JP5000476B2 - Fillet welded joints with excellent fatigue crack initiation characteristics - Google Patents

Description

  The present invention relates to a method for improving the fatigue strength of a welded structure applied to the manufacture of automobile underbody members, building materials, machine parts, and the like, and more specifically, fillet welds a thin plate or a thick plate having a thickness of 1 to 20 mm. It is related with the welded joint excellent in the fatigue strength which increased the generation | occurrence | production life of the fatigue crack in the weld toe part of the welded joint obtained by this.

  In recent years, the strength of structural steel plates has been increasing rapidly in response to demands for weight reduction of structures such as automobile undercarriage members, building materials, and machine parts.

  In general, the fatigue strength of a steel sheet is improved with an increase in the steel sheet strength, but it is common technical knowledge that the fatigue strength of a steel structure manufactured by welding the steel sheet hardly increases even when the steel sheet strength is increased. This is because, in general, fatigue failure of a welded structure may occur from the weld toe where the weld bead and the steel plate surface intersect, and stress is likely to concentrate and weld defects are likely to occur. This is because the fatigue crack generation characteristics hardly affect the composition and structure of the steel sheet.

  Therefore, in welded steel structures that are used in environments subject to repeated loads and where fatigue strength is a problem, fatigue strength is improved in addition to the yield strength and tensile strength of welded joints in terms of safety and durability of the steel structure. Is anxious.

  Since the fatigue strength of a welded structure is mainly governed by the shape of the toe portion of the welded portion, a method for ensuring the improvement of the fatigue strength of the welded joint by the toe end treatment of the welded portion has been conventionally applied. For example, a method of grinding the weld toe portion by a grinder to increase the toe radius, a method of remelting the toe end by TIG welding and plasma treatment and smoothing the toe shape (see, for example, Patent Document 1), A typical toe processing method is a method in which the weld toe shape is improved by shot peening to generate compressive residual stress.

  However, the weld toe treatment not only increases the number of man-hours for constructing the structure, but there are many cases where the toe part treatment cannot be carried out depending on the welded part. Yes.

  In addition, for thick plates used in large structures such as ships and marine structures, the fatigue characteristics of the base steel plate in welded joints are improved, and fatigue cracks that occur at the weld toe are moved in the thickness direction of the base metal plate. It is also known that delaying propagation is effective to improve the fatigue life of welded structures. As a steel plate excellent in such fatigue crack propagation resistance, in a matrix mainly composed of ferrite, pearlite, bainite, and the like, a hard phase such as martensite that is flat in the rolling direction has a predetermined existence interval, Steel sheets (see, for example, Patent Documents 2 to 4) that are present at a predetermined volume ratio and reduce the crack propagation speed in the thickness direction by the hard phase have been proposed.

  However, although these steel plates with excellent fatigue crack propagation resistance are effective in reducing the propagation speed of fatigue cracks generated when the plate thickness is large and increasing the fatigue life of welded joints, they are automotive steel plates. The effect cannot be expected with a joint in which a thin plate having a small thickness is welded.

  Further, as a method of improving the joint fatigue strength from the weld surface, the average hardness A in the range of 1 mm from the fusion boundary (FL) at the weld toe to the weld metal side, and the weld heat affected zone (HAZ) from the fusion boundary (FL). ), The hardness difference (AB) of the maximum hardness B in the range of 1 mm is 0 to 15 or 26 to 39, and the weld metal or weld heat affected zone (HAZ) in the vicinity of the melting boundary (FL). Has been proposed (for example, see Patent Documents 5 and 6).

  In these methods, the hardness of the weld metal or weld heat affected zone (HAZ) in the vicinity of the fusion boundary (FL) of the weld toe of the turned fillet welded joint with a base metal plate thickness of about 20 mm is compared with the surrounding hardness. In order to suppress strain concentration due to local lowering, the hardness of the weld metal is made higher (overmatching) than the weld heat affected zone (HAZ).

  However, as a result of examinations by the inventors' experiments, fillet welded joints in which steel plates of 1 to 20 mm in thickness, such as automobile steel plates, building materials, and steel plates for machine parts, are pile fillet welded or turned fillet welded, It has been confirmed that when the hardness is made higher (overmatching) than the weld heat affected zone (HAZ), the fatigue strength of the welded joint may be greatly reduced.

