JP5505369B2 - Laser welded joint with excellent joint strength and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lap joint in which a plurality of high-strength steel plates are overlapped and joined by laser, and more particularly to a laser weld joint that improves the quality of weld beads and has excellent joint strength.

  In recent years, in order to meet the demands of improving the fuel efficiency and safety of automobiles, high-strength thin steel sheets are often used in automobile bodies, and these steel sheets must be welded using laser welding. Is required. Furthermore, a laser welding method that stably obtains high strength of the joint is desired in the method of superposing and welding high-strength thin steel plates.

  Since laser welding uses laser light as a heat source, control of the heat input is more reliable and easier than arc welding such as TIG welding and MIG welding. For this reason, thermal deformation can be reduced by appropriately setting welding conditions such as welding speed, laser beam irradiation output, and shield gas flow rate. Also, laser welding can be welded from one side, and is therefore suitable for assembly welding of complex members such as automobile bodies. Furthermore, in recent years, high-efficiency remote welding in which a laser beam is positioned at a high speed by a mirror and the movement between welding points is performed in a short time is becoming widespread.

  In fact, laser welding is widely used for welding members formed from thin steel sheets in the automobile manufacturing industry, electrical equipment manufacturing industry, and other fields. In relation to this, a laser welding method of a lap joint excellent in weld joint strength has been proposed.

  For example, Patent Document 1 discloses a method of preventing easy bead breakage during molding by improving the quality by tempering the first bead with the heat of the second bead and improving the quality. Yes.

JP 2009-000721 A

  However, as the strength of the steel plate increases, further strength improvement of the welded portion is required, and the strength of the welded portion may be insufficient with conventional techniques.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser-welded joint that is superior in joint strength to the conventional one.

  The present inventors diligently studied a laser welding method for increasing the strength of the welded portion. As a result, by forming a second bead for the purpose of tempering the first bead after forming the first bead in welding for forming a closed loop or a closed loop bead. It was found that a laser welded joint excellent in joint strength can be obtained.

  However, in particular, in the lap welding of steel sheets in which the P content [P] and the S content [S] satisfy [P] +5 [S] ≧ 0.026 mass%, a bead for simply tempering is formed. In this case, vertical cracks may occur in the bead, and the fracture surface was observed and found to be solidification cracks. The reason is considered as follows.

  When the first bead of closed loop or closed loop shape is formed, a tensile residual stress field is generated in a direction perpendicular to the plate thickness direction in a region surrounded by the bead in a plane perpendicular to the plate thickness direction. That is, when the second bead is formed, welding is performed in a state where a tensile residual stress field is generated. Therefore, the second bead is cracked before solidification is completed.

  In addition, the tendency of cracking becomes more prominent as the closed loop or closed loop bead length becomes longer, that is, as the circumscribed circle of the closed loop or closed loop bead becomes larger, particularly when the diameter of the circumscribed circle is 10 mm or more. I understood that. The occurrence of cracks is particularly noticeable when the C content of the steel sheet is 0.05% or more, but it has been found that cracks may occur even when the C content is about 0.002%.

  The present inventors have further studied to solve this problem. As a result, before forming a bead intended for tempering the first bead, a compressive strain is formed by thermal expansion inside a position where the bead intended for tempering is formed. It was found that cracking of a tempered bead can be prevented by forming a bead of.

  That is, the tensile strain due to the first bead can be canceled or greatly reduced by the compressive strain due to the bead for forming the compressive strain, so that the bead is tempered without causing cracking. It has been found that a bead intended for that purpose can be formed.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) A laser welded joint in which a plurality of steel plates satisfying [P] +5 [S] ≧ 0.026 mass% in P content [P] and S content [S] are overlapped and joined by laser Because
An average bead width of W, and a closed loop or closed loop bead;
A closed loop or closed loop tempered bead in which the inner toe is disposed at a distance of more than W and not more than 2.2 W from the outside toe of the bead;
Laser welding characterized by having a closed loop or closed loop compression field applying bead in which the outer toe is disposed at a distance of more than 1.5 W and not more than 4.0 W from the outside toe of the main bead. Fittings.

  (2) The laser weld joint according to (1), wherein the compression field imparting bead is arranged without overlapping the tempering bead.

  (3) Each of the closed-loop main bead, the tempered bead, and the compression field imparting bead has one or more openings, and the total length of the openings in each bead is the circumscribing of each bead. The laser welded joint according to (1) or (2) above, which is 3/4 or less of the equivalent circle diameter.

  (4) The laser welded joint according to any one of (1) to (3), wherein an average Vickers hardness of the bead is lower than an average Vickers hardness of the tempered bead.

  (5) The laser welded joint according to any one of (1) to (4), wherein an average Vickers hardness of the main bead is 15 or more lower than an average Vickers hardness of the tempered bead.

  (6) The laser welded joint according to any one of (1) to (5) above, wherein there are two or more sets of the compression field imparting bead and the tempered bead.

(7) A method for producing a laser welded joint according to any one of (1) to (5),
(A) forming a closed-loop or closed-loop main bead having an average bead width of W;
(B) After the temperature of the main bead becomes Ms point −50 ° C. or lower, the outer toe is arranged at a distance of more than 1.5 W and 4.0 W or less inward from the outer toe of the main bead. And a closed loop or closed loop compression field applying bead,
(C) During the period from 0.3 to 3.5 sec from the formation start time of the compression field imparting bead, the outside of the main bead is adjusted so that the temperature of the main bead is 400 ° C. or higher and Ac point + 50 ° C. or lower. A method of manufacturing a laser welded joint comprising sequentially forming a closed loop or a closed loop tempered bead in which an inner toe is disposed at a distance of greater than W and not more than 2.2 W from the toe to the inside. .

(8) This bead and tempered bead are closed loop,
The laser according to (7), wherein an angle formed by a line segment connecting the center of the bead and the start and end of the bead and a line segment connecting the center of the bead and the start and end of the tempered bead is 10 ° or more. A method for manufacturing a welded joint.

(9) The method for producing a laser welded joint according to (6) above,
The step (7) or (8), further comprising one or more steps of the step (b) and the step (c) in order after the step (c). Manufacturing method of laser welded joint.

(10) When forming a plurality of welded joints on a plurality of stacked steel plates,
Form multiple beads and then
The laser weld joint according to any one of (7) to (9) above, wherein a compression field imparting bead and a tempering bead are formed on the bead having a temperature of Ms point −50 ° C. or lower. Production method.

  According to the present invention, it is possible to obtain a laser welded joint having a joint strength superior to that of the conventional one.

It is a figure which shows the outline of the laser welding joint which concerns on this invention, (a) is a perspective view, (b) is a figure which shows the A-A 'cross section of (a). It is a figure explaining the measuring method of the width | variety of this bead of the laser welded joint which concerns on this invention. It is a figure which shows the example of the bead shape in the closed loop laser welded joint which concerns on this invention. It is a figure explaining the method of the hardness measurement of this bead and tempering bead of the welded joint concerning the present invention. It is a figure explaining the measuring method of the average bead width | variety of this bead when this bead and a temper bead fuse | melt and united with the welded joint which concerns on this invention. It is a figure which shows the outline of the other example of the laser welded joint which concerns on this invention.

  Hereinafter, the welded joint of the present invention will be described in detail with reference to the drawings. Hereinafter, when simply described as “%”, it means “mass%”.

  FIG. 1 is a diagram showing an outline of a laser welded joint of the present invention. FIG. 1B is a cross-sectional view taken along the line A-A ′ in FIG.

  The laser welded joint 1 of the present invention is formed by superposing and joining a plurality of high-tensile steel plates 5. FIG. 1 shows an example in which two sheets are overlapped.

  In particular, when a steel sheet in which the P content [P] and the S content [S] satisfy [P] +5 [S] ≧ 0.026 mass% is used as the high-tensile steel plate 5, Solidification cracks in the thickness direction are likely to occur. When a load is applied to a welded part where solidification cracking has occurred, the stress value generated near the intersection of the bead fusion line and the overlapped part of the steel sheet will be higher than when no cracking occurs. It is difficult to ensure strength, particularly strength against a load in the peeling direction.

  The welded joint of the present invention improves joint strength compared to the prior art. The beads forming the laser welded joint 1 are in a closed loop or closed loop shape, and have a main bead 10, a tempered bead 20, and a compression field imparting bead 30 from the outside.

  In the tempered bead 20, when the average bead width of the main bead 10 is W, the toe 20 b on the inner side of the tempered bead 20 is inward from the outer toe 10 a of the main bead 10 to be more than W and 2.2 W or less. Placed in.

  The average bead width W of the bead 10 is measured at the overlapping portion of the steel plates. When a plurality of steel plates are overlapped and welded, the average value of the bead widths of the respective overlapping portions is used. FIG. 2 shows an example in which four steel plates are overlapped and welded, and the average bead width W is an average value of W1, W2, and W3.

  The temper bead 20 is formed for the purpose of tempering the bead 10. Thereby, the ductility of the overlap part of welding improves and the stress concentration of the overlap part at the time of load addition can be relieved. Since the tempered bead 20 is formed inside the main bead 10, the stress received when a load is applied is lower than that of the main bead 10.

  The compression field imparting bead 30 is disposed at a position where the outer toe 30a of the compression field imparting bead 30 is more than 1.5 W and less than 4.0 W inward from the outer toe 10a of the bead 10.

  The compression field imparting bead 30 is formed before the tempering bead 20 is formed in order to cancel the tensile strain generated by the formation of the main bead 10 by the compression strain. Thereby, since the tempered bead 20 is formed without a tensile strain, cracks do not occur when the tempered bead is formed.

  If the compression field imparting bead 30 and the tempering bead 20 are arranged in an overlapping manner, the region that has expanded greatly at a high temperature in the vicinity of the compression field imparting bead is melted by the tempering bead, so that the effect of imparting compressive strain is reduced. As a result, there is a high possibility that cracks will occur in the tempered bead. Therefore, it is preferable that the compression field imparting bead 30 and the tempered bead 20 are arranged without overlapping.

  For example, as shown in FIG. 3, the closed-loop bead has one or more openings in which no beads are formed in the main bead, the tempered bead, and the compression field imparting bead, and the length of the opening. Is a bead having a shape that is 3/4 or less of the circumscribed circle equivalent diameter of each bead.

  FIG. 3 shows an example of a bead shape having an opening, where a solid line indicates a bead and a broken line indicates an opening. For simplicity, only this bead is depicted in FIG. The bead shape shown in FIG. 3 is an example, and the closed loop bead shape is not limited to this.

  Further, for example, when the galvanized steel sheets are overlapped and welded, the plating between the steel sheets reaches the boiling point and evaporates, and the volume rapidly expands. The reason for providing the opening is that if there is no metal or metal plating path in the region surrounded by the bead, the pressure between the steel plates increases, the molten pool blows out during welding, and the bead is defective. is there.

  In the welded joint of the present invention, the average Vickers hardness of the present bead is lower than the average Vickers hardness of the tempered bead. The average Vickers hardness of the bead is an average value of hardness measured at a position (excluding the boundary) obtained by dividing the interval between the melt boundaries of each bead into four equal parts.

  This will be described with reference to FIG. The average Vickers hardness of this bead is the average value of the hardness measured at h11, h12, and h13 obtained by dividing the space between the melt boundaries 10c of the main bead in FIG. 4 (a) into four equal parts, and the average Vickers hardness of the tempered bead is The average value of the hardness measured by h21, h22, and h23 obtained by dividing the space between the melting boundaries 20c of the tempered beads in FIG.

  The bead and the tempered bead may be melted together. When the bead and the tempered bead are integrated, the hardness of the integrated bead is the center of the integrated bead (the position where the gap between the melting boundary of the bead and the tempering bead is divided into two equal parts) and each bead. It is set as the average value of the hardness measured in the position (except for a boundary) which equally divided between the fusion boundaries.

  This will be described with reference to FIG. The average Vickers hardness of the bead is an average value of the hardness measured at h14, h15, and h16, which is obtained by dividing the space between the central portion 60 of the integrated bead and the melting boundary 10c of the bead into four equal parts. . The average Vickers hardness of the tempered bead is the average value of the hardness measured at h24, h25, and h26, which is divided into four equal parts between the central portion 60 of the integrated bead and the melting boundary 20c of the tempered bead. .

  Even when the bead and the tempered bead are integrated, the effect of the present invention can be obtained if the hardness of the bead defined above is lower than the hardness of the tempered bead. May be integrated as a melt.

  When the main bead and the tempered bead are integrated, as shown in FIG. 4B, it appears as one bead in appearance. However, when welding is performed to form only one bead, the hardness distribution as described above cannot be obtained, and the effects of the present invention cannot be obtained.

  From the viewpoint of improving the joint strength, the average Vickers hardness of the bead and the average Vickers hardness of the tempered bead are such that the average Vickers hardness of the bead is 15 or more smaller than the average Vickers hardness of the tempered bead. It is preferred to form a tempered bead.

  In general, during laser welding, the bead solidifies from the melting boundary during cooling, and the crystal grows toward the center of the bead, so it is easy to know the center of the bead that will be the final solidified part by observing the microstructure. is there.

Therefore, even when the main bead and the tempered bead are integrated and the main bead is hidden by the tempered bead, when the main bead remains more than half in the bead width direction as shown in FIG. The average bead width W of this bead can be obtained by doubling the distance from the melting boundary 10c. For example, in the case of FIG. 5A, the average value of 2W _h1 , 2W _h2 , 2W _h3 is obtained.

  On the other hand, in laser welding, in keyhole welding in which a laser beam penetrates a plurality of plates in the plate thickness direction, the bead width is mainly determined by the beam diameter. From the viewpoint of cost, it is generally considered that the present bead and the tempered bead are often formed using the same optical system, so that it may be considered that they have substantially the same beam diameter. Moreover, in order to temper this bead, it is necessary to weld with the tempering bead which can form bead width comparable as this bead.

  Accordingly, as shown in FIG. 5B, when the bead center portion of the main bead is hidden by the tempered bead and the bead width of the original main bead is not known, the bead width of the main bead is set to the average bead width of the tempered bead. Can be substituted.

  As shown in FIG. 6, the welded joint of the present invention may have two or more sets of compression field imparting beads and tempering beads. Thereby, the tempering of the present bead is further advanced, and the first tempered bead is also tempered by the second tempered bead, so that the joint strength in the peeling direction can be further improved. Moreover, since the cross-sectional area of the beads connecting the steel plates increases, it is possible to improve the shear strength when a load is applied in a direction perpendicular to the plate thickness direction.

  In FIG. 6, the tempering bead 20, the second tempering bead 21, the compression field imparting bead 30, and the second compression field imparting bead 31 are sequentially arranged inside the main bead 10. The positional relationship between the first tempering bead, the second tempering bead, the first compression field imparting bead, and the second compression imparting bead is not limited to this.

  Next, the welding method according to the present invention will be described.

  The apparatus used for manufacturing the laser welded joint of the present invention can be the same apparatus as the apparatus for manufacturing a conventional laser welded joint.

  In the production of the laser welded joint according to the present invention, after forming the bead, the temperature of the bead is waited until the Ms point −50 ° C. (Ms point: martensite transformation start temperature) or lower, and then the compression field imparting bead And the formation of tempered beads is started.

  If the temperature of this bead is Ms point −50 ° C. or less, a certain amount or more of martensite is generated in the steel sheet. Thereafter, by forming the compression field imparting bead and the tempering bead in order, the martensite is tempered and softened, and the joint strength is increased.

  If the formation of the compression field imparting bead and the tempering bead is started while the temperature of the bead is higher than the Ms point −50 ° C., sufficient martensite is not generated in the bead. Furthermore, since the wide area | region near a welding part softens with the heat | fever of the compression field provision bead and tempering bead which were irradiated subsequently, joint strength falls significantly.

  The lower limit of the temperature of the bead when starting the formation of the compression field imparting bead is not particularly defined, but is preferably set to Ms point −250 ° C. or higher. This is because a general steel plate finishes the martensitic transformation at an Ms point of −250 ° C. There is no merit by waiting until the Ms point is less than −250 ° C., the tact time is increased, and the production cost is increased.

  The outer toe of the compression field imparting bead is disposed at a distance of more than 1.5 W and not more than 4.0 W inward from the outer toe of the bead. When the distance from the outer toe of the bead to the outer toe of the compression field imparting bead is 1.5 W or less, the region expanded greatly at a high temperature in the vicinity of the compression field imparting bead is melted by the tempering bead, and the compressive strain is imparted. Therefore, the possibility of cracking in the bead increases during tempering.

  When the outer toe of the compression field imparting bead exceeds 4.0 W from the outer toe of the main bead, the total bead length of the compression field imparting bead located inside the main bead is shortened. The total amount of heat input to is reduced. As a result, the total amount of compressive strain is reduced, so that the tensile strain generated in the vicinity of the bead cannot be offset.

  Further, the tempered bead is formed under the condition that the temperature of the bead becomes 400 ° C. or higher and the Ac point + 50 ° C. or lower within 0.3 to 3.5 sec from the formation start time of the compression field imparting bead.

  When formation of a tempered bead is started in less than 0.3 sec from the formation start time of the compression field imparting bead, a compressive strain is not sufficiently imparted, and a tempered bead is formed without canceling out the tensile strain generated during the formation of this bead. Therefore, there is a high possibility that the bead is cracked.

  When it exceeds 3.5 sec from the formation start time of the compression field imparting bead, the temperature increased by the formation of the compression field imparting bead is lowered, and the thermal expansion is changed to contraction. Then, tensile strain is generated at the position where the tempered bead is formed, and there is a high possibility that the bead is cracked when the tempered bead is formed.

The temperature measured at the toe on the steel sheet surface can be used as a representative value for the bead temperature. The temperature can be measured using a radiation thermometer or a thermocouple. The Ms point is derived from the steel plate components.
Ms (° C.) = 550-361 × (% C) −39 × (% Mn) −35 × (% V)
−20 × (% Cr) -17 (% Ni) -10 × (% Cu)
−5 × (% Mo +% W) + 15 × (% Co) + 30 × (% Al)
Can be estimated. (% C) and the like are values indicating the content of each element in the steel sheet in mass%.

  Further, the tempered bead is formed by heating under the conditions that the temperature of the bead is in the range of 400 ° C. or higher and Ac1 point + 50 ° C. or lower. The bead temperature can be measured on the surface of the steel sheet as described above.

  When forming the tempered bead, if the average temperature of the bead is less than 400 ° C., the bead is not sufficiently tempered and does not soften, so that sufficient joint strength cannot be obtained. When the temperature of the bead exceeds the Ac1 point ° C. + 50 ° C., the proportion of austenite formed in the structure in the bead increases, and it is quenched again during cooling, causing martensitic transformation and not softening. I can't get it. A more preferable temperature range is 400 ° C. or more and less than Ac1 point.

Ac1 point is from the component of the steel plate,
Ac1 (° C.) = 723-10.7 × (% Mn) −16.9 × (% Ni)
+ 29.1 × (% Si) + 16.9 × (% Cr) + 290 × (% As)
+ 6.38 × (% W)
Can be estimated. (% C) and the like are values indicating the content of each element in the steel sheet in mass%.

  Further, the inner toe of the tempered bead is arranged so that the distance from the outer toe of the bead to the inside is greater than W and 2.2 W or less. If the distance from the outer toe of the bead to the inner toe of the tempered bead is equal to or less than W, the temperature of the outer toe and the overlapping portion of the bead becomes Ac3 or higher when forming the tempered bead. The joint strength is not improved because it is baked again.

  If the distance from the outer toe of the bead to the inner toe of the tempered bead is larger than 2.2 W, the heat generated when forming the tempered bead is not sufficiently conducted to the bead, so that the tempering of the bead is not performed. The joint strength cannot be improved.

Power density of the laser used to form the laser weld joint of the present invention, 0.5 MW / cm ² or more, 500 MW / cm ² is preferably in a range of about. Power density 0.5 MW / cm ² or more, if 500 MW / cm ² or less, it is possible to keyhole welding as laser beam penetrates in the thickness direction, tempering of the bead can be performed with a wide welding speed range .

If the power density is lower than 0.5 MW / cm ² , the keyhole is not formed, so that the tempering of the beads cannot be realized unless the moving speed of the laser beam, that is, the welding speed is significantly reduced, which is disadvantageous in actual production. . On the other hand, when the power density is higher than 500 MW / cm ² , evaporation is dominant in the beam irradiating portion, so that it cannot be used as a heat source for fusion welding, and it becomes difficult to form a bead.

The power density of the laser beam can be calculated by dividing the output of the laser beam by the beam area. Further, the beam area can be calculated by dividing the beam diameter (the distance from the beam center to the point where the intensity decreases to 1 / e ² of the beam center intensity ( Radius)).

  When forming a closed-loop bead whose start end and end end coincide with each other, the heat at the start end is superposed on the end, resulting in overheating, and the molten steel may sag or blow off. Moreover, when the start and end positions of the bead and the tempered bead are brought close to each other, dripping and blowing off of the molten steel of the tempered bead may be promoted.

  If dripping of molten steel occurs, it will lead to a decrease in joint strength. Therefore, it is preferable to shift the start and end positions of the main bead and the tempered bead in order to prevent the molten steel from dripping and blowing off.

  Specifically, the bead is formed so that the angle formed by the line segment connecting the start and end of the bead from the center of the bead and the line segment connecting the start and end of the tempered bead from the center of the bead is 10 ° or more. It is preferable to do. Here, the center of the bead means the center of the circumscribed circle of the bead.

  By forming the weld bead as described above, a welded joint having good joint strength can be manufactured.

  In addition, when forming two or more welded joints of this invention, it is not necessary to form each welded joint in order. That is, first, only a plurality of main beads may be formed, and then a compression field imparting bead and a tempering bead may be formed on the main beads having the highest point of Ms point −50 ° C. or lower.

  By forming the bead in this way, the time until the temperature of the main bead is reduced can be used for forming another main bead, so that a plurality of weld joints can be formed efficiently.

  The laser welded joint of the present invention is suitable for lap welding of high-tensile steel sheets having a plate thickness in the range of 0.5 to 3.0 mm. Even if the plate thickness is less than 0.5 mm, the effect of improving the strength of the welded portion can be obtained, but the strength of the joint is governed by the plate thickness, so the effect of improving the strength of the entire joint is reduced, and the application of the member The range is limited. Moreover, even if the plate thickness exceeds 3.0 mm, the effect of improving the strength of the welded portion can be obtained, but the application range of the member is limited from the viewpoint of reducing the weight of the member.

  The main component of the steel sheet with a plate thickness of 1.0 mm is C: 0.12%, Si: 0.5%, Mn: 2.0%, P: 0.01%, S: 0.004% Two tension steel plates were superposed and joined by laser welding to produce a joint. The bead shape of the welded portion was a closed circle, or the shape shown in FIGS.

  The welds of the inventive examples all have three beads: a regular bead, a tempered bead, and a compression field imparting bead.

  From the steel sheet components, the Ms point and Ac1 point are estimated to be 429 ° C. and 716 ° C., respectively.

  A plurality of types of laser welded joints were produced in which the shape and manufacturing conditions of the weld beads were changed as shown in Table 1. The bead shapes (a) to (e) indicate the shapes of (a) to (e) in FIG. Moreover, the underline in Table 1 shows that it is outside the range prescribed | regulated by this invention.

  The other welding conditions were a laser output of 4.0 kW, a focal position at the surface of the upper steel plate, and a beam spot diameter at the focal position of 0.5 mm. The welding speed was constant at 4 m / min for the main bead and the tempered bead and 2 m / min for the bead applied with the compression field.

  The temperature of the bead was measured by attaching a thermocouple in the vicinity of the outer end of the bead.

  The cross tensile strength of the produced laser welded joint and the average Vickers hardness of the bead and the tempered bead were measured.

  The method of measuring the cross tensile strength and the joint shape were in accordance with JIS Z 3137 defined for spot welded joints. A cruciform joint was produced by laser welding, a tensile test was performed using a predetermined tensile jig and a tensile speed of 10 mm / min constant, and the maximum load at that time was defined as a cross tensile strength.

  Table 2 shows these results. Based on the cross tensile strength when only one bead is formed (No. 8), it is good when it is 1.2 times or more of this, and the ratio of cross tensile strength is less than 1.2 times. Judged as bad.

  As can be seen from the results in Table 2, according to the present invention, a laser welded joint excellent in joint strength can be obtained without causing cracks in the bead.

  No. 8 is a comparative example in which a welded joint is formed with a single circular bead, and the cross tensile strength is inferior to that of the inventive example.

  No. In No. 9, since the compression field imparting bead was too far from the main bead, the tensile strain generated in the vicinity of the main bead could not be offset, and the tempered bead was cracked.

  No. In No. 10, since the tempered bead was too far from the main bead, heat was not sufficiently conducted to the main bead, the tempering of the main bead was insufficient, and the joint strength was not improved.

  No. No. 11 formed a compression field imparting bead before the temperature of the bead was sufficiently lowered, so that sufficient martensite was not generated in the bead and was excessively softened, and the cross tensile strength was greatly reduced.

  No. No. 12, since it takes a long time from forming the compression field imparting bead to forming the tempering bead, the temperature raised by the formation of the compression field imparting bead is lowered, and tensile strain is generated at the position where the tempering bead is formed. Cracking occurred in the tempered bead.

  No. In No. 13, since the compression field imparting bead was too close to the main bead, the region greatly expanded at a high temperature in the vicinity of the compression imparting bead was melted by the tempering bead, the effect of imparting compressive strain was reduced, and the tempering bead was cracked.

  No. In No. 14, since the tempered bead was too close to the actual bead, the temperature of the actual bead was increased too much, so that tempering occurred again, tempering was insufficient, and the cross tensile strength was not improved.

  No. No. 15, since the time from the formation of the compression field imparting bead to the formation of the tempered bead is short, the compressive strain is not sufficiently imparted, and as a result, the tensile strain generated during the formation of this bead is offset. The cracks occurred in the tempered beads.

  According to the present invention, a laser-welded joint having a joint strength superior to that of the conventional one can be obtained and applied to a member for an automobile and the like, and thus industrial applicability is great.

DESCRIPTION OF SYMBOLS 1 Laser welded joint 5 High-tensile steel plate 10 Bead 10a Toe outside this bead 10b Toe inside this bead 10c Melting boundary of this bead 10d Final solidification part (center part) of this bead
20, 21 Tempered bead 20a Outer end of tempered bead 20b Inner end of tempered bead 20c Melting boundary of tempered bead 30,31 Compressed field imparting bead 30a Outer toe 30b of compressive field imparted bead 30b Toe 50 Opening 60 Central part of integrated bead

Claims (10)

A laser welded joint in which a plurality of steel sheets satisfying P content [P] and S content [S] satisfying [P] +5 [S] ≧ 0.026 mass% are joined by laser. ,
An average bead width of W, and a closed loop or closed loop bead;
A closed loop or closed loop tempered bead in which the inner toe is disposed at a distance of more than W and not more than 2.2 W from the outside toe of the bead;
A joint strength characterized by having a closed loop or closed loop compression field imparting bead in which the outer toe is disposed at a distance of more than 1.5 W and not more than 4.0 W on the inside from the outer toe of the bead. Excellent laser welded joint.   The laser weld joint with excellent joint strength according to claim 1, wherein the compression field imparting bead is disposed without overlapping the tempering bead.   Each of the closed-loop bead, the tempered bead, and the compression field imparting bead has one or more openings, and the total length of the openings in each bead is a diameter corresponding to the circumscribed circle of each bead. The laser welded joint excellent in joint strength according to claim 1 or 2, wherein the joint is 3/4 or less.   4. The laser welded joint having excellent joint strength according to claim 1, wherein an average Vickers hardness of the main bead is lower than an average Vickers hardness of the tempered bead.   The laser welded joint with