JP2004131744A - Method for improving fatigue strength of lap welded joint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more improve the reliability of a welded structure obtained by using metallic sheets by providing a method for improving the fatigue strength of a lap welded joint obtained by lapping two sheets of metallic sheets and welding the central parts, and a method for producing a welded structure using the same. <P>SOLUTION: In the method for improving fatigue strength, the vicinities of the welded toe parts in a lap welded joint obtained by lapping two sheets of metallic sheets with sheet thicknesses t1 and t2, and welding the central parts are struck with ultrasonic vibration terminals. In the surface of the metallic sheet with the sheet thickness t1, provided that either welded toe part is defined as a first original point, the other welded toe as a second original point, and the diameter D of the ultrasonic vibration terminals is 2 to 8 mm, also, the relative position of the central axis of the ultrasonic vibration terminals to the first original point is defined as x1, and the relative position thereof to the second original point as x2, as to the metallic sheet with the sheet thickness t1, both the positions of that satisfying -t1≤x1≤t1 and that satisfying -t1≤x2≤t1 are struck with the ultrasonic vibration terminals. <P>COPYRIGHT: (C)2004,JPO

Description

【００２１】
【発明の効果】
本発明によれば、２枚の金属板を重ね合わせて中央部を溶接した重ね溶接継手の重ね合わせ面におけるボンド端部近傍の特定位置を超音波振動端子で打撃することにより、従来と比較し良好な疲労強度を得ることができる重ね溶接継手の疲労強度向上方法を提供することにより、金属板を用いた溶接構造物の信頼性を一層向上させることができ、産業上有用な著しい効果を奏する。
【図面の簡単な説明】
【図１】本発明の重ね溶接継手において超音波振動端子による打撃範囲を示す模式図である。
【図２】本発明における超音波振動端子による打撃位置と疲労限強度の関係を表す図面である。
【符号の説明】
１：金属板（上側）、
２：金属板（下側）、
３：溶接金属、
４：溶接止端部（超音波振動端子に近い側）、
５：溶接止端部（超音波振動端子から遠い側）、
６：溶接止端部（超音波振動端子に近い側）、
７：溶接止端部（超音波振動端子から遠い側）、
８：超音波振動端子、
９：超音波振動端子の直径Ｄ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for improving the characteristics of a welded portion of a welded structure using a metal plate such as a bridge, an automobile, or a motorcycle part. Specifically, the center portion is welded by overlapping two metal plates. The present invention relates to a method for improving fatigue strength of a lap weld joint.
[0002]
[Prior art]
In recent years, with the aim of reducing the weight of welded structures, the development of technology that enhances the fatigue strength of welds in a reproducible and effective manner is strongly desired as the strength and thickness of steel materials used are increasing. .
In general, as a welded joint shape in a welded structure using a metal plate such as an automobile or a motorcycle part, a lap weld joint in which two metal plates are overlapped and the central portion is welded is often used. As welding methods, various welding methods such as arc welding, laser welding, plasma welding, and electron beam welding are applied.
[0003]
Obtaining good fatigue strength as a characteristic of the lap weld joint is extremely important, and various techniques for improving fatigue strength have been proposed so far.
For example, in Japanese Patent Application Laid-Open No. 5-156213, in two adherends, a plate on one side or both sides of an overlapped portion is processed in advance so as to have a concave shape, and then bonded while pressing the joint portion, A method has been disclosed in which a crack closing load is generated at an adhesive end as a starting point to improve static tensile strength and fatigue strength.
However, the joining method of this method is based on the premise of adhesion, and other welding methods are not considered. In addition, durability and cost of the adhesive sometimes become a problem.
Further, in JP-A-10-193164, in a lap fillet welded joint using two thin steel plates, a heat source such as a TIG arc is used, and the vicinity of the welded portion of the lower steel plate is parallel to the welded portion. A method for improving fatigue strength by changing the residual stress in the vicinity of the toe portion, which is a fatigue crack generation point, from tension to compression by heating to such an extent that the steel sheet does not melt is disclosed.
However, this method can be applied to a lap fillet joint, and cannot be applied to the lap weld joint in the present specification.
[0004]
Further, in Japanese Patent Laid-Open No. 5-329672, when manufacturing a load wheel having a lap weld joint by joining a rim and a disk, the ratio of the joint width W of the joint overlap portion to the plate thickness T, W / Weld so that T is 0.3 or more, the ratio of bead length L to plate thickness T, and L / T is 1.5 or more, and the bead does not break on the overlapping surface under repeated load. A method for manufacturing a road wheel having fatigue strength is disclosed.
