart from each other in the longitudinal direction of the welding line.

[0030] In one specific embodiment, the device may further include a control unit that controls one or more factors selected from a rotational speed of the circular disk, a conveying speed by the conveying unit, and a load applied to the circular disk by the load applying unit.

[0031] The method according to the start of work can quickly relieve the welding residual stress around the weld line through mechanical deformation.

[0032] The device according to the initiation of work can quickly form highly controlled compressive residual stress in the target area, thereby relieving welding residual stress.

[0033] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0034] Figure 1 is a schematic diagram showing a cross-section perpendicular to a weld line in a steel structure having a weld line and showing a residual stress profile according to the position.

[0035] Figure 2 is a schematic diagram of a process for forming an indentation in a deformation target area in a method for reducing welding residual stress according to one specific example.

[0036] FIG. 3 is a schematic diagram showing a cross-section perpendicular to a welding line in a steel structure after forming an indentation, and a residual stress profile according to a position, in a method for reducing welding residual stress according to one specific example.

[0037] FIG. 4 is a schematic diagram illustrating a process in which an indentation is extended in a continuous line shape in a deformation target area in a method for reducing welding residual stress according to one specific example.

[0038] FIG. 5 is a schematic diagram showing a cross-sectional view of an edge shape of a circular disk and an indentation shape that can be formed in a deformation target area by a circular disk having each edge shape in a method for reducing welding residual stress according to one specific example.

[0039] FIG. 6 is a schematic diagram illustrating a process in which a rotating circular disk forms an indentation in a deformation target area in a method for reducing welding residual stress according to one specific example.

[0040] Fig. 7 is another schematic diagram illustrating a process in which a rotating circular disk forms an indentation in a deformation target area in a method for reducing welding residual stress according to one specific example.

[0041] FIGS. 8 to 11 are another schematic diagram illustrating a process in which a rotating circular disk forms an indentation in a deformation target area in a method for reducing welding residual stress according to one specific example.

[0042] Hereinafter, preferred embodiments of the present invention will be described with reference to the attached drawings. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0043] In addition, the embodiments of the present invention are provided to more completely explain the present invention to a person having average knowledge in the relevant technical field.

[0044] The shape and size of elements in the drawing may be exaggerated for clearer explanation.

[0045] In describing the embodiments of the present invention, if a detailed description of a known technology related to the present invention is judged to unnecessarily obscure the gist of the present invention, such detailed description will be omitted. In addition, the terms described below are terms defined in consideration of their functions in the present invention, and these may vary depending on the intention or custom of the user or operator. Therefore, the definitions should be made based on the contents throughout this specification. The terminology used in the detailed description is only for the purpose of describing the embodiments of the present invention and should in no way be limiting. Unless clearly defined otherwise, expressions in the singular form include plural meanings.

[0046] In this description, expressions such as “including” or “having” are intended to indicate certain features, numbers, steps, operations, elements, portions or combinations thereof, and should not be construed to exclude the presence or possibility of one or more other features, numbers, steps, operations, elements, portions or combinations thereof other than those described.

[0047] Unless otherwise specified in the specification of the present invention, the % unit means weight %.

[0048] Additionally, throughout the specification, when we say that a part is 'connected' to another part, this includes not only cases where it is 'directly connected', but also cases where it is 'indirectly connected' with other elements in between.

[0049] Below, the present invention will be described in detail through each embodiment or example of the present invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may be combined with other embodiments or examples. Therefore, the citation of a claim in the patent claims is only an example of an embodiment, and the technical concept of the present invention should not be interpreted solely as a combination with the cited claim, and combinations with various claims also fall within the scope of the technical concept of the present invention.

[0050] The terms used herein are for the purpose of describing the invention and are not intended to limit the invention. Furthermore, the singular forms used herein also include the plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0051] The meaning of 'comprising' as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0052] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0053] In this specification and the appended claims, a steel structure means a body in which at least two steel members are joined together by welding.

[0054] In this specification and the appended claims, a weld bead refers to a weld metal formed by welding and bonding two steel members together. The weld metal refers to metal that is melted by the welding energy during welding and then solidified.

[0055] In this specification and the appended claims, a weld line means a weld bead having a macroscopic line shape created by continuous welding.

[0056] In this specification and the appended claims, "base material" refers to the material (substance) of the member to be welded. The base material includes not only the raw material of the member to be welded, but also materials whose mechanical properties, etc., have been altered by the welding heat, such as their structure.

[0057] Fig. 1 is a schematic diagram showing a cross-section perpendicular to a weld line in a steel structure having a weld line, and showing a residual stress (σ) profile according to a position (Y) in the direction of increasing Y-axis from the boundary (interface) between the weld bead and the base material in the cross-section as the origin (0) of the Y-axis. The steel structure is a single piece of two steel materials welded together, but the two steel materials are not distinguished and are collectively referred to as the base material (100).

[0058] As shown in an example in Fig. 1, a steel structure includes a base material (100) and a weld bead (210) formed by welding. At the interface (IL) between the weld bead (210) and the base material (100), in the direction toward the base material, the largest tensile residual stress is formed at the interface (IL), and a compressive residual stress capable of offsetting the tensile residual stress is formed in an area away from the influence of welding. In the cross-sectional schematic diagram of Fig. 1, a high-tensile residual stress region where a high tensile residual stress is formed near the interface (IL) is also depicted as 'TRSZ'.

[0059] In detail, welding residual stress occurs after the weld bead and the base material become a single continuum after welding, when the base material (base material not affected by the welding heat) restrains the shrinkage of the weld bead and heat-affected zone, which are both at a higher temperature. In other words, the stress that occurs when the base material restrains the shrinkage of the weld bead, whose temperature has risen to a molten state immediately after welding, is the welding residual stress, and it is a state in which 'the base material tightly restrains the shrinkage of the weld bead.' Accordingly, the welding residual stress is basically a tensile residual stress, and as shown in the welding residual stress profile of Fig. 1, the maximum tensile residual stress occurs at the interface (IL) between the deposited metal (weld bead) and the base material, and a compressive residual stress corresponding to the balance of the tensile residual stress force occurs in the base material outside the weld heat-affected zone. For example, in the case of mild steel, a maximum tensile residual stress equivalent to the yield stress level of mild steel may occur at the boundary between the deposited metal (weld bead) and the mild steel base material.