  Further, as a method for improving the brittle fracture resistance Kc of a butt weld joint used for large structures such as ships and marine structures, the hardness of the weld metal is set to 70 to 110 of the hardness of the base steel plate. %, A welded joint excellent in brittle fracture resistance has been proposed (see, for example, Patent Documents 7 to 9).

  In these techniques, the hardness of the weld metal of a butt welded joint of a thick plate having a thickness of 15 to 100 mm is not reduced compared to the weld heat affected zone (HAZ) (70% of the hardness of the base steel plate). (Undermatching) suppresses local stress at the melting boundary (FL) that becomes the most fragile portion due to coarsening of crystal grains, and prevents a decrease in fracture toughness value. In the butt welded joint, the melt boundary (FL), which is the weakest part, covers the entire plate thickness range, the hardness of the entire plate thickness range is lower than the weld heat affected zone (HAZ), and the local stress on the HAZ side is welded. The brittle fracture toughness of the butt weld joint can be improved by preventing the metal from being dragged and increased.

  However, the area that causes the fatigue strength reduction of fillet welded joints is not the entire thickness range of the welded part, but the stress concentration in the vicinity of the weld toe part on the front and back sides, which causes brittle fracture of the butt welded joint. It is different from the generation mechanism. In addition, from the results of the study by the inventors, fillet welding produced by laminating fillet welding or turning fillet welding of steel plates having a relatively thin plate thickness of 1 to 20 mm, such as automobile steel plates, building materials, and steel plates for machinery. When joints are targeted, it has been confirmed that sufficient effects cannot be obtained to improve the fatigue strength of the joints.

  Furthermore, as a method of improving the joint fatigue strength from the weld surface, in a fillet welded joint of a thin steel plate having a thickness of 1 to 4 mm, the martensite volume ratio in the weld metal is set to 50% or more, and the angle of the weld toe A high fatigue strength fillet welded joint has been proposed in which the angle is 110 to 150 ° (see, for example, Patent Document 10). This joint generates martensitic transformation expansion on the low temperature side in the process of cooling the weld metal after fillet welding of a high strength steel sheet with a tensile strength of 680 to 980 MPa or less, and effectively compresses residual stress at the weld toe. In order to reduce the stress concentration at the toe portion of the weld joint, the angle of the weld toe portion is controlled within a predetermined range.

  However, in order to introduce compressive residual stress to the weld toe using this method, a high-strength steel sheet having a tensile strength of 680 MPa or more is used, and the component composition is set in order for the weld metal to undergo martensitic transformation expansion at low temperatures. It is necessary to use an adjusted welding material, and it is difficult to sufficiently improve the joint fatigue strength with a steel sheet having a tensile strength of 680 MPa or less or a welding material having a general component composition.

JP 07-242992 A Japanese Patent Laid-Open No. 06-271985 Japanese Patent Application Laid-Open No. 07-090478 JP 07-242992 A Japanese Patent Laid-Open No. 07-171679 JP-A-11-104838 JP 2005-125348 A JP 2005-144552 A JP 2006-088184 A JP-A-2005-238305

  In order to prolong the fatigue life of a fillet welded joint obtained by laminating fillet welding or turning fillet welding of a thin plate or thick plate having a thickness of 1 to 20 mm, based on the current state of the prior art, It is an object of the present invention to provide a joint design method having improved fatigue crack generation characteristics at the weld toe and excellent in fatigue strength.