However, the application of this method is limited to road wheels, and fatigue strength could not be sufficiently exhibited with these weld sizes under different part shapes, materials, and stress application conditions.
JP-A-2000-202563 discloses that 0.2% proof stress, tensile strength, and hardness depend on steel plate characteristics when a lap weld joint is formed using a high strength steel plate having a tensile strength of 430 to 1000 MPa. A method for producing a joint having high static tensile strength and fatigue strength is disclosed, in which a rivet having a specified range is used, and after the rivet is driven, the tip of the rivet is deformed and mechanically joined. Has been.
However, since this method is a point-like joint assuming the use of rivets, the fatigue strength is lower than a welded joint having continuous welds such as laser, plasma, and electron beam welding. In addition, the cost of rivets becomes a problem, and there are cases where it cannot be adopted.
[0005]
Also, in US Pat. No. 6,171,415, as a prior art related to a method for improving fatigue strength by applying ultrasonic vibration to a welded portion, ultrasonic vibration is applied along a weld seam portion heated by a welding arc immediately after arc welding. A method is disclosed.
However, since this conventional technology is premised on applying ultrasonic vibration to the weld bead in a high temperature state immediately after welding, the yield strength is low due to the high temperature of the weld metal and the surrounding base metal, and the impact stress is not applied. Also, it is difficult to introduce compressive residual stress to the weld toe, and once the compressive residual stress is introduced, it is canceled by thermal contraction during the subsequent cooling to room temperature. The effect of improving fatigue strength by introducing residual stress cannot be obtained sufficiently. In addition, since there is no disclosure of specific conditions for introducing compressive residual stress at the weld toe such as the range where the ultrasonic vibrator strikes, fatigue strength above a predetermined level can be stably achieved using this method. It is difficult to improve.
In addition, a method of applying compressive residual stress by performing shot peening near the weld toe is known.
This method is a technique in which a large number of steel balls of less than 1 mm are struck on the surface of a site where fatigue cracks start, such as the weld toe, and a compressive residual stress is applied to the inside. Since the crack initiation point is the bond edge on the steel sheet overlap surface and does not exist on the surface, this method has the effect of introducing compressive residual stress to the bond edge existing on the steel sheet overlap surface. Was not obtained.
[0006]
In addition, it is said that the maximum depth of residual stress applied by shot peening is about several hundred μm, and a fatigue strength improvement cannot be obtained with a metal plate having a plate thickness of 0.5 mm or more. Furthermore, this method has a problem that the allowance for improving fatigue strength is not stable.
Further, it is known that the weld toe shape can be improved or the tensile residual stress can be reduced by heating and remelting the weld metal. However, this is applicable only when the stress concentration portion exists on the surface of the welded member as in the case of the lap fillet joint, and cannot be employed in the lap joint of the present specification.
As described above, it has been difficult to employ conventional fatigue strength improving techniques for lap joints that are widely used in automobile bodies and the like. Even if it could be adopted, the fatigue strength improvement cost was not stable or remained at a low level.
[0007]
[Patent Document 1] JP-A-5-156213 [Patent Document 2] JP-A-10-193164 [Patent Document 3] JP-A-5-329672 [Patent Document 4] JP-A 2000-202563 [Patent Document 3] Document 5: US Pat. No. 6,171,415
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, and uses a metal plate by providing a method for improving the fatigue strength of a lap weld joint in which two metal plates are overlapped and the center portion is welded. It is an object to further improve the reliability of the welded structure.
[0009]
[Means for Solving the Problems]
The present invention has been made as a result of intensive studies to solve the above-mentioned problems, and ultrasonic vibration is applied to the vicinity of the weld bead toe of a lap weld joint in which two metal plates are overlapped and the center is welded. The present invention provides a method for improving fatigue strength by hitting with a terminal and a method for producing a welded structure using the same, and the gist of the method is as follows.
(1) A method for improving fatigue strength in which an ultrasonic vibration terminal is used to strike the vicinity of a weld bead toe of a lap weld joint in which two metal plates having thicknesses t1 and t2 are overlapped and the center portion is welded. ,
On the surface of the metal plate having the thickness t1, any one weld toe is a first origin, and the other weld toe is a second origin,
When the diameter D of the ultrasonic vibration terminal is 2 to 8 mm, the relative position of the central axis of the ultrasonic vibration terminal with respect to the first origin is x1, and the relative position with respect to the second origin is x2.