[0060] The present disclosure relates to a method for reducing residual stress, which can mechanically relieve welding residual stress, particularly welding tensile residual stress, formed in a weld including a weld line and a weld heat affected zone.

[0061] A method for reducing residual stress according to the initiation of work includes a step of contacting a circular disk with a deformation target area, which is a base material area adjacent to an interface between a weld bead of a weld line and a base material in a steel structure having a weld line, and applying a compressive load to the deformation target area with the rotating circular disk to plastically deform the deformation target area, and a step of moving the steel structure or the circular disk so that an indentation due to plastic deformation extends in a continuous linear form along the weld line in the deformation target area.

[0062] In one specific example, the deformation target area may be an area within 0.1 cm to 20 cm from the interface between the base material and the welding bead (weld line) toward the base material side (one base material side), specifically, an area within 1 cm to 10 cm from the interface toward the base material side.

[0063] FIG. 2 is a schematic diagram of a process step of forming an indentation (410) in a deformation target area (110) by applying a compressive load to the deformation target area (110) using a rotating circular disk (310) in a method for reducing welding residual stress according to one specific example. At this time, the compressive load may be applied to the rotation axis (ra) of the circular disk.

[0064] The circular disk (310) can contact the deformation target area (110) adjacent to the interface (IL) between the weld bead (210) of the weld line and the base material (100). The circular disk (310) can contact the deformation target area (110) while rotating, or can rotate after contacting the deformation target area (110).

[0065] The deformation target area (110) in contact with the circular disk (310) can be heated by frictional heat generated by the rotation of the circular disk (310), and when the rotating circular disk (310) applies a compressive load (indicated by an arrow in 310) to the heated deformation target area (110), the rotating circular disk (310) digs into the deformation target area (110) and plastically deforms it, so that an indentation (410) can be formed at the area in contact with the circular disk (310).

[0066] As described above, the method for reducing welding residual stress according to one specific example can be performed by heating a deformation target area using a rotating circular disk and applying a compressive load substantially simultaneously to plastically deform the deformation target area. By heating and plastically deforming the deformation target area by the circular disk, a designed compressive stress can be quickly and precisely generated in the deformation target area, and welding (tensile) residual stress can be relieved through this compressive stress.

[0067] FIG. 3 is a schematic diagram showing a cross-section perpendicular to a weld line in a steel structure having a weld line after plastic deformation (indentation formation) is performed by a compressive load applied from a rotating circular disk, and a residual stress (σ) profile according to a position (Y) is shown, with the boundary (interface, IL) between the weld bead (210) and the base material (100) as the origin (0) of the Y-axis and the direction toward the base material where the indentation is formed as the increasing direction of the Y-axis. At this time, the cross-section perpendicular to the weld line means the cross-section of the steel structure perpendicular to the longitudinal direction of the weld line, and the weld line on the cross-section is referred to as a weld bead and described above. The width direction of the weld line is the same as the width direction of the weld bead. The width direction of the weld bead at a point on the weld line may be a direction perpendicular to the longitudinal direction of the weld line at that point. The longitudinal direction of the weld line is the same as the extension direction of the weld bead.

[0068] In the residual stress (σ) profile of Fig. 3, the residual stress due to welding is depicted as a solid line, and the residual stress resulting from plastic deformation under compressive load is depicted as a dotted line. In addition, the area where high compressive residual stress remains around the indentation is depicted as 'CRSZ'.

[0069] As shown in the example of the residual stress profile in Fig. 3, the tensile residual stress formed by conventional welding can be offset by the compressive residual stress caused by plastic deformation, thereby reducing the welding residual stress.

[0070] During plastic deformation, the deformation target area may be heated at least by frictional heat generated by the rotation of the circular disk, and the depth of the indentation created by plastic deformation may be controlled by a compressive load.

[0071] A rotating circular disc can contact a base material over a large area, allowing for rapid heating of a relatively large area of ​​the base material to a desired temperature. The frictional heat generated by the rotating circular disc can be controlled by one or more factors selected from the diameter of the circular disc, the rotational speed of the circular disc, and the compressive load applied to the circular disc.

[0072] As a relatively large area of ​​the parent material is heated to the desired temperature very quickly, compressive residual stresses can be formed over a wider range with a smaller compressive load (smaller indentation), and by controlling the magnitude of the compressive load applied through the circular disk, the depth of the indentation and the magnitude of the compressive residual stresses formed can be easily controlled.

[0073] In addition, when the circular disk rotates and applies a compressive load to the parent material, even if the circular disk or steel structure moves, the surface of the parent material that comes into new contact with the circular disk due to the movement is already heated by heat conduction, so that the tensile residual stress in the deformation target area along the welding line can be relieved by rapidly moving or continuously moving the rotating circular disk or steel structure.

[0074] In order to stably realize the above-described effect, the diameter of the circular disc is preferably 100 mm or more, more preferably 200 mm or more, and even more preferably 250 mm or more. The circular disc may have a diameter of substantially 500 mm or less, but it is obvious that the present invention is not limited by the upper limit of the diameter of the circular disc.

[0075] The element being "transported" during the indentation extension phase, i.e. the "moving" entity, can be the steel structure, the circular disc, or both the steel structure and the circular disc. Since "movement" is a relative concept, even if both the steel structure and the circular disc move, if the moving speed of the steel structure and the circular disc are different, the faster-moving element can be interpreted as moving relative to the slower-moving element.

