JP5599653B2 - Welded joint - Google Patents

Description

  The present invention is a welded joint suitable for steel structures that require excellent fatigue properties such as steel bridges, and improves fatigue strength by introducing compressive residual stress without deforming the welded part as a new stress concentration part. About what

  In recent years, with the aging of steel bridges, reports of damage cases due to corrosion and fatigue are increasing. In order to prevent these problems, it is necessary to establish an inspection system. In particular, in the case of fatigue damage, it is important to reduce the external force of the passing vehicle and improve the welding quality from the design and production aspects.

  If the weld has defects such as cracks, or if the shape of the weld toe is inadequate and becomes a stress concentration part, the effect of welding residual stress is superimposed on the repeated stress and fatigue notches are likely to occur, resulting in fatigue failure Therefore, proposals from various viewpoints have been made to prevent this.

  Patent Document 1 relates to a method for improving the fatigue strength of a welded portion and a welded structure using the welded portion, and is a processing device that performs plastic deformation by striking the vicinity of the weld toe while ultrasonically oscillating. It is described that the fatigue strength can be stably improved without depending on the skill level of the worker at high speed by processing under the striking condition.

  Patent Document 2 relates to a laser shock peening method, which uses a pulsed laser beam from a laser light source to instantaneously vaporize a surface thin layer or a surface coating that forms a plasma, and a part of the surface due to its explosive force. Describes a method of introducing a compressive residual stress into a fan rotor blade of a gas turbine engine by a method of locally generating a compressive force.

  Patent Document 3 relates to a method and apparatus for improving the fatigue characteristics of a welded joint, and a device for the same, and using a striking pin whose tip is a specific size, the steel plate surface is formed so that a groove having a specific size is formed at the weld toe. It is described that compressive residual stress is introduced into the weld by compressing.

  Non-Patent Document 1 relates to a method for improving the fatigue strength of a welded joint portion of high strength steel (SM570) by hammer peening and TIG treatment, and since fatigue strength may decrease when hammer peening is applied, the stress concentration at the weld toe And the results of studies on a new hammer peening method that reduces residual stress.

  Normally, hammer peening is performed by the operator holding the peening device in hand so that the tip of the tip (also referred to as a vibration terminal) strikes the welding toe from obliquely above and entrusting the load of the peening device to the welding toe. To reduce the workload.

  Therefore, when hammer peening is performed on the out-of-plane gusset joint in which the rib 2 is erected on the base material 1 shown in FIG. 9, a deep groove serving as a stress concentration point is formed at the weld toe by the tip of the chipper 5 of the peening apparatus. In some cases, a fatigue crack 7 is generated from the tip of the weld bead 3.

  Non-Patent Document 1 introduces that grinding a part of the weld toe with a grinder before hammer peening is effective in preventing the occurrence of fatigue notches, and proposes that hammer peening be performed multiple times of about 3 passes. doing.

JP 2006-175512 A JP 2006-159290 A JP 2010-29897 A

IMPROVING FATIGUE STRENGTH OF WELDED JOINTS BY HAMMER PEENING TIG-DRESSING: Kengo ANAMI, Chitoshi MIKI, Hideki TANI, Haruto YAMActu, Str. / Earthquake Eng. , JSCE, Vol. 17, NO. 1, 57s-68s. 2000 April)

  By the way, when manufacturing a welded structure, a welding method is selected in consideration of the work environment, work efficiency, and welded joint performance. In order to improve the fatigue strength of the welded portion, the fatigue strength of the welded portion described in Patent Document 1 is improved. Although the method is applied, if the characteristics of the welded joint having excellent fatigue characteristics are clear, it is possible to select an optimal fatigue strength improving method in the same manner as the selection of the welding method.

  The hitting processing method described in Patent Document 3 uses a hitting pin having a radius of curvature of the tip of ½ or less of the thickness of the metal material and 2 to 10 mm, and the base material up to a range where the hitting pin does not touch the weld metal during hitting. Although it gives a hit mark on the surface of the metal material, it is difficult to efficiently introduce compressive residual stress.