The lap welding is characterized in that, for the metal plate having a thickness of t1, both the position satisfying -t1≤x1≤t1 and the position satisfying -t1≤x2≤t1 are hit with an ultrasonic vibration terminal. A method for improving the fatigue strength of joints.
[0010]
(2) On the surface of the metal plate having a thickness of t2, any one weld toe is the third origin, and the other weld toe is the fourth origin.
When the diameter D of the ultrasonic vibration terminal is 2 to 8 mm, the relative position of the central axis of the ultrasonic vibration terminal with respect to the third origin is y1, and the relative position with respect to the fourth origin is y2.
The metal plate having the thickness t2 is hit with an ultrasonic vibration terminal at both the position satisfying −t2 ≦ y1 ≦ t2 and the position satisfying −t2 ≦ y2 ≦ t2 (1) The fatigue strength improvement method of the lap weld joint as described in 2.
(3) The lap weld joint according to (1) or (2), wherein the two metal plates are high-tensile steel plates having a carbon equivalent Ceq of the following formula (A) exceeding 0.35. Fatigue strength improvement method.
Ceq = C + Si / 24 + Mn / 6 (A)
[0011]
(4) The lap weld joint according to any one of (1) to (3), wherein when the thicknesses of the two metal plates are t1 and t2 and t1 ≧ t2, the bead width W is t2 or more. Of improving fatigue strength of steel.
(5) The method for improving fatigue strength of a lap weld joint according to any one of (1) to (4), wherein the temperature of the lap weld joint when striking with the ultrasonic vibration terminal is 300 ° C. or lower.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described in detail with reference to FIG.
FIG. 1 is a diagram showing an embodiment of a method for improving fatigue strength of a lap weld joint according to the present invention.
In FIG. 1, the metal plate 1 and the metal plate 2 that are overlapped are welded by a weld metal 3, 4 is one weld toe (first origin) on the surface of the metal plate 1, and 5 is a metal plate. 1 is the other weld toe (second origin) on the surface of 1, 6 is one weld toe (third origin) on the surface of the metal plate 2, and 7 is the other weld toe on the surface of the metal plate 2. End (fourth origin), 8 is the ultrasonic vibration terminal, 9 is the diameter D of the ultrasonic vibration terminal, x1 is the relative position of the central axis of the ultrasonic vibration terminal with respect to the first origin, and x2 is the second origin The relative position of the central axis of the ultrasonic vibration terminal with respect to, y1 indicates the relative position of the central axis of the ultrasonic vibration terminal with respect to the third origin, and y2 indicates the relative position of the central axis of the ultrasonic vibration terminal with respect to the fourth origin.
[0013]
In the present invention, the thicknesses of the two metal plates are t1 and t2, and on the surface of the metal plate with the plate thickness t1, one of the weld toes is the first origin and the other weld toe is The second origin is set, the diameter D of the ultrasonic vibration terminal is 2 to 8 mm, the relative position of the central axis of the ultrasonic vibration terminal with respect to the first origin is x1, and the relative position with respect to the second origin is x2. And when
With respect to the metal plate having the thickness t1, the ultrasonic vibration terminal is used to strike a position satisfying −t1 ≦ x1 ≦ t1 and a position satisfying −t1 ≦ x2 ≦ t1. However, for x1, the direction from the first origin to the metal plate side is negative (-), and the opposite direction, that is, the direction from the first origin to the weld metal side is positive (+). In addition, x2 assumes that the direction from the second origin to the weld metal side is negative (−), and the opposite direction, that is, the direction from the second origin to the metal plate side is positive (+).
First, the diameter D of the ultrasonic vibration terminal is set within a range of 2 to 8 mm.
The reason why D is set to 2 mm or more is that if D is smaller than 2 mm, the strength of the ultrasonic vibration terminal is insufficient, so that the terminal may buckle when hit.
Also, the reason why D is 8 mm or less is that when D is larger than 8 mm, the contact area of the terminal is large, so that the impact pressure is insufficient and sufficient compressive residual stress cannot be applied, and a sufficient fatigue strength improvement effect is obtained. Because it disappears. In addition, when the object to be struck is a thin metal, the impact of the ultrasonic vibration terminal may cause the metal plate not to be locally deformed but to cause macro plastic deformation, resulting in a decrease in fatigue strength due to poor shape or increased stress concentration. This is because it is preferable.