[0076] As described above, since the 'movement' is caused by the relative movement difference between the steel structure and the circular disc, in the following detailed description of the method for reducing welding residual stress, the circular disc is assumed to be the subject of the movement (the element being transported). However, even when the steel structure, not the circular disc, is the subject of the movement, the following content is substantially equally applicable. In addition, even when both the circular disc and the steel structure move, if there is a net movement due to the difference in transport speed, the following content is substantially equally applicable. For example, when the circular disc is the subject of the movement, the circular disc moves (is transported) along the welding line within the deformation target area, which corresponds to the steel structure moving (being transported) such that the area in contact with the circular disc moves along the welding line within the deformation target area when the steel structure is the subject of the movement.

[0077] Fig. 4 is a schematic diagram illustrating a process in which a circular disk (rotating circular disk, 310) moves along a welding line (200) within a deformation target area (110), and an indentation extends in a continuous linear shape along the welding line (200). Hereinafter, the indentation extending in a linear shape may also be collectively referred to as an indentation line (400). In the schematic diagram of Fig. 4, the direction of movement of the circular disk (310) is indicated by an arrow.

[0078] As shown in the schematic diagram of Fig. 4, the circular disk (310) can move along the welding line (200) within the deformation target area (110). Specifically, the circular disk (310) can move in the longitudinal direction of the welding line (200) within the deformation target area (110).

[0079] The circular disk (310) may be in a rotating state when moving, and at the same time, may be in a state where a compressive load is applied toward the parent material (100). When the circular disk (310) moves, an indentation may extend in the form of a continuous line along the movement trajectory of the circular disk (310). As a result, an indentation line may be formed that is located within the deformation target area (110) and is in contact with or spaced from the weld line, but has a shape corresponding to at least a portion of the shape of the weld line. Here, it is appropriate to interpret the shape of the weld line as the shape of a simple line when the weld line is replaced in a steel structure, rather than being interpreted as the three-dimensional shape of the weld line.

[0080] FIG. 5 is a cross-sectional view showing the edge shape of a circular disk (310) and a schematic diagram showing the shape of an indentation (410) that can be formed in a deformation target area by a circular disk (310) having each edge shape.

[0081] FIG. 5(a) is an example of a circular disk (310) having a tapered edge. Due to the tapered edge shape, a tapered indentation (410) having a deep indentation on one side and a continuously shallower indentation depth can be formed in the deformation target area. When a circular disk (310) having a tapered edge is used, the size of the indentation (410), the depth of the indentation (410), and / or the degree of tapering of the indentation can be controlled by adjusting one or more factors selected from the thickness of the circular disk (310), the taper angle (θ), and the compressive load applied to the circular disk (310).

[0082] FIG. 5(b) is an example of a circular disk (310) having a round edge, and due to the gently curved and protruding edge shape, a gently curved and fine indentation (410) can be formed in the deformation target area. When using a circular disk (310) having a round edge, the size of the indentation (410), the depth of the indentation (410), and / or the degree of curvature of the indentation can be controlled by adjusting one or more factors selected from the thickness of the circular disk (310), the curvature of the round edge, and the compressive load applied to the circular disk (310).

[0083] Fig. 5(c) is an example of a circular disk (310) having an eccentric edge, which is an example of having both a gently curved protruding portion and a gently curved engraved portion. When using a circular disk (310) having such an eccentric edge, it is possible to form various shapes of indentations by controlling the depth at which the circular disk (310) is pressed, from a gently curved fine indentation (shown in Fig. 5(c)) similar to a round edge, to an eccentric indentation (not shown).

[0084] In the case where only the gently curved protruding portion of the eccentric edge is indented into the parent material, by controlling the degree of protrusion from the eccentric edge and the position where the protruding portion is formed, not only the depth of the indentation within the deformation target area but also the position where the indentation is formed can be controlled more precisely. In addition, the eccentric edge can act substantially the same as a rounded edge having a thickness corresponding to the thickness of the protruding portion. That is, a circular disc having an eccentric edge can form an indentation with a narrower width substantially independent of the thickness of the disc.

[0085] When a gently curved, engraved portion of an eccentric edge is pressed into the parent material, the shape of the indentation can be precisely controlled through the size of the deeply engraved portion, the degree of deep engraving, the size of the shallowly engraved portion, and the degree of shallow engraving. This precise control of the shape of the indentation means that the compressive residual stress profile formed in the parent material including the deformation target area can be precisely controlled.

[0086] In detailing a specific example based on FIGS. 2, 4 and 5, an example in which the circular disk is perpendicular to the surface of the deformation target area is illustrated, but the present invention is not limited thereto. For example, as in the example illustrated in FIG. 6, the circular disk (310) may be perpendicular to the surface of the deformation target area where an indentation is formed, or may be tilted at an angle (β) of 65° or less (within). At this time, the tilting angle (β) is an angle based on the case (0°) in which the circular disk is perpendicular to the surface of the deformation target area.

[0087] Based on FIG. 5, the size, shape, and maximum fine depth of the indentation formed in the deformation target area can be controlled by the tilting angle itself, together with or independently of the shape of the edge described above. This means that by controlling the tilting angle together with the shape of the edge, or even controlling the tilting angle even for the shape of the same edge, as well as the compressive load applied to the parent material by the circular disk, it is possible to precisely control the compressive residual stress profile formed in the parent material including the deformation target area.

[0088] FIG. 7 is a schematic diagram illustrating a process of forming continuous indentations (310A, 310B) in each of the deformation target areas (110A, 110B) by positioning circular disks (310A, 310B) on each of the deformation target areas (110A, 110B) on both sides of the welding line (200) in a method for reducing welding residual stress according to one specific example.

[0089] As an interface (IL) with the base material (100) is formed on both sides of the welding line (200), a deformation target area (110A, 110B) can be located on each side of the welding line (200) in the steel structure.

[0090] As shown in an example in Fig. 7, circular disks (310A, 310B) are positioned for each deformation target area (110A, 110B), so that compressive residual stress can be formed in both deformation target areas (110A, 110B) around the welding line (200) substantially simultaneously. By forming compressive residual stress substantially simultaneously in the deformation target areas where welding tensile residual stress relief is required, as shown in Fig. 7, the time required for reducing residual stress can be significantly reduced, and productivity can be improved.