  In addition, the ultrasonic peening method described in Patent Document 1 is expensive and difficult to obtain as compared with a conventional device that drives a chip with air pressure. The laser shock peening method described in Patent Document 2 requires pretreatment of the material, and the apparatus is expensive and large, and is difficult to apply to steel bridge manufacturing.

  Then, in order to solve the said subject, this invention aims at providing the welded joint excellent in the fatigue characteristic.

In order to improve the fatigue strength of the welded joint, the present inventors have intensively studied a method for reducing the tensile residual stress due to welding of the toe portion where fatigue cracks are likely to occur, and the hammer peening damage mark is welded to the weld toe. It has been found that the maximum compressive residual stress due to impact can be introduced into the weld toe when it is formed farther toward the base metal side. The present invention has been made based on the above findings and further studies. In mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.02 %, And the balance is substantially made of Fe, and the microstructure is a two-phase structure of ferrite and pearlite having the characteristics of (1) to (3), and is manufactured using a steel material having ferrite with an area ratio of 65 to 85%. A welded joint of steel material that has been hammer peened or ultrasonic shock treated,
The vibration terminal has an impact mark continuously formed on the surface of the steel material along the weld bead, and the vibration terminal is a flat quadrilateral whose area is 4 mm ² or more, and the impact mark is formed by the vibration terminal. A weld joint having a maximum depth of 0.03 mm or more and less than 0.40 mm in a region from the weld toe to 2 mm on the base metal side.
(1) L-plane and T-plane pearlite average spacing: 15-30 μm
(2) L-plane and T-plane ferrite average particle diameter: 10 to 20 μm
(3) Perlite lump shape: L (L) ≦ 3Z (L), T (T) ≦ 3Z (T)
Here, L (L): L-direction average length of the pearlite block on the L plane, Z (L): Z-direction average length of the pearlite block on the L plane T (T): The pearlite block on the T plane 1. T-direction average length, Z (T): Z-direction average length of pearlite blocks on the T plane 2. The welded joint according to 1, wherein the square is a rectangle.

  According to the present invention, a welded joint excellent in fatigue strength of a welded portion is obtained, which is extremely useful industrially.

Schematic explaining the principle that fatigue characteristics are improved by the welded joint according to the present invention. It is a figure which shows the influence which the shape of the front-end | tip part of a vibration terminal exerts on the compressive residual stress which arises by impact, (a) is a shape of a front-end | tip part, (b) is a figure which shows the distribution state of the compressive residual stress from an impact center. The figure explaining the X and Y cross-sectional direction of the vibration terminal shown to Fig.2 (a). The figure explaining the L, T, Z surface and L, T, Z direction of the steel materials prescribed | regulated by this invention. The figure which shows the relationship of a pearlite average space | interval and (actual crack length) / (linear distance of a crack propagation direction). The figure which shows the relationship between (actual crack length) / (linear distance of a crack propagation direction) and fatigue crack propagation velocity. The figure which shows the relationship between a pearlite lump shape and a fatigue crack propagation rate ratio. The fatigue test body of an Example is shown, (a) is a top view, (b) is a side view. The figure explaining the condition which performs hammer peening to an out-of-plane gusset joint.

  The present invention is a welded joint of steel material, and is compressed to the weld toe by continuously forming a striking trace along the weld bead on the surface of the steel material around the weld toe, excluding the weld metal and the weld toe. The residual stress is introduced. In the following description, the toe (also referred to as a weld toe) is defined as a line of intersection between the surface of the member and the surface of the weld metal (the illustrated welding terminology dictionary, Nikkan Kogyo, September 20, 1971, 4th edition).

  FIG. 1 is a schematic view for explaining the principle of improving fatigue characteristics by a welded joint according to the present invention, and shows a side view of a welded joint welded by welding a rib 2 around a base material 1. The surface of the base material 1 at a distance d from the toe 4 of the weld bead 3 is plastically deformed (indicated by a dotted line) by pressing the base material surface with a tip (not shown) having a width B in a direction perpendicular to the base material surface. It is a blow mark that caused.

  The position of the impact mark on the surface of the base material 1 (specified by the distance d from the toe 4) is determined by the compressive residual stress generated in the base material 1 due to the welding of the toe 4 when the impact mark is formed by the tip having the width B. The tensile residual stress is canceled out so that the toe 4 has a compressive residual stress.

  In the welded joint according to the present invention, the base material surface is pressurized as an index of the influence of the compressive residual stress generated in the base material 1 when the impact mark is formed on the tensile residual stress at the toe 4. The shape of the vibration terminal used for forming the hitting trace on the surface and the maximum depth of the hitting trace in a specific width region from the weld toe to the base metal side are used. In addition, in the welded joint according to the present invention, it is a principle that the weld bead is not hit including the weld toe, but the weld bead is temporarily hit to the extent that plastic deformation is not given to the weld bead by adjustment of the work start or the like. Is fine.

Hammer peening using a vibration terminal having a flat square with an area of 4 mm ² or more at the tip or an ultrasonic impact treatment device so that the central axis of the vibration terminal is perpendicular to the surface of the base material (flat plate). The maximum depth to the bottom is 0.03 mm or more and less than 0.40 mm on the base material 1 side from the toe 4. The vibration terminal strikes the base material surface in the vertical direction.

  FIG. 2 shows a state in which the center axis of the vibration terminal is perpendicular to the surface of the base material (flat plate, 12 mm thick steel plate having a yield strength of 294 MPa and a tensile strength of 445 MPa) by applying a load to the vibration terminal model shown in FIG. The stress distribution was obtained by FEM analysis by simulating the case where the load was released after the concave deformation was applied to the base metal surface side by pushing 0.1 mm so that the compression from the hit center The distribution state of the residual stress is shown in (b).

  Since the compressive residual stress by the vibration terminal is symmetrical, FIG. 2B shows the right half from the axis center of the vibration terminal. The XZ cross section and YZ cross section of FIG. The vertical axis in FIG. 2 (b) indicates the residual stress, and the horizontal axis in (b) indicates the distance from the axis center of the vibration terminal. When the tip shape of the vibration terminal is rectangular (1. tip rectangle), the X coordinate of 2 mm is the position of the right side surface of the vibration terminal having a width of 4 mm, and the X coordinate of 1.5 mm is the right end portion of the hitting mark.

  When the tip shape of the vibration terminal is rectangular (1. tip rectangle), the range in which compressive residual stress is introduced to the steel sheet surface with a single impact is wider than when the tip is hemispherical (2. tip sphere). In addition, a value close to the maximum compressive residual stress is maintained in a wide range. In FIG. 2B, the width in which the compressive residual stress of 300 to 400 MPa is generated is about 3 mm, but the tip having a hemispherical tip has a width of about 1 mm.

  Therefore, in the case where the tip shape of the vibration terminal is a flat quadrangle (1. tip rectangle), it is possible to repeatedly hit the same place with a smaller number of times compared to a hemisphere tip (2. tip sphere), Efficient, stable and deep strike mark shape can be obtained. Depending on the strength of the base material, a crack may occur around the square due to the impact, so that the corners around the flat part of the square may be chamfered to the extent that efficiency is not impaired.

Flat rectangular portion of the vibration terminal tip, when the area is less than 4 mm ^2, it becomes difficult to pressurize surface of the base material and the vertical direction, and 4 mm ² or more.

  If the maximum depth from the toe 4 to the bottom of the striking trace formed on the base material 1 side is less than 0.03 mm, compressive stress cannot be applied to the toe, whereas if 0.40 mm or more, the impact is not achieved. The plastic deformation around the scar becomes excessive and becomes a new stress concentration source.

  In the present invention, the hitting trace having the above-described depth is formed in a region of 2 mm from the weld toe to the base metal side. From FIG. 2, it is possible to introduce compressive residual stress into the weld toe even when a hitting mark is formed in contact with the weld toe (when the weld toe is located at an X coordinate of 1.5 mm).

  In addition, since the steel structure is subjected to loads from various directions, it is preferable to form the striking traces continuously along the weld bead, but in particular, the striking is performed only along the part where the weld bead is likely to cause fatigue damage. A mark may be formed.

  When the tip of the vibration terminal is rectangular, at the time of hitting, it is preferable to hit the long side of the tip of the hitting trace parallel to the weld toe. This is because the compressive residual stress is distributed in a wider range as the portion parallel to the weld toe becomes longer. When hitting against the weld toe, the long side on the base metal side of the tip of the strike mark is hit against the weld toe.

In the welded joint according to the present invention, since the deformation that becomes a stress concentration source does not occur in the weld metal or the weld toe, the shape of the tip of the vibration terminal, so as not to form an impact mark on the weld metal or the weld toe, A position where a hitting mark is formed in an area of 2 mm from the weld toe to the base material side is appropriately combined. The tip of the vibration terminal changes the length of the long and short sides in a flat quadrilateral with an area of 4 mm ² or more.

  The hitting trace is preferably formed by hammer peening or an ultrasonic impact treatment device that repeatedly impacts the base metal surface. The hitting trace is preferably formed by hitting a plurality of times so that part or all of the hitting marks overlap each other.

  Before pressing the base metal surface with the vibration terminal, if the r part is provided by the grinder grinding etc. at the boundary between the weld toe and the base metal, the deformation of the base material surface does not reach the weld toe, and a larger compressive residual It is possible and preferable to introduce stress into the weld toe. In addition, the function and effect of the present invention can be obtained by either hammer peening or ultrasonic impact treatment.

In the welded joint according to the present invention, the safety of a fatigue crack occurrence risk part is remarkably improved by using a steel material having excellent fatigue crack propagation resistance as a base material. The microstructure of steel with excellent fatigue crack propagation resistance is mainly composed of a two-phase structure of ferrite and pearlite, and cracks can be controlled by controlling the area ratio and form on the L, T, and Z planes. It has small propagation anisotropy and excellent fatigue crack propagation resistance.
[Microstructure]
In order to obtain performance with small crack propagation anisotropy and excellent fatigue crack propagation resistance, the area ratio of ferrite and pearlite in steel materials and the L, T, and Z planes of these structures (specified in Fig. 4) As representative three-dimensional microstructures, pearlite spacing, ferrite grain size, and pearlite three-dimensional shape are defined as follows.

1. Considering the area ratio ductility and bending workability of ferrite and pearlite, the microstructure of the microstructure is a two-phase structure composed of ferrite and pearlite. When the ferrite area ratio is less than 65%, ductility and bending workability are lowered. On the other hand, when it exceeds 85%, sufficient strength cannot be obtained.

  When the pearlite area ratio is less than 10%, the effect of improving the fatigue crack propagation characteristics described later is not exhibited. On the other hand, when it exceeds 30%, weldability, ductility, and bending workability deteriorate.

  In the present invention, bainite and martensite are allowed to be mixed in as the remaining structure. However, in consideration of ductility and bending workability, the area fraction of the remaining structure is preferably 5% or less.

2. Perlite average interval between L plane and T plane The pearlite interval effectively arranges pearlite in the direction in which fatigue cracks propagate, and this pearlite controls the formation of a plastic zone at the crack tip. In order to induce crack bending due to wraparound, the average interval is set to 15 μm or more.

  However, if the average spacing exceeds 30 μm, the frequency of wraparound decreases and the crack propagation resistance decreases, so the thickness is set to 15 to 30 μm. The average interval is desirably secured for any surface of the steel sheet so as not to cause crack propagation anisotropy, and is defined by the L and T planes as representatives.

3. The ferrite average grain diameter of the L face and the T face is 20 μm or less so that the number of times the fatigue crack collides with the ferrite grain boundary is sufficient to obtain the effect of reducing the crack growth rate. To do.

  However, if the particle size is too small, the formation of the plastic zone between the above-mentioned pearlites is hindered. It is desirable to secure an arbitrary surface of the steel plate so as not to cause the anisotropy of grain size crack propagation, and it is defined by the L plane and the T plane as a representative.

4). In the present invention, the shape change of the plastic zone at the crack tip that occurs when the fatigue crack approaches the pearlite is controlled, and the bending of the crack due to the pearlite wrapping around the fatigue crack is utilized to eliminate fatigue. Improved crack propagation resistance. The three-dimensional shape of pearlite is defined as follows so that good fatigue crack propagation resistance can be exhibited in all directions without causing anisotropy in the crack propagation direction. To do.

That is, the main object of the present invention is to provide equally excellent fatigue crack propagation resistance characteristics with respect to fatigue crack propagation in various directions with respect to a steel sheet, which is a concern in actual structures. The pearlite three-dimensional shape is defined by Equations (1) and (2) so that the fatigue crack propagation characteristics in each direction are equal.
L (L) ≦ 3Z (L) (1) Formula T (T) ≦ 3Z (T) (2) where L (L) is an average in the L direction on the L plane of the pearlite block. Length, Z (L) is the average length in the Z direction of the pearlite block on the L plane, T (T) is the average length in the T direction of the pearlite block on the T plane, and Z (T) is the pearlite block on the T plane The average length in the Z direction is shown. 5 to 7 show the effect of the microstructure that satisfies the above-described regulations 1 to 3.

  FIG. 5 shows the relationship between the average pearlite interval and (actual crack propagation length) / (linear distance in the crack propagation direction). The ferrite area ratio, pearlite area ratio, ferrite average particle diameter, and pearlite lump shape are shown in FIG. It was obtained from the results of a fatigue crack propagation test (test condition: ΔK = 20 MPa√m constant) in the T direction of the steel with only the pearlite average interval being changed within the range.

  Here, when (actual crack propagation length) / (straight distance in the crack propagation direction) is large, cracks and wraparound often occur in the pearlite part described above, resulting in an increase in the actual propagation distance and fracture. This means that surface irregularities are increasing.

  (Actual crack propagation length) is defined by the total length of the crack propagated. If the crack is meandering, the length is along the meandering. If the crack is branched, the length is the length of the main crack (the crack that propagates the longest).

  (Linear distance in the crack propagation direction) is defined as the length obtained when a crack is projected in the crack propagation direction. If the crack is meandering, the crack propagation direction is the direction in which the crack advances while meandering, and the length obtained by projecting the crack in that direction. If the crack is branched, the crack propagation direction is the direction in which the main crack proceeds, and the distance obtained by projecting the main crack in that direction.

  The steel material aims to have a value of (actual crack propagation length) / (straight distance in the crack propagation direction) of 1.1 or more, and this value is achieved at an average pearlite interval of 15-30 μm. It is recognized that

  FIG. 6 shows the relationship between the actual crack propagation length / the linear distance in the crack propagation direction and the fatigue crack propagation speed obtained in the test performed in FIG.

  As a result of the increase in the actual propagation distance and the increase in the fracture surface irregularities due to the control of the average pearlite spacing described above, this value is particularly 1. with the increase in (actual crack propagation length) / (linear distance in the crack propagation direction). In the case of 1 or more, a decrease in fatigue crack propagation speed, that is, an improvement in fatigue crack propagation resistance is observed.

  FIG. 7 shows L (L) / Z (L): ratio of L direction average length on L plane to Z direction average length on L plane, T (T) / Z (T): T The ratio of the average length in the T direction on the plane and the average length in the Z direction on the T plane, and these and the velocity (Z) / velocity (L): the fatigue crack propagation velocity in the Z direction and the crack propagation velocity in the L direction. Ratio, speed (Z) / speed (T): The relationship between the fatigue crack propagation speed in the Z direction and the crack propagation speed in the T direction is shown.

  The results shown in FIG. 7 are the results of a fatigue crack propagation test (test condition: ΔK) of steel in which the ferrite area ratio, pearlite area ratio, ferrite average particle diameter, and pearlite average interval are within the scope of the present invention, and only the pearlite lump shape is changed. = 20 MPa√m constant).

  By setting L (L) / Z (L) and T (T) / Z (T) to 3 or less, fatigue crack propagation characteristics with less directionality (small anisotropy) can be obtained. Equally excellent fatigue crack propagation resistance is realized for fatigue cracks that propagate in various directions with respect to the steel sheet.

[Ingredient composition] In the description, “%” means “mass%”.
C
C is added in an amount of 0.04% or more to ensure strength. If it exceeds 0.20%, weldability is impaired, so 0.04 to 0.20%, preferably 0.06 to 0.18% is added.

Si
Si is added in an amount of 0.05% or more to ensure deoxidation and strength. If added over 0.50%, weldability and toughness deteriorate, so 0.05 to 0.50%, preferably 0.10 to 0.40%.

Mn
Mn is added in an amount of 0.5% or more in order to ensure strength and toughness by increasing hardenability. If it exceeds 1.8%, the weldability deteriorates, so 0.5 to 1.8%, preferably 0.8 to 1.6% is added.

P
P is an impurity and degrades toughness. Therefore, its content is preferably as small as possible, and is 0.05% or less, preferably 0.03% or less in terms of manufacturing cost.

S
Since S is an impurity and degrades toughness, the content is preferably as small as possible, and is 0.02% or less, preferably 0.01% or less in terms of manufacturing cost.

  The above is the basic component composition of the steel according to the present invention, but when further improving the strength, toughness, weldability, or imparting weather resistance, Cu, Ni, Cr, Mo, Nb, V, Ti, B Add one or two or more.

Cu
Cu increases the strength by solid solution and improves the weather resistance, so it is added according to the desired properties. When added, if it exceeds 0.4%, the weldability is impaired, and flaws are likely to occur during the production of the steel material, so 0.4% or less, preferably 0.3% or less.

Ni
Ni improves low-temperature toughness and weather resistance, and improves hot brittleness when Cu is added, so it is added according to desired characteristics. When added, if it exceeds 0.8%, the weldability is impaired and the steel material cost increases, so the content is made 0.8% or less, preferably 0.6% or less.

Cr
Cr increases the strength and improves the weather resistance, so it is added according to the desired properties. When added, if it exceeds 0.4%, weldability and toughness are impaired, so 0.4% or less, preferably 0.3% or less.

Mo
Since Mo increases strength, it is added according to desired characteristics. When added, if it exceeds 0.4%, weldability and toughness are impaired, so 0.4% or less, preferably 0.2% or less.

Nb
Nb suppresses austenite recrystallization during rolling to achieve finer grains, and at the same time, precipitates during air cooling after accelerated cooling and increases strength. Therefore, Nb is added according to desired characteristics. When added, if it exceeds 0.05%, the toughness is impaired, so 0.05% or less, preferably 0.03% or less.

V
V precipitates during air cooling after accelerated cooling and increases the strength, so it is added according to the desired characteristics. When added, if it exceeds 0.05%, weldability and toughness are impaired, so 0.05% or less, preferably 0.03% or less.

Ti
Ti increases strength and improves weld toughness, so it is added according to desired properties. When adding, if it exceeds 0.03%, the steel material cost increases, so 0.03% or less, preferably 0.02% or less.

B
B increases the hardenability and increases the strength, so it is added according to the desired properties. When adding, if it exceeds 0.003%, the weldability deteriorates, so 0.003% or less, preferably 0.002% or less.

Steel components according to the steel according to the present invention described above, 1000 ° C. or higher, then heated to 1300 ° C. or less to complete the rolling at a cumulative reduction of 50% or more of the rolling performed Ar ₃ point or more by Ar ₃ or more points Then, it is obtained by accelerated cooling at 10 ° C./s or more from Ar ₃ to Ar ₃ −60 ° C. to 650 ° C. or less and 450 ° C. or more.

The above temperature is the surface temperature of the steel material, and the cooling rate is the average cooling rate in the thickness direction of the steel material. The Ar ₃ point is determined by Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (where the element symbol represents the content in mass% of each element in the steel). be able to.

A rib 2 (SM490Y) of 75 mm × 50 mm is welded to a base material 1 (SM490Y) having a width of 150 mm × a length of 500 mm × a plate thickness of 12 mm (wire MXZ200-1.2Φ, 100% CO ₂ , 240A-30V-40CPM, Hammer peening (with a pneumatic pressure of about 6 kg / cm ² , a frequency of 90 Hz, and a moving speed of 0.25 mm / second) using a vibration terminal having a rectangular flat portion with a tip of 3 mm × 4 mm on a specimen welded at 10.8 KJ / cm) ) Was repeatedly struck vertically while preventing it from hitting the weld toe. Some test pieces were hit so that the base metal part remained between the weld toe and the hitting marks. After forming hitting marks, it was subjected to a fatigue test.

The base material 1 (SM490Y: symbols A to E) has a component composition within the scope of the present invention, but a steel material having a slow fatigue crack propagation rate (× 10 ⁻⁶ cm / cycle) and excellent fatigue crack propagation resistance ( Tested by changing the fatigue crack propagation rate (× 10 ⁻⁶ cm / Cycle) between steel materials of the same component composition (symbols B and E) by adjusting the microstructures (symbols A, C, D) did. Symbol B is a steel material (example of the present invention) having a slow fatigue crack propagation rate (× 10 ⁻⁶ cm / cycle) and excellent fatigue crack propagation resistance, and symbol E is a fatigue crack propagation rate (× 10 ⁻⁶ cm). / Cycle) is a steel material (comparative example) that is fast and inferior in fatigue crack propagation resistance.

  The fatigue test was performed by chucking both ends of the base material 1 and repeatedly applying stress to the longitudinal direction of the rib 2 with respect to the test body. FIG. 8 shows a plan view (a) and a side view (b) of the specimen.

  Table 1 shows the composition of the base material 1 (SM490Y), the fatigue crack propagation speed, the maximum depth and width of the impact mark, and the fatigue test results. The measurement of the striking scar depth was performed every 0.1 mm along the steel plate surface using a laser displacement meter to obtain the maximum depth.

It can be seen from Table 1 that the welded joint according to the present invention has excellent fatigue characteristics. Symbol D is a steel material having a slow fatigue crack propagation rate (× 10 ⁻⁶ cm / cycle), but the maximum depth of the hitting marks is outside the scope of the present invention and is inferior in fatigue characteristics. Symbol E is fatigue crack propagation. The speed (× 10 ⁻⁶ cm / Cycle) was high, and the maximum depth of the hitting mark was also inferior in fatigue characteristics outside the scope of the present invention.

1 Base Material 2 Rib 3 Weld Bead 4 Weld Toe 5 Chipper (Vibration Terminal)

Claims (2)

In mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.02 %, And the balance is substantially made of Fe, and the microstructure is a two-phase structure of ferrite and pearlite having the characteristics of (1) to (3), and is manufactured using a steel material having ferrite with an area ratio of 65 to 85%. A welded joint of steel material that has been hammer peened or ultrasonic shock treated,
The vibration terminal has an impact mark continuously formed on the surface of the steel material along the weld bead, and the vibration terminal has a tip having a flat quadrangle with an area of 4 mm ² or more and 12 mm ² or less. A welded joint having a maximum depth of 0.03 mm or more and less than 0.40 mm in a region from the weld toe to 2 mm from the weld toe by the vibration terminal.
(1) L-plane and T-plane pearlite average spacing: 15-30 μm
(2) L-plane and T-plane ferrite average particle diameter: 10 to 20 μm
(3) Perlite lump shape: L (L) ≦ 3Z (L), T (T) ≦ 3Z (T)
Here, L (L): L-direction average length of the pearlite block on the L plane, Z (L): Z-direction average length of the pearlite block on the L plane T (T): The pearlite block on the T plane T direction average length, Z (T): Z direction average length of pearlite block on T plane   The welded joint according to claim 1, wherein the square is a rectangle.