[0014]
Next, the hit position by the ultrasonic vibration terminal is set to both a position satisfying the above-mentioned -t1≤x1≤t1 and a position satisfying -t1≤x2≤t1 for the following reason.
FIG. 2 is a diagram showing the relationship between the striking position by the ultrasonic vibration terminal and the fatigue limit strength in the present invention.
FIG. 2 shows one weld toe on the surface of the metal plate 1 in a lap weld joint comprising a metal plate 1 having a plate thickness t1 of 1.2 or 0.8 mm and a metal plate 2 having a plate thickness t2 of 1.2 mm. (First origin) The relative position x1 of the center axis of the vibration terminal is fixed to 0 mm, and the relative position x2 of the center axis of the ultrasonic vibration terminal with respect to the ultrasonic wave with respect to the other weld toe (second origin) is changed. Then, x2 (mm) when each position was hit with an ultrasonic vibration terminal was taken as the horizontal axis, and the fatigue limit strength (kN) at the time of the lap weld joint was taken as the vertical axis. For x2, the direction from the second origin to the weld metal side was negative (-), and the opposite direction, that is, the direction from the second origin to the metal plate side was positive (+).
From FIG. 2, the optimum striking position of the ultrasonic vibration terminal for improving the fatigue limit strength of the lap weld joint is that of the ultrasonic vibration terminal with respect to one weld toe (first origin) on the surface of the metal plate 1. It is determined by the relationship between the relative position x1 of the central axis, the relative position x2 of the central axis of the ultrasonic vibration terminal with respect to the other weld toe (second origin), and the thickness t1 of the metal plate 1, t1 = In the case of 1.2 mm, a position satisfying −1.2 mm ≦ x2 ≦ 1.2 mm, and in the case of t1 = 0.8 mm, a position satisfying −0.8 mm ≦ x2 ≦ 0.8 mm, respectively. By hitting with a terminal, the fatigue limit strength (kN) is remarkably improved, and if it is out of this range, the fatigue strength is hardly improved.
Here, the optimum striking position of the ultrasonic vibration terminal depends on the thickness t1 of the metal plate 1. From the investigation results of the inventors, the spread of the load and impact force due to the striking of the ultrasonic vibration terminal is in the striking direction. The reason is that the direction is limited to about ± 45 degrees.
The first origin of the position x1 of the center axis of the ultrasonic vibration terminal is the one closer to the center axis of the striking terminal out of both toes on one surface of the metal plate 1, and the second origin of x2 Is the one that is farther from the center axis of the striking terminal out of both toes on one surface of the metal plate 1. In the present invention, based on FIG. 2 as a technical basis, the hit position by the ultrasonic vibration terminal is set to both the position satisfying −t1 ≦ x1 ≦ t1 and the position satisfying −t1 ≦ x2 ≦ t1 as described above. Stipulated.
In a normal lap weld joint, not only the weld toes on the front and back surfaces of the steel sheet, but also the weld metal generated by heat input during welding in the vicinity of the bond part between the weld metal and the base metal in the thickness direction of the two metal plates Tensile residual stress is generated due to the difference in thermal expansion from the steel plate, but especially on the steel plate overlap surface, in addition to this tensile residual stress, it becomes a stress concentration part due to the notch shape, so fatigue load is applied It becomes the starting point of fatigue cracks, and is the biggest cause of reducing the fatigue strength of lap welded joints.
In the present invention, by striking with the ultrasonic vibration terminal, stress can be applied in the vicinity of the bond end portion on the steel sheet overlapping surface, which tends to be a fatigue crack generation point particularly in the lap weld joint. The fatigue strength can be improved by reducing the tensile residual stress or changing to a compressive residual stress.
Since the occurrence point of the fatigue crack is in the vicinity of the bond end portion of the overlapping surface, in order to apply stress to that portion, it is desirable to strike the surface of the metal plate at the shortest distance from it with an ultrasonic vibration terminal, From this point of view, the origin of the striking position should be the position immediately above the bond end of the overlapping surface, but the bond end cannot be identified from the surface of the metal plate. A weld toe was used. In a normal lap weld joint, there is no practical problem because the weld toe portion on the surface of the metal plate and the position immediately above the bond end portion are very close.