[0091] When the circular disks are positioned in each deformation target area, the two circular disks (310A, 310B) may be identical in one or more factors selected from the applied compressive load, disk edge shape, disk thickness, disk tilting angle, disk diameter, and disk rotation speed, or alternatively, one or more factors may be different from each other.

[0092] As mentioned above, the compressive load, edge shape, thickness of the circular disc, and tilting angle are major factors affecting the compressive residual stress profile, and the compressive load, disc diameter, and disc rotation speed are major factors affecting the degree of frictional heat generation. The fact that these can be different from each other means that the indentations (indentation lines) can be formed by heating to different temperatures depending on the deformation target area, purpose, and design, and having different compressive residual stress profiles.

[0093] The fact that one or more of the factors selected from the diameter of the disc and the rotational speed of the disc may be different, either together with or separately from the compressive load, means that the frictional heat generated by each circular disc may be different from each other.

[0094] When members of different parent materials (first parent material and second parent material) are welded to form a steel structure, the welding residual stress profile formed on the first parent material side centered on the weld line and the welding residual stress profile formed on the second parent material side are bound to be different from each other.

[0095] A method for reducing welding residual stress according to one specific example can control the heating degree of the deformation target area on both sides of the weld line differently to suit the properties of the base material by independently adjusting the diameter of the disk and / or the rotation speed of the disk, together with or separately from the compressive load, for each circular disk located in the deformation target area on both sides of the weld line, even when the base materials on both sides of the weld line are different from each other and have different properties.

[0096] In addition, the method for reducing welding residual stress according to one specific example can reduce welding residual stresses that are different from each other substantially simultaneously and independently according to the design by independently adjusting the compressive load, edge shape, thickness of the circular disk, and tilting angle of the circular disk located in the deformation target area on both sides of the weld line, even when the base materials on both sides of the weld line are different from each other and have different welding tensile residual stress profiles.

[0097] An example of Fig. 7 is an example in which a steel structure has one welding line (200), but the present invention is not limited thereto. The steel structure may have multiple welding lines. If the steel structure has Q welding lines (Q is a natural number greater than or equal to 1), 2Q deformation target areas may be defined. At this time, circular disks are positioned in each of 1 to 2Q deformation target areas, so that indentations (lines) can be formed in 1 to 2Q deformation target areas substantially simultaneously.

[0098] The specific examples of FIGS. 2 to 7 are examples in which a single circular disk is provided in a single deformation target area. However, the method for reducing welding residual stress according to the initiation of work includes not only a case in which a single circular disk is positioned in a single deformation target area, but also a case in which multiple circular disks are positioned.

[0099] In one specific example, n indentations may be formed in the deformation target region by n circular disks (where n is a natural number from 2 to 5) arranged and spaced apart from each other in the width direction of the weld line. It should be understood that when the distance between them is close, adjacent indentations may be connected to each other, and thus, in terms of physical continuity, the n indentations may appear as a single indentation.

[0100] Fig. 8 is a schematic diagram illustrating a process of forming three indentations by positioning three circular disks in one deformation target area in a method for reducing welding residual stress according to one specific example.

[0101] As shown in an example in Fig. 8, a plurality of circular disks (321, 322, 323) may be arranged spaced apart from each other in the width direction (W) of the welding bead (210) and positioned in one deformation target area (110). In the example of Fig. 8, the compressive load applied to each disk is different, and the size of the compressive load applied to the circular disk is shown by the length of the arrow.

[0102] A plurality of circular disks (321, 322, 323) are positioned in a deformation target area (110), and by independently controlling one or more factors selected from the compressive load (an example of FIG. 8), the thickness of the circular disk, the edge shape of the circular disk, the tilting angle of the circular disk, the diameter of the circular disk, and the rotational speed of the circular disk, it is possible to form a wider and more precise compressive residual stress profile under a temperature profile formed and controlled by the frictional heat controlled for each disk.

[0103] The tensile residual stress formed by welding can vary depending on the material of the base metal, welding conditions, and the physical shape of the weld. Therefore, to relieve the tensile residual stress in the weld, precise control of the stress profile of the compressive residual stress formed is required.

[0104] As described above, by controlling one or more factors selected from the thickness of the circular disk, the edge shape of the circular disk, the compressive load applied to the circular disk (the degree of depression of the circular disk), the tilting angle of the circular disk, the diameter of the circular disk, and the rotational speed of the circular disk, the size (width) of the indentation, the shape of the indentation, the fine depth, etc., of each of the n indentations formed in the deformation target area can be controlled.

[0105] This means that the compressive residual stress profiles of each of the n indentations can be controlled independently. At this time, it goes without saying that the n compressive residual stress profiles of the n indentations accumulate with each other to form the final compressive residual stress profile formed in the parent material including the deformation target region.

[0106] FIG. 9 is a drawing showing an example in which a first disk set (320A) including j (j is a natural number from 2 to 5, j=3 in the example of FIG. 9) circular disks arranged and spaced apart from each other in the width direction of the weld line (the width direction of the weld bead) and a second disk set (320B) including k (k is a natural number from 2 to 5, k=2 in the example of FIG. 9) circular disks arranged and spaced apart from each other in the width direction of the weld line are arranged with a weld line (200) therebetween, so that j indentations are formed in one deformation target area (110A) among deformation target areas (110A, 110B) on both sides of the weld line (200), and k indentations are formed in the other deformation target area (110B).

[0107] The j circular disks constituting the first disk set (320A) may have the same or different factors from each other in terms of the compressive load (an example in FIG. 9), the thickness of the circular disk, the edge shape of the circular disk, the tilting angle of the circular disk, the diameter of the circular disk, and the rotational speed of the circular disk. In addition, independently of the first disk set (320), the k circular disks constituting the second disk set (320B) may have the same or different factors from each other in terms of the compressive load (an example in FIG. 9), the thickness of the circular disk, the edge shape of the circular disk, the tilting angle of the circular disk, the diameter of the circular disk, and the rotational speed of the circular disk.

[0108] As in an example of Fig. 9, by positioning the first disk set (320A) and the second disk set (320B) in the deformation target areas (110A, 110B) on both sides of the welding line and forming an indentation, a wider and more precisely adjusted compressive residual stress profile can be formed under a temperature profile controlled for each deformation target area, while significantly reducing the time required for reducing the residual stress and improving productivity.

[0109] Furthermore, as in the examples of FIGS. 8 and 9, when a set of disks including a plurality of circular disks spaced apart in the width direction of the weld line is positioned in the deformation target area, it is advantageous because the desired compressive residual stress profile can be formed even if the depth of the indentation is formed shallowly.

[0110] Considering the intended use of a steel structure having weld lines, the depth of the indentation may need to be controlled to a level that does not significantly adversely affect the intended use. For example, if the steel structure is a fluid storage vessel that stores pressurized fluid, it is advantageous for the maximum depth of the indentation to be within 100 μm. However, when the depth of the indentation is controlled, it may be difficult to form a single indentation by a single circular disk, thereby forming the desired level of compressive residual stress. However, when a disk set including a plurality of circular disks spaced apart in the width direction is positioned, multiple shallow indentations can be formed, and the compressive residual stress profiles of each indentation can accumulate to form a compressive residual stress (the final compressive residual stress applied to the base material), so that the desired level of compressive residual stress can be easily formed even with shallow indentations.

[0111] In one specific example, l (l is a natural number from 2 to 4) circular disks arranged spaced apart from each other in the longitudinal direction of the weld line may be positioned in one deformation target area.

[0112] FIG. 10 is a schematic diagram illustrating a process in which two circular disks (341, 342) are positioned in a deformation target area (110) but are spaced apart in the longitudinal direction of a welding line (200) in a method for reducing welding residual stress according to one specific example.

[0113] At this time, based on the movement direction of the circular disks (341, 342) (as shown by arrows in Fig. 10), the movement trajectories of the preceding circular disk (341) and the following circular disk may overlap each other.

[0114] By the overlapping movement trajectories between the circular disks (341, 342), the deformation target area (110) is first formed by the first indentation line (510) by the preceding circular disk (341), and then the second indentation line (520) is formed so as to overlap at least a portion of the first indentation line (510), and finally, an indentation line (500) having a designed shape (cross-sectional shape) can be formed.

[0115] An example of Fig. 10 is an example in which the width of the first pressure mark line (510) is larger than that of the second pressure mark line (520), and the second pressure mark line (520) is formed again within the line-formed first pressure mark line (510), but is not limited thereto.

[0116] By overlapping the first indentation line (510) and the second indentation line (520) to form the final indentation line (500), the compressive residual stress profile formed in the parent material can be controlled more precisely than when the indentation line (500) is formed with a single circular disk.

[0117] For example, the width of the first indentation line may be narrower than that of the second indentation line, and in an extreme example, the width of the second indentation line may be large and deep, so that when only the physical shape of the ultimately formed indentation line is examined, it may appear that only the second indentation line exists. However, even in cases where the first indentation line is formed pre-formed and the second indentation line is formed post-formed to cover the first indentation line and the trace of the first indentation line disappears, the compressive residual stress profile ultimately formed in the parent material by the pre-formed first indentation line may be different from the compressive residual stress profile by the second indentation line alone.

[0118] In addition, the first indentation line (510) is formed, and after the deformation target area is heated by the frictional heat between the circular disk (341) forming the first indentation line (510) and the parent material (100), the circular disk (342) forming the second indentation line (520) can generate frictional heat. That is, while a wide area of ​​the parent material is heated by the preceding circular disk (341), an area where a deep indentation is to be formed can be selectively heated further by the succeeding circular disk (642). By this thermal control, the circular disk can be moved more quickly, and at the same time, excessive heat can be prevented from being applied to an area of ​​the parent material other than the intended area.

[0119] FIG. 11 is a process diagram showing an example in which a third disk set (340A) including l (l is a natural number from 2 to 4, l=2 in the example of FIG. 11) circular disks (341A, 342A) spaced apart from each other in the longitudinal direction of a weld line (200) and a fourth disk set (340B) including m (m is a natural number from 2 to 4, m=2 in the example of FIG. 11) circular disks (341B, 342B) spaced apart from each other in the longitudinal direction of a weld line (200) are spaced apart from each other with a weld line (200) therebetween, so that an indentation line (500A or 500B) in which at least two or more indentations are overlapped is formed in each of the deformation target areas (110A, 110B) on both sides of the weld line (200).

[0120] As shown in the example in Fig. 11, a set of disks including a plurality of circular disks spaced apart in the longitudinal direction are positioned in each deformation target area with a weld line in between to form an indentation line, thereby forming a more precisely controlled compressive residual stress profile under a controlled thermal profile, and can quickly relieve welding residual stress.

[0121] Even when a plurality of circular disks are arranged spaced apart in the longitudinal direction of the weld line, as in the examples of FIGS. 10 and 11, one or more selected factors may be different or the same for each circular disk, including the compressive load, the thickness of the circular disk, the edge shape of the circular disk, the tilting angle of the circular disk, the diameter of the circular disk (an example of FIGS. 10 and 11), and the rotational speed of the circular disk. In addition, one or more selected factors may be controlled for each circular disk.

[0122] An example of FIGS. 8 and 9 is a case where a plurality of circular disks are arranged spaced apart in the width direction of the weld line in one deformation target area, and an example of FIGS. 10 and 11 is a case where a plurality of circular disks are arranged spaced apart in the length direction of the weld line in one deformation target area. However, the direction of spacing of the circular disks is not limited to the width direction or the length direction.