The present invention solves the above-mentioned problems in fillet welded joints by further developing a method of suppressing strain concentration occurring in the weld metal or weld heat affected zone (HAZ) in the vicinity of the above-mentioned melt boundary (FL). The gist of the invention is as follows.
(1) In a fillet welded joint of steel plate, the hardness Hv (FL-0.5) of the weld metal at a position 0.5 mm away from the base point to the weld metal side with the melting boundary FL at the weld toe as the base point Is 105 to 254, the hardness Hv (FL + 0.5) of the heat affected zone at a position 0.5 mm away from the base point toward the heat affected zone is 183 to 390, and the hardness of the weld metal and the heat affected zone The ratio of hardness [Hv (FL−0.5) / Hv (FL + 0.5)] is 0.3 or more and 0.9 or less, and has excellent fatigue crack generation characteristics. Fillet welded joint.
(2) The fillet welded joint having excellent fatigue crack generation characteristics according to (1) above, wherein a curvature radius ρ of the weld toe is 1 mm or more.
(3) The fillet welded joint excellent in fatigue crack resistance as described in (1) or (2) above, wherein the angle θ of the weld toe is 100 degrees or more.
(4) The fatigue resistance according to any one of (1) to (3) above, wherein at the weld toe portion, the weld bead toe and the melt boundary are separated by at least 0.05 mm or more. Fillet welded joint with excellent crack initiation characteristics.
(5) The fillet welded joint excellent in fatigue crack resistance according to any one of (1) to (4), wherein the steel sheet has a thickness of 1 to 20 mm.

  According to the present invention, the fatigue life of the welded joint is improved compared with the conventional one by improving the fatigue crack generation characteristics of the weld toe particularly in a thin plate or thick plate fillet welded joint having a thickness of 1 to 20 mm. Can be lengthened. Therefore, by applying the present invention to the manufacture of automobile underbody members, building materials, machine parts, etc., the fatigue strength of these welded structures can be improved, and the durability and safety can be improved. The contribution of is tremendous.

  In the present invention, a target welded joint form is a fillet welded joint obtained by laminating a plurality of steel plates and performing fillet welding or rotating fillet welding, which is often applied in the automotive field and the like. The plate thickness of the steel plate used in the above is 1 to 20 mm, preferably from the viewpoint of improving joint fatigue strength, and is intended for a steel plate having a relatively thin plate thickness of 1 to 10 mm.

  The design of conventional fillet welded joints is to make the strength and hardness of the weld metal higher than the base metal in order to prevent deformation and distortion from concentrating on the weld toe, The welding material used in fillet welding has been selected such that its strength is overmatched compared to the base metal strength.

However, according to the study by the present inventors, a lap fillet welded joint or a turned fillet welded joint of a steel plate having a thickness of about 1 to 20 mm is welded as compared with a fillet welded joint of a steel plate thicker than this. degree of concentration of strain and stress of the toe portion (the degree of stress concentration) is relatively low, and because concentration regions distortion is Ru extensive der, the hardness of the weld metal by applying the method of higher than the base material However, the joint fatigue improvement effect was not sufficiently obtained. In particular, this phenomenon was remarkable in a lap fillet welded joint of steel plates having a thickness of about 1 to 10 mm.

  In order to confirm the influence on the fatigue strength of the fillet welded joint by the steel plate strength and the hardness of the weld metal, the inventor has a plate thickness t of 1 to 20 mm and a tensile strength of 270 to 980 MPa class. Using a steel plate and a welding material selected so that the strength and hardness of the weld metal change with respect to the strength of the steel plate, a lap fillet welded joint as shown in FIG. 1 is manufactured, and a fatigue test is performed. The fatigue life of the joint was evaluated.

  As a result, it was confirmed that the fatigue life of the welded joint was hardly improved even if only the tensile strength of the steel sheet was increased within the range of the tensile strength. Further, the fatigue crack occurrence positions of the welded joints were all in the heat affected zone of the weld toe or the weld metal portion.

  Next, the relationship between the fatigue life strength [SF100] of fillet welded joints and the tensile strength of fillet welded joints when the strength of the welding material is varied with respect to the tensile strength (TS (B)) of the steel sheet is investigated. did.

  FIG. 2 shows the ratio [Hv (FL−0.5) / Hv (FL + 0.5)] (horizontal axis) of the hardness of the weld metal to the hardness of the base metal heat affected zone (HAZ) in the fillet weld joint, 1 million times fatigue life strength of fillet welded joint [SF100] (left vertical axis) and ratio of joint tensile strength to steel plate tensile strength [TS (J) / TS (B)] (right vertical axis) Show the relationship.

  Note that the hardness of the weld metal is the hardness [Hv (FL-0) at a position 0.5 mm away from this base point to the weld metal side (− side) with the melting boundary FL at the weld toe as the base point (0). .5)] is measured, and the hardness of the base material heat-affected zone (HAZ) is the hardness [Hv (FL + 0.5)] at a position 0.5 mm away from the base point to the heat-affected zone side (+ side). Was measured.

  In the present invention, the hardness measurement position of the weld metal is set at a position 0.5 mm away from the melting boundary (FL) to the weld metal side, and the hardness measurement position of the base metal heat affected zone is determined from the melting boundary (FL). The reason why the position is 0.5 mm away from the material heat affected zone side is as follows.

  In order to evaluate the local stress increase and local strain concentration at the weld toe of fillet welded joints, the fatigue crack is the size of one crystal grain during the fatigue crack initiation and growth process. It is necessary to accurately grasp the hardness distribution in a range corresponding to the initial process of being formed to a degree and growing to a length of about 1 mm.

  When measuring micro Vickers hardness, the indentation size is smaller than structural components such as ferrite and bainite, and the measured values vary greatly at the location where the indentation is made. Therefore, the hardness near the melting boundary (FL) It is difficult to accurately evaluate the strength characteristics of the crack tip region by catching a rapid change in thickness.

  Therefore, in the present invention, the measurement position of the micro Vickers hardness is set to 0 from the melting boundary (FL) to the molten metal side, in which the change in hardness of the melting boundary (FL) can be reliably captured and interference between measurement pitches can be ignored. The position of 0.5 mm and the position of 0.5 mm from the fusion boundary (FL) to the weld heat affected zone side were used as the hardness measurement positions.

  The indentation condition in this case is not problematic even if an indentation of 500 g or less is used as long as the microscopic tissue near the measurement position is uniform, but the indentation with a weight of about 500 g to 1 kg is usually used. preferable. There is no problem with the number of measurement points of hardness since there is almost no variation even at one point, but it is preferable to measure about 3 to 5 points and use the average value. The hardness measurement method may be based on JIS or ASTM.

  Regarding the fatigue strength of the fillet welded joint, as shown by “●” in FIG. 2, the hardness [Hv (FL−0.5)] of the weld metal is set to the hardness [Hv (FL + 0) of the base metal heat affected zone. .5)] can be reduced to 90% or less to achieve the effect of improving the fatigue life strength [SF100] of fillet welded joints (about 30% or more in the improvement allowance for 150 MPa). it can.

  As a result of analyzing the local stress distribution at the weld toe of each fillet welded joint by a three-dimensional finite element method, (1) the hardness of the weld metal [Hv (FL-0.5)] is the base metal welding heat. Under a condition higher than the hardness [Hv (FL + 0.5)] of the affected zone (HAZ), the local stress at the weld toe increases significantly in the vicinity of the weld metal or the fusion boundary FL of the HAZ zone, (2) On the contrary, if the hardness [Hv (FL−0.5)] of the weld metal is lower than the hardness [Hv (FL + 0.5)] of the base metal welding heat affected zone (HAZ), the above boundary of the local stress As a result, it was confirmed that not only a remarkable increase in the strain can be suppressed, but also the strain distribution generated in the weld metal part spreads over a wide range, and as a result, the local strain concentration can also be suppressed.

  From these findings, in the present invention, in order to sufficiently increase the fatigue life strength of the fillet welded joint by preventing local stress increase and local strain concentration at the weld toe of the fillet welded joint. Further, the hardness Hv (FL−0.5) at a position 0.5 mm away from the base point to the weld metal side with respect to the melting boundary FL at the weld toe portion, and 0. The upper limit of the ratio to the hardness Hv (FL + 0.5) at a position 5 mm away was set to 0.9.

  On the other hand, if the hardness of the weld metal of the fillet welded joint is made lower than the hardness of the base metal heat-affected zone, there is a risk that the weld joint tensile strength equivalent to the base metal strength required for the structure cannot be secured. Arise. As for the tensile strength of the fillet welded joint, as shown by ◇ in FIG. 2, the hardness [Hv (FL−0.5)] of the weld metal is equal to the hardness [Hv (FL + 0. 5)], it was found that the tensile strength of the welded joint can be equal to or higher than the base metal strength.

  Therefore, in the present invention, in order to suppress the decrease in the joint tensile strength due to the reduction in the hardness of the weld metal of the fillet welded joint, and to maintain the sufficient tensile strength of the fillet welded joint, Hardness Hv (FL-0.5) at a position 0.5 mm away from the base point to the weld metal side from the fusion boundary FL, and hardness at a position 0.5 mm away from the base point to the heat affected zone side The lower limit of the ratio to Hv (FL + 0.5) was set to 0.3.

  In addition, since the stress distribution at the weld toe changes depending on the shape of the weld toe of the fillet welded joint, the effect of improving the joint fatigue strength by reducing the local stress at the weld toe is more stable. In addition to the above conditions, it is preferable that the radius of curvature ρ and / or the angle θ (see FIG. 1) of the weld toe portion of the fillet welded joint described below be defined within an appropriate range.

  FIG. 3 shows the relationship between the radius of curvature ρ (the radius of the toe in FIG. 1) of the fillet welded joint and the fatigue life strength [SF100] of 1 million times, and FIG. 4 shows the fillet welded joint. The relationship between the angle θ of the weld toe (flank angle in FIG. 1) and the fatigue life strength [SF100] of 1 million times is shown.

  From FIG. 3, when the radius of curvature ρ of the weld toe of the fillet welded joint is less than 1 mm, the 1 million times fatigue life strength [SF100] varies greatly, and the target 1 million times fatigue life strength [SF100]. The improvement effect (about 30% or more in the improvement allowance for 150 MPa) cannot be achieved stably. Therefore, in the present invention, in order to prevent local stress increase and local strain concentration at the weld toe of the fillet weld joint, and to improve the fatigue life strength of the fillet weld joint stably, In addition to the ratio between the hardness Hv (FL-0.5) of the weld metal and the hardness Hv (FL + 0.5) of the heat affected zone, the curvature radius ρ of the weld toe is 1 mm or more. Is desirable.

  Further, from FIG. 4, when the angle θ of the weld toe of the fillet weld joint becomes less than 100 degrees, the 1 million times fatigue life strength [SF100] varies, and the target 1 million times fatigue life strength [SF100]. ] Improvement effect (about 30% or more in the improvement allowance for 150 MPa) cannot be achieved stably. Therefore, in the present invention, in order to prevent local stress increase and local strain concentration at the weld toe of the fillet weld joint, and to improve the fatigue life strength of the fillet weld joint stably, In addition to the ratio between the hardness Hv (FL-0.5) of the weld metal and the hardness Hv (FL + 0.5) of the heat affected zone, the angle θ of the weld toe is 100 degrees or more. Is desirable.

  Furthermore, in the present invention, it is desirable that the distance d between the weld bead toe P and the fusion boundary FL is at least 0.05 mm or more at the weld toe as shown in FIG. In addition, the weld bead toe P refers to a portion where the surface of the steel plate (or a surface obtained by extending the surface of the steel plate) and the surface of the weld bead formed on the steel plate intersect.

As described above, the present invention controls the hardness distribution in a range corresponding to the initial process in the generation and growth process of fatigue cracks in a welded joint, thereby increasing the fatigue life of the welded joint. Fatigue cracks in welded joints occur from the point where the stress concentration at the weld toe is the largest, so the weld bead toe tends to be the starting point for fatigue. For this reason, in order to effectively suppress local stress increase and local strain concentration at the weld toe, the position of the fusion boundary FL is separated by 0.05 mm or more from the position of the weld bead toe P. It is desirable that
Particularly preferred is 0.2 mm or more. The upper limit of the distance d is not particularly defined, but is 1 mm that can be realized due to welding restrictions.

  In the present invention, the component composition of the welding material is mainly adjusted in accordance with the component composition of the steel sheet for welded structure, and the hardness of the weld metal is controlled within the range specified in the present invention, preferably the welding conditions are further adjusted. By controlling the weld toe portion to a preferable shape, the fatigue crack generation characteristics can be greatly improved particularly in a fillet welded joint of a steel plate having a thickness of 1 to 20 mm.

  The steel plate used in the fillet welded joint of the present invention may be manufactured from a structural steel for welding having a known component composition. For example, in mass%, C: 0.02 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.20%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less as basic components, depending on required properties such as improvement of base material strength and joint toughness, Ni, Cr, Steels containing one or more of Mo, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, and B are preferred.

  Also, the welding material is not particularly limited in its chemical composition and welding method. For example, as chemical components of the welding material, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 0.5 to 2.5%, Ni: 0 to 4.0% , Mo: 0 to 0.30%, Al: 0 to 0.3%, Mg: 0 to 0.30%, Ti: 0.02 to 0.25%, B: 0 to 0.050% Desirably, it may be appropriately selected in consideration of the chemical composition and hardness of the steel sheet.

As a welding method, a normal gas shielded arc welding (MAG welding, MIG welding, TIG) using CO ₂ gas, Ar gas, or a mixed gas thereof used when fillet welding a steel plate having a thickness of about 1 to 20 mm is performed. Welding) or covering arc welding (SMAW welding) can be applied, and the welding conditions at that time are appropriately determined so as to satisfy the conditions of the hardness and weld toe shape of the weld metal specified in the present invention. do it. For example, when welding the steel sheet using a CO ₂ arc welding, you can use the welding wire of 1.4mm diameter, to weld the current at about 200～450A.

  However, when laser welding or electron beam welding is used, welding material is not used in normal welding, but when using a steel material with high strength, it is difficult to control the hardness of the weld metal within a predetermined range. Therefore, it is necessary to control the hardness of a predetermined weld metal using a welding material.

  There are various methods for separating the distance d between the weld bead toe P and the fusion boundary FL by 0.05 mm or more. For example, there are the following methods.

  When performing fillet welding, as shown in FIG. 6, if the combined welding members are tilted and welded by about 45 degrees, the weld toe portion tends to be smooth, and as a result, the shape characteristics of the welded joint The position of FL, which is the boundary between the weld metal portion and the weld heat affected zone, can be shifted from the weld bead toe.

Also, by welding using a welding material that improves the wettability of the molten metal forming the weld bead to the base steel plate, the weld toe portion is similarly smoothed, so that the same effect can be exhibited.
Further, it can be realized by appropriately adjusting the current and voltage of the welding conditions. For example, the welding bead toe against the melting boundary is reduced by reducing the amount of welding penetration by reducing the voltage to a little less than normal, and as a result, spreading the molten metal to the periphery of the weld by expanding the arc spread. Can be controlled.

  The effects of the present invention will be described below based on examples. The conditions in the following examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to the one condition example. The present invention can adopt various conditions or combinations of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
A steel plate having a thickness of 2 to 25 mm was prepared, and the characteristics and performance of the lap fillet welded joint were tested and investigated. The results are shown in Table 3. The chemical components of the steel materials used and the chemical components of the welding materials are shown in Tables 1 and 2.

Regarding the welding methods in Table 3, CO ₂ represents carbon dioxide welding, ArCO ₂ represents argon carbon dioxide mixed gas welding, TIG represents TIG welding, and SMAW represents manual welding.

  The hardness is measured by setting the weight of the indentation weight to 1 kg, first stamping on the fusion boundary (FL), using this as the reference point, 0.5 mm pitch from the reference point to the weld metal side, and the reference From the point, the micro Vickers hardness was continuously measured at a pitch of 0.5 mm from the base material side.

  Hv (FL-0.5) is a position 0.5 mm from the weld toe into the plate thickness direction inside the joint and 0.5 mm from the fusion boundary (FL) to the weld metal side (-direction). It shows the hardness of the weld metal measured by. Hv (FL + 0.5) is the position entering the plate thickness direction inside the 0.5mm joint from the weld toe, measured by 1kg indentation, and the base metal heat-affected zone (HAZ) side from the melting boundary (FL) The hardness of the base material heat-affected zone measured at a position of 0.5 mm is shown in (+ direction). HVR in Table 3 represents the ratio of the hardness of the weld metal to the hardness of the base metal heat-affected zone (Hv (FL−0.5) / Hv (FL + 0.5)).

  SF100 in Table 3 indicates a repeated stress range that does not break even in a fatigue test even 1 million times. 1 million times fatigue life strength: SF100 is a value improved by 30% or more of this value based on 150 MPa. The case was evaluated as having good joint fatigue strength.

  TS (J) / TS (B) is the joint tensile strength TS (J) with respect to the base material tensile strength TS (B). When this value is 1 or more, the joint strength is evaluated as good.

  Example No. in Table 3 1 to 18 are examples of the present invention that satisfy the conditions of the hardness ratio HVR: Hv (FL−0.5) / Hv (FL + 0.5) between the weld metal and the weld heat affected zone defined in the present invention. is there.

In any of the inventive examples, the value of SF100 (MPa), which is a repeated stress range that does not break even one million times, is maintained while maintaining a joint tensile strength that is higher than the base metal strength (TS (J) / TS (B) ≧ 1). A value of 200 MPa or more improved by 30% or more of 150 MPa, which is a value of SF100 generally known in the art, and has excellent joint fatigue strength. Among the inventive examples, Example No. In the invention examples 1, 3, 6-14 and 16-18, the curvature radius ρ of the weld toe portion and the angle θ of the weld toe portion satisfy the preferred range of the present invention. As a result, the joint fatigue strength was higher than that in the results.

  On the other hand, Example No. deviating from the range of the HVR defined in the present invention. In the comparative example of R1-R6, although the joint tensile strength above the base metal strength (TS (J) / TS (B) ≧ 1) can be maintained, the value of SF100 is 200 MPa or less, and sufficient joint fatigue The strength could not be obtained.

(Example 2)
A steel plate having a thickness of 2 to 18 mm was prepared using the steel materials shown in Tables 1 and 4, and a lap fillet welded joint was formed using the welding material shown in Table 2 in the same manner as in Example 1. Features and performance were tested and investigated in the same manner as in Example 1. The results are shown in Table 5.
At that time, the angle d of the lap fillet weld member was changed to change the distance d from the weld bead toe P to the fusion boundary FL.

In Table 3, No. 20 to 22 , 24 to 28 , 31 , 33 and 36 are examples satisfying d ≧ 0.05 mm which is a more preferable condition of the present invention.
In any example, while maintaining the joint tensile strength at or above the base material strength (TS (J) / TS (B) ≧ 1), the value of SF100 (MPa) shows an excellent value of 200 MPa or more. In particular, when d ≧ 0.2 mm was satisfied, the value of SF100 (MPa) was superior to the other examples.

It is a figure which shows a welded joint and its test piece shape. It is a figure which shows the relationship between the hardness of a weld toe part vicinity, and a 1 million times fatigue life strength. It is a figure which shows the relationship between a welding toe part radius ((rho)) and a million times fatigue life strength. It is a figure which shows the relationship between the flank angle ((theta)) of a weld toe part, and a million times fatigue life strength. It is a figure for demonstrating the distance between the weld bead edge part and fusion boundary in a weld toe part. It is a figure for demonstrating the welding method for separating between a weld bead toe and a fusion boundary.

Explanation of symbols

ρ Curvature radius of weld toe θ Angle of weld toe (flank angle)
FL Melting boundary P Weld bead toe d Spacing between weld bead toe and melting boundary

Claims (5)

In the fillet welded joint of a steel plate, the weld metal hardness Hv (FL-0.5) at a position 0.5 mm away from the base to the weld metal side with respect to the fusion boundary FL at the weld toe is 105 to 254, the hardness Hv (FL + 0.5) of the heat affected zone at a position 0.5 mm away from the base point toward the heat affected zone is 183 to 390, and the hardness of the weld metal and the hardness of the heat affected zone The ratio [Hv (FL−0.5) / Hv (FL + 0.5)] is 0.3 to 0.9 and fillet welds with excellent fatigue crack resistance Fittings.   The fillet weld joint excellent in fatigue crack generation characteristics according to claim 1, wherein a curvature radius ρ of the weld toe portion is 1 mm or more.   The fillet welded joint having excellent fatigue crack resistance according to claim 1 or 2, wherein an angle θ of the weld toe is 100 degrees or more.   In the said weld toe part, it is excellent in the fatigue crack generation characteristics in any one of Claims 1-3 characterized by the distance between the weld bead toe and a fusion boundary being at least 0.05 mm or more. Fillet welded joint.   The fillet welded joint having excellent fatigue crack generation characteristics according to any one of claims 1 to 4, wherein the steel plate has a thickness of 1 to 20 mm.

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