[0015]
In the present invention, any one of the weld toes 4, 5 on the surface of the metal plate 1 and the weld toes 6, 7 on the surface of the metal plate 2 shown in FIG. It is possible to improve the fatigue strength of the lap weld joint by striking with.
The striking positions y1 and y2 when striking both weld toes 6 and 7 on the surface of the metal plate 2 with the ultrasonic vibration terminal are also in relation to the plate thickness t2 of the metal plate 2 for the same reason as described above. The positions satisfy t2 ≦ y1 ≦ t2 and −t2 ≦ y2 ≦ t2.
Moreover, in order to further improve the fatigue strength of the lap weld joint, it is preferable to perform a hammering treatment with ultrasonic vibration terminals on both weld toes on the surfaces of both metal plates, especially when the plate thickness is thick The effect is even greater.
[0016]
In addition, the metal plate in this invention means a normal steel plate, a stainless steel plate, an aluminum steel plate, etc.
The ultrasonic vibration generator used in the present invention is not particularly limited. After ultrasonic waves are oscillated by an oscillator, the frequency is converted to 20 to 60 kHz by a transducer, and the amplitude is amplified by a waveguide. Then, the ultrasonic vibration terminal comprising a pin having a diameter of 2 mm to 8 mmφ is mechanically vibrated with an amplitude of 30 to 40 μm, so that the surface of the hitting portion is deeper than the surface before hitting while maintaining smoothness. An indentation of about several hundred μm can be formed.
Since the ultrasonic vibration has a frequency of 20 kHz or higher, sufficient compressive residual stress can be applied without impairing surface smoothness compared to conventional techniques such as shot peening, and the shape of the surface of the weld bead. Can be improved.
Since the impact pressure depends on the frequency, these effects improve as the frequency increases at 20 kHz or higher. However, if the ultrasonic frequency exceeds 60 kHz, a very high ultrasonic output device is required and the cost of the device is increased. From the aspect of this, it is not practical and not preferable.
[0017]
The two metal plates are a high-tensile steel plate having a carbon equivalent Ceq of the following formula (A) exceeding 0.35, and a remarkable effect is obtained.
Ceq = C + Si / 24 + Mn / 6 (A)
For example, in automobiles, high-strength steel sheets that satisfy the above-described component ranges are being adopted to promote weight reduction of vehicle bodies.
In the case of a high-tensile metal plate having a Ceq value of (A) exceeding 0.35, the notch sensitivity is increased. Therefore, when the bond end portion on the overlap surface becomes a stress concentration portion like a lap weld joint, the joint The fatigue limit does not depend on the strength of the steel material, and is almost the same level as that of a low-strength general material, and the merit of using a high-tensile steel plate is lost.
In the impact treatment method using an ultrasonic terminal of the present invention, it is possible to introduce residual compressive stress in the vicinity of the bond end portion on the overlapping surface of the steel plates, which is impossible with conventional methods such as shot peening. The fatigue strength improvement effect is remarkably obtained with respect to the lap weld joint to which a high strength steel plate having a Ceq value of 0.35 exceeding 0.35 is applied.
[0018]
The welding method applicable to the lap weld joint in the present invention is not particularly limited, and a deeply welded lap weld joint is produced with a heat source having a high energy density, such as laser welding, plasma welding, and electron beam welding, which are generally used. It can be applied to possible welding methods.
Regarding the weld bead width W of the lap weld joint in the present invention, while maintaining the fatigue strength improvement effect of the lap weld joint, it further prevents fracture of the weld when a static tensile load is applied, and provides sufficient joint strength. In order to ensure, it is preferable to set it as the thickness of the thin metal plate of the two metal plates. That is, when the relationship between the thicknesses t1 and t2 of the two metal plates 1 and 2 is t1 ≧ t2, the bead width W preferably satisfies the relationship W ≧ t2.
In addition, in order to stably obtain the effect of improving fatigue strength by introducing compressive residual stress into a predetermined range in the vicinity of the bond end on the overlap surface by striking with the ultrasonic vibration terminal, the tensile strength of the lap weld joint is The temperature is preferably 300 ° C. or lower. When the temperature of the lap weld joint is 300 ° C or higher, the yield stress of the weld metal and the metal plate is low at the time of impact by the ultrasonic vibration terminal, so the residual stress reaches the yield due to the external force at the impact and is redistributed. In addition, there is a high possibility that a new tensile residual stress is generated due to thermal contraction in the process of lowering the temperature to room temperature after the impact treatment.
Further, when the temperature of the lap weld joint is −10 ° C. or lower, the toughness of the joint deteriorates due to the low temperature. Therefore, it is more preferable to perform the striking treatment at −10 ° C. or higher.
By using a lap welded joint to which the method of the present invention is applied to manufacture a welded structure such as an automobile or a motorcycle, a welded structure having a high fatigue strength and a long life can be manufactured. it can.
[0019]
【Example】
The effect will be described below using an embodiment of the present invention.
After superposing two steel plates having the thickness and tensile strength shown in Table 1 and welding the overlapped central portion with a laser, a striking process was performed with the ultrasonic vibration terminal of the present invention. The joint size was 40 mm (width) × 250 mm (length), and the overlap allowance was 40 mm.
The upper steel plate 1 has a thickness t1 of 1.2 mm and a steel strength of 805 MPa, and the upper steel plate 1 has a thickness t1 of 0.8 mm and a steel strength of 603 MPa, and the lower steel plate 2 has a thickness t2. A steel material having a steel material strength of 598 MPa at 0 mm and a steel material strength of 805 MPa in a plate thickness t2 of 1.2 mm were used.
A YAG laser was used for laser welding, the processing point output was 3.5 kW, the welding speed was 4.0 m / min, and the focal beam diameter was 0.45 mm. The shield method was a center shield, and nitrogen was used as the shield gas. The focal position of the beam was the surface of the upper steel plate 1. At this time, the bead width W was 1.8 mm.
The ultrasonic vibration device had a power supply of 1 kW, a frequency of 25 kHz, an ultrasonic vibration terminal diameter D of 3.0 mm, an amplitude of 30 to 40 μm, and an impact treatment speed of 50 cm / min.
[0020]
The fatigue characteristics after performing the impact treatment with the ultrasonic vibration terminal are measured, and the measured values are compared with the measured values before the impact treatment is performed. When the fatigue limit strength is improved by 10% or more, “OK” ( “Good” and lower are “NG” (bad).
The fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.05 and a repetition rate = 10 Hz.
No. in Table 1 1-4 are examples of the present invention. No. Nos. 1 and 4 are the cases where both the weld toes (x1 = x2 = 0) of the upper steel plate 1 having a plate thickness t1 of 1.2 mm and 0.8 mm were subjected to a striking process. 2 shows both weld toes (x1 = x2 = y1 = y2 = 0) in both the upper and lower steel plates 1 and 2 where the plate thickness t1 of the upper steel plate 1 is 1.2 mm and the plate thickness t2 of the lower steel plate 2 is 2.0 mm. No.) is the case of hitting processing. 3 is a case where a hammering process is performed on both weld toes (y1 = y2 = 0) in the lower steel plate 2 having a thickness t2 of 2.0 mm. In any case, since the diameter D of the ultrasonic vibration terminal and the striking position x1, x2 and / or y1, y2 of the ultrasonic vibration terminal are within the range defined in the present invention, good fatigue characteristics can be obtained.
On the other hand, no. Nos. 5 to 14 are the ultrasonic vibration terminal diameter D, the ultrasonic vibration terminal striking position x1, x2 and / or y1, y2 is out of the range defined in the present invention, or is not welded at all. This is a comparative example.
No. 5 and no. 6 is a position separated by x1 = −1.5 mm from the weld toe of the upper steel plate 1 having a plate thickness t1 of 1.2 mm, a weld toe (x2 = 0), a weld toe (x1 = 0), and welding. The striking treatment was performed with an ultrasonic vibration terminal having a diameter D of 3 mm at a position x2 = + 1.5 mm away from the toe, but the striking relative position (x1 = −1.5, x2 = + 1.5) was out of the scope of the present invention, and both fatigue strength evaluations were poor.
No. 7 and no. 8 is a position where y1 = −2.5 mm away from the weld toe of the lower steel plate 2 having a thickness t2 of 2.0 mm, a weld toe (y2 = 0), and a weld toe (y1 = 0). The striking process was performed with an ultrasonic vibration terminal having a diameter D of 3 mm at a position y2 = + 2.5 mm away from the weld toe, but the striking position from one of the weld toes was outside the scope of the present invention. Both fatigue strength evaluations were poor.
No. 9 is a position at a distance of x1 = −1.5 mm and x2 = + 1.5 mm from both weld toes of the upper steel plate 1 with a thickness t1 of 1.2 mm, and the lower steel plate 2 with a thickness t2 of 2.0 mm. This is a case where an ultrasonic vibration terminal having a diameter D of 3 mm is hit at positions where y1 = −2.5 mm and y2 = + 2.5 mm away from both weld toes. However, the fatigue strength evaluation was poor.
No. 10 and no. No. 13 was a case where no impact treatment was performed by the ultrasonic vibration terminal, and the fatigue strength evaluation was poor.
No. 11 and no. 12 is a position separated by x1 = −1.0 mm from the weld toe of the upper steel plate 1 having a thickness t1 of 0.8 mm, the weld toe (x2 = 0), the weld toe (x1 = 0) and the weld. The striking process was performed with an ultrasonic vibration terminal having a diameter D of 5 mm at a position x2 = + 1.0 mm away from the toe, but the striking relative position (x1 = −1.5, x2 = + 1.5) was out of the scope of the present invention, and both fatigue strength evaluations were poor.
No. No. 14 was subjected to a striking process using an ultrasonic vibration terminal having a diameter D of 10 mm on both weld toes (x1 = 0, x2 = 0) of the upper steel plate 1 having a thickness t1 of 0.8 mm. The diameter D was out of the scope of the present invention, and both fatigue strength evaluations were poor.
[Table 1]

[0021]
【The invention's effect】
According to the present invention, the ultrasonic vibration terminal is used to strike a specific position in the vicinity of the bond end on the overlap surface of a lap weld joint in which two metal plates are overlapped and the center portion is welded. By providing a method for improving the fatigue strength of a lap weld joint capable of obtaining a good fatigue strength, the reliability of a welded structure using a metal plate can be further improved, and there are significant industrially useful effects. .
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a striking range by an ultrasonic vibration terminal in a lap weld joint of the present invention.
FIG. 2 is a diagram showing a relationship between a hitting position by an ultrasonic vibration terminal and fatigue limit strength in the present invention.
[Explanation of symbols]
1: Metal plate (upper side),
2: Metal plate (lower side),
3: Weld metal
4: Weld toe (side near ultrasonic vibration terminal),
5: Weld toe (the side far from the ultrasonic vibration terminal),
6: Weld toe (side near ultrasonic vibration terminal),
7: weld toe (side far from the ultrasonic vibration terminal),
8: Ultrasonic vibration terminal,
9: Diameter D of the ultrasonic vibration terminal

Claims (5)

It is a fatigue strength improving method of hitting the vicinity of the weld toe of a lap weld joint obtained by superimposing two metal plates having a thickness of t1 and t2 and welding the central portion with an ultrasonic vibration terminal,
On the surface of the metal plate having the thickness t1, any one weld toe is a first origin, and the other weld toe is a second origin,
When the diameter D of the ultrasonic vibration terminal is 2 to 8 mm, the relative position of the central axis of the ultrasonic vibration terminal with respect to the first origin is x1, and the relative position with respect to the second origin is x2.
The lap welding is characterized in that, for the metal plate having a thickness of t1, both the position satisfying -t1≤x1≤t1 and the position satisfying -t1≤x2≤t1 are hit with an ultrasonic vibration terminal. A method for improving the fatigue strength of joints. On the surface of the metal plate having a thickness of t2, any one weld toe is the third origin, and the other weld toe is the fourth origin,
When the diameter D of the ultrasonic vibration terminal is 2 to 8 mm, the relative position of the central axis of the ultrasonic vibration terminal with respect to the third origin is y1, and the relative position with respect to the fourth origin is y2.
The ultrasonic vibration terminal is used to strike both the position satisfying -t2≤y1≤t2 and the position satisfying -t2≤y2≤t2 for the metal plate having the thickness t2. The fatigue strength improvement method of the lap weld joint as described in 2. The fatigue strength of the lap weld joint according to claim 1 or 2, wherein the two metal plates are high-tensile steel plates having a carbon equivalent Ceq of the following formula (A) exceeding 0.35. Method.
Ceq = C + Si / 24 + Mn / 6 (A) The fatigue of a lap weld joint according to any one of claims 1 to 3, wherein when the thicknesses of the two metal plates are t1 and t2 and t1≥t2, the weld bead width W is t2 or more. Strength improvement method. The method for improving the fatigue strength of a lap weld joint according to any one of claims 1 to 4, wherein the temperature of the lap weld joint when striking with the ultrasonic vibration terminal is 300 ° C or lower.

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