[0123] When a plurality of circular disks (a set of disks) are spaced apart in a deformation target area, the spacing may include spacing in the width direction of the weld line; spacing in the length direction of the weld line; and / or spacing in the width direction of the weld line and spacing in the length direction of the weld line.

[0124] 'Spacing in the width direction of the weld line and spacing in the length direction of the weld line' may mean that the set of discs may include both circular discs spaced apart in the width direction of the weld line with respect to one circular disc (A) and circular discs spaced apart in the length direction of the weld line with respect to one circular disc (A). Together or independently of this, 'spacing in the width direction of the weld line and spacing in the length direction of the weld line' may mean that the set of discs may include at least one circular disc spaced apart in the width direction of the weld line with respect to one circular disc (A) by a given distance and also spaced apart in the length direction of the weld line by a given distance.

[0125] When a set of disks is positioned in the deformation target area, at least, the moving speeds of each circular disk forming a set of disks may be substantially the same. In addition, the moving speeds of the first set of disks and the second set of disks may be substantially the same; however, depending on the purpose, the moving speeds of the first set of disks and the second set of disks may be controlled differently.

[0126] At the point where the indentation is formed by the circular disc or set of circular discs, the temperature in the deformation target area may be set to a temperature that can obtain the desired compressive residual stress profile and allow application of a significantly reduced compressive load. However, it is preferable that the temperature (maximum temperature) in the deformation target area be higher than the temperature of unwanted phase transformations, etc., so as not to adversely affect the parent material. For example, when the steel is a high-strength steel having a high yield strength (e.g., YS690 grade), it is preferable that the temperature (maximum temperature) in the deformation target area be controlled below the eutectoid transformation point (A1 transformation point). As a practical and non-limiting example, when the steel is a high-strength steel having a high yield strength (e.g., YS690 grade), the maximum temperature in the deformation target area (the area in contact with the circular disc in the deformation target area) may be 500 to 600°C.

[0127] The moving speed of the circular disc or set of circular discs may be sufficient to form the desired indentation line, taking into account the properties of the steel material. As a practical and non-limiting example, if the steel material is a high-strength steel with a high yield strength (e.g., YS690 grade), the moving speed may be, but is not limited to, 10 cm / min to 60 cm / min.

[0128] The thickness of the circular disc is sufficient to ensure stable rotation of the circular disc, prevent buckling, and have the desired edge shape, taking into account the material of the circular disc. As a practical and non-limiting example, when the circular disc is made of carbon steel, the thickness of the circular disc may be on the order of 1 mm to 10 mm, but is not limited thereto. In addition, as described above, depending on the shape of the edge, the thickness of the circular disc and the size of the indentation may be substantially unrelated to each other.

[0129] The rotational speed of the cylindrical disc may be a rotational speed suitable for obtaining the desired thermal profile, designed in consideration of the specific properties of the base material, the welded structure (joint shape), the size of the region where compressive residual stress is to be formed in the deformation target region, and the arrangement of the spheres within the disc set if the configuration of the disc set is such. As a practical and non-limiting example, if a single cylindrical disc with a diameter of 100 mm or more is located in the deformation target region and the steel material is high-strength steel with high yield strength (e.g., YS690 grade), the rotational speed of the cylindrical disc may be 10 rpm to 600 rpm. In this case, it is obvious that the rotational speed may vary outside the presented range depending on whether a disc set is used or the arrangement of the spheres within the disc set.

[0130] The compressive load applied to the circular disc may be a compressive load that can form a designed compressive residual stress profile in consideration of the specific properties of the parent material, the edge shape of the circular disc, the tilting angle of the circular disc, the target temperature profile of the deformation target area, the weld residual stress profile formed in the deformation target area, the weld structure (joint shape), and the arrangement of the spheres within the disc set if the disc set has a configuration. As a practical and non-limiting example, if a single circular disc is positioned in the deformation target area, is a butt joint, is welded by single-pass SAW (submerged arc welding), has a rounded edge, has a tilting angle of 0° (perpendicular to the surface of the deformation target area), and the steel is a high-strength steel having a high yield strength (e.g., YS690 grade), the compressive load applied to the circular disc may be a load that applies a pressure of 250 to 350 MPa to the parent material, taking into account the contact area between the disc and the parent material.

[0131] According to one specific example, the welding residual stress reduction method can be performed after welding while welding is being performed between parent materials.

[0132] In this case, frictional heat generated by the circular disk is added to the deformation target area with residual welding heat, and the temperature profile of the deformation target area can be controlled. The residual welding heat significantly reduces the amount of frictional heat generated by the circular disk, enabling faster movement of the circular disk and more precise selective heating of the target area (additional heating by frictional heat).

[0133] A method for reducing welding residual stress according to one specific example may further include a step of annealing a steel structure in which an indentation is formed after formation of a continuous linear indentation is completed; and a surface treatment step of removing surface irregularities caused by the indentation after the annealing.

[0134] Annealing is intended to relieve stress in a portion deformed plastically by a circular disk, and can be performed by applying heat of about 200°C or less, or substantially 100 to 200°C, to the area where the indentation line is located. At this time, the application of heat can be performed through irradiation of heat including near-infrared or infrared rays, application of hot air, contact with a heating element, charging into a constant-temperature space, etc., but the present invention is not limited to the specific method of annealing.

[0135] Surface treatment may include, but is not limited to, physical surface treatments such as grinding.

[0136] The aforementioned method for reducing welding residual stress is particularly useful when considering the specific intended use of a steel structure where tensile residual stress has a detrimental effect. Examples of steel structural applications where tensile residual stress has a detrimental effect include fluid storage containers such as high-pressure gas or liquid containers, ship structures, marine structures, offshore wind monopiles, and automobile bodies. However, the present invention is not limited to the intended use of steel structures.

[0137] Hereinafter, a device for reducing welding residual stress in a steel structure having a weld line is described in detail.

[0138] In the reduction device, the weld line, steel structure, base material, interface between the weld bead and base material, deformation target area, circular disk, disk set, indentation, indentation line, formation of indentation line, compressive load (axial load), compressive residual stress profile formed in the base material, etc. are similar to or identical to those described above in the welding residual stress reduction method. Accordingly, the welding residual stress reduction device includes all of the contents described above in the welding residual stress reduction method.

[0139] A device according to the invention is a device for reducing welding residual stress of a steel structure having a weld line, comprising: a deformation unit including a circular disk that is provided to be rotatable while in contact with a base material in a deformation target area, which is a base material area adjacent to an interface between a weld bead of a weld line and the base material; a load applying unit that applies an axial load (compressive load) in the direction of the base material to the circular disk of the deformation unit; and a transfer unit (I) that transfers the deformation unit along the weld line in the longitudinal direction of the weld line so that the circular disk is positioned within the deformation target area.

[0140] Another device according to the disclosure is a device for reducing welding residual stress of a steel structure having a weld line, comprising: a deformation unit including a circular disk that is arranged to be rotatable while in contact with the base material in a deformation target area, which is a region of the base material adjacent to the interface between a weld bead of the weld line and the base material; a load applying unit that applies an axial load (compressive load) in the direction of the base material to the circular disk of the deformation unit; and a transport unit (II) that transports the steel structure in the longitudinal direction of the weld line so that the circular disk is positioned within the deformation target area.

[0141] As described above, the welding residual stress reduction device includes two aspects: a case where the element conveyed by the conveying unit is a steel structure and a case where the element is a deformation unit.

[0142] The deformation unit may include a conventional rotation means capable of rotating a circular disk. For example, it may include a magnetically-based rotation means that is fixed by a magnetic coupling and can be rotated by a rotating magnetic field, or a mechanical rotation means that includes a rotation shaft that is rotated by a mechanical force such as an electric motor drive or air pressure.

[0143] In terms of precise rotational speed control and easy and independent rotational speed control for each circular disk, the rotation means may be a magnetic-based rotation means. The magnetic-based rotation means may include a driving magnet and a rotating magnet that is formed at a 90° angle with a constant gap from the driving magnet. The rotating magnet can rotate through an attractive and repulsive force generated between the rotating magnet and the driving magnet, and the circular disk can be coupled to the rotating magnet via a rotation shaft so that it can rotate integrally with the rotating magnet.

[0144] The load application unit may include a conventional pressurizing means configured to apply pressure to a circular disk, specifically to the rotational axis of the circular disk, so that the circular disk applies a compressive load to the parent material. Examples of the pressurizing means include, but are not limited to, a pneumatic drive device, a hydraulic drive device, or a mechanical displacement drive device.

[0145] The transport unit (I) may include a conventional transport means (I) that allows a deformation unit including a circular disk to move along a weld line of a steel structure. The specific transport means (I) may be determined and provided considering the physical shape of the weld line in the steel structure. For example, when the weld line is a straight line, the transport means (I) may include an X-axis unit that operates back and forth. As another example, when the weld line is a curved shape in one plane, the transport means (I) may include a Y-axis unit that operates left and right, an X-axis unit that operates back and forth, and a rotation unit that rotates horizontally about a rotation axis. As yet another example, when the weld line is a three-dimensional curved shape, the transport means (I) may include an X-axis unit that operates back and forth, a Y-axis unit that operates left and right, a Z-axis unit that operates up and down, and a rotation unit that rotates horizontally about a rotation axis. In this case, regardless of the physical shape of the weld line, the transport means (I) may include a Z-axis unit, and the elevation and descent of the deformation unit may be controlled by the Z-axis unit. That is, when the residual stress reduction device is driven, the deformation unit can be moved to contact the deformation target area through the Z-axis unit, and when the residual stress reduction device is finished being used, the deformation unit can be moved to not contact the deformation target area through the Z-axis unit.

[0146] The transport unit (II) may include a conventional transport means (II) that allows the deformation target area of ​​the steel structure to move along the weld line below the deformation unit including a circular disk fixed at a predetermined position. The specific transport means (II) may be determined and provided in consideration of the physical shape of the weld line in the steel structure. For example, when the weld line is in the shape of a straight line or a curve, the transport unit may include a belt conveyor or a roller conveyor driven by a power source. As another example, when the weld line is in the shape of a three-dimensional curve, the transport means (II) may include an X-axis unit that operates back and forth, a Y-axis unit that operates left and right, a Z-axis unit that operates up and down, and a rotation unit that rotates horizontally around a rotation axis. When the transport unit transports the steel structure, the welding residual stress reduction device may further include a lift member that moves the deformation unit up and down. The lifting member may control the elevation and descent of the deformation unit, and may bring the deformation unit into contact with / separate from the steel structure when the device is operated / ended.

[0147] Similar to or identical to that described above based on FIG. 7, the deformation unit may include two circular disks arranged spaced apart from each other and contacting deformation target areas on both sides of the weld line with the weld line interposed therebetween.

[0148] Similar to or identical to the above-described based on FIGS. 8 and 9, the deformation unit includes a first disk set and a second disk set that are arranged spaced apart from each other and contact deformation target areas on both sides of the weld line, respectively, with the weld line interposed therebetween, and the first disk set may include j (j is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line, and the second disk set may include k (k is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line.

[0149] Similar to or identical to the above-described based on FIGS. 10 and 11, the deformation unit includes a third disc set and a fourth disc set that are arranged spaced apart from each other and contact deformation target areas on both sides of the weld line with the weld line in between, and the third disc set may include l (l is a natural number from 2 to 4) circular discs arranged spaced apart from each other in the longitudinal direction of the weld line, and the fourth disc set may include m (m is a natural number from 2 to 4) circular discs arranged spaced apart from each other in the longitudinal direction of the weld line.

[0150] If necessary, the device may further include a tilting unit for tilting the circular disc at a predetermined angle.

[0151] In one embodiment, the device may further include a control unit. The control unit may control the rotational speed of the circular disks by controlling the mechanical rotation means or the magnetic rotation means, control the transport means (I, II) to control the transport speed of the deformation unit or the steel structure, and / or control the load application unit to control the load applied to the circular disks by controlling the load application unit.

[0152] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

[0153] *Explanation of symbols*

[0154] 100: Mother material

[0155] 110, 110A, 110B: Transformation target area

[0156] 210: Welding bead

[0157] 200: Welding line

[0158] 310, 310A, 310B, 321. 322, 323, 341, 342: Circular discs

[0159] 320A, 320B, 340A, 340B: Disc Set

[0160] 410: Impression

[0161] 400, 400A, 400B, 500A, 500B: Indentation lines

Claims

1. A method for reducing welding residual stress in steel structures having weld lines. A step of contacting a circular disk with a deformation target area, which is a region of the base material adjacent to the interface between the weld bead of the welding line and the base material, and applying a compressive load to the deformation target area with the rotating circular disk to plastically deform the deformation target area; and A method comprising the step of transporting the steel structure or the circular disk so that, in the deformation target area, the indentation caused by the plastic deformation extends in a continuous linear form along the welding line.

2. In paragraph 1, A method in which the deformation target area is heated at least by frictional heat caused by rotation of the circular disk during the plastic deformation.

3. In paragraph 2, A method in which the depth of the above indentation is controlled by the heating temperature of the deformation target area and the compressive load applied by the circular disk.

4. In paragraph 2, A method wherein the diameter of the above circular disc is 100 mm or more.

5. In paragraph 2, The edge of the above circular disc is of tapered type, round type or eccentric type.

6. In paragraph 1, The above circular disk is perpendicular to the surface of the deformation target area where the indentation is formed, or is tilted at an angle of 65° or less.

7. In paragraph 1, A method in which n indentations are formed in the deformation target area by n (n is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the welding line.

8. In paragraph 1, A method in which two circular disks are spaced apart and arranged with the welding line interposed therebetween, so that an indentation is formed in each of the deformation target areas on both sides of the welding line.

9. In paragraph 1, A method in which a first disk set including j (j is a natural number from 2 to 5) circular disks arranged and spaced apart from each other in the width direction of the weld line, and a second disk set including k (k is a natural number from 2 to 5) circular disks arranged and spaced apart from each other in the width direction of the weld line are arranged with the weld line interposed therebetween, so that j indentations are formed in one deformation target area among the deformation target areas on both sides of the weld line, and k indentations are formed in the other deformation target area.

10. In paragraph 1, A method comprising l (l is a natural number from 2 to 4) circular disks arranged spaced apart from each other in the longitudinal direction of the above welding line.

11. In paragraph 10, A method in which a third disk set including the l circular disks and a fourth disk set including m (m is a natural number from 2 to 4) circular disks arranged at a distance from each other in the longitudinal direction of the welding line are arranged at a distance from each other with the welding line interposed therebetween, so that at least two indentations are formed in each of the deformation target areas on both sides of the welding line.

12. In paragraph 9 or paragraph 11, The circular discs forming the above disc set are the same or different in at least one factor selected from the diameter of the disc, the thickness of the disc, the shape of the disc edge, the tilting angle of the disc, the compressive load applied to the disc, and the rotational speed of the disc.

13. In paragraph 1, The above deformation target area is a region within 0.1 cm to 20 cm from the interface toward the parent material.

14. In paragraph 2, A method in which frictional heat is added to the deformation target area having welding residual heat.

15. In paragraph 1, After the formation of continuous linear impressions is completed, A step of annealing the steel structure on which the above-mentioned impression is formed; and A method further comprising a surface treatment step for removing surface roughness caused by indentation after the above-mentioned annealing treatment.

16. In paragraph 1, The above steel structure is a fluid storage container, a ship structure, an offshore structure, an offshore wind monopile or an automobile body.

17. A device for reducing residual welding stress in steel structures having weld lines. A deformation unit including a circular disk that is configured to rotate while in contact with the base material in a deformation target area, which is a base material area adjacent to the interface between the weld bead of the above welding line and the base material; A load applying unit that applies an axial load in the direction of the parent material to the circular disk of the above deformation unit; and A transport unit that transports the steel structure in the longitudinal direction of the welding line so that the circular disk is positioned within the deformation target area; A device comprising:

18. A device for reducing welding residual stress in steel structures having weld lines. A deformation unit including a circular disk that is configured to rotate while in contact with the base material in a deformation target area, which is a base material area adjacent to the interface between the weld bead of the above welding line and the base material; A load applying unit that applies an axial load in the direction of the parent material to the circular disk of the above deformation unit; and A transfer unit that transfers the deformation unit along the welding line in the longitudinal direction of the welding line so that the circular disk is positioned within the deformation target area; A device comprising:

19. In paragraph 17 or 18, The above deformation unit is a device including two circular disks arranged spaced apart from each other and contacting deformation target areas on both sides of the welding line with the welding line interposed therebetween.

20. In paragraph 17 or 18, The above deformation unit includes a first disk set and a second disk set that are arranged spaced apart from each other and contact deformation target areas on both sides of the welding line with the welding line interposed therebetween, The first disk set includes j (j is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the welding line, A device wherein the second disk set includes k (wherein k is a natural number from 2 to 5) circular disks arranged spaced apart from each other in the width direction of the weld line.

21. In paragraph 17 or 18, The above deformation unit includes a third disk set and a fourth disk set that are arranged spaced apart from each other and contact deformation target areas on both sides of the welding line with the welding line interposed therebetween, respectively. The third disc set comprises l (l is a natural number from 2 to 4) circular discs spaced apart from each other in the longitudinal direction of the welding line, The above fourth disk set is a device including m (m is a natural number from 2 to 4) circular disks arranged spaced apart from each other in the longitudinal direction of the welding line.

22. In paragraph 17 or 18, A device further comprising a control unit that controls one or more factors selected from the rotational speed of the circular disk, the transport speed by the transport unit, and the load applied to the circular disk by the load application unit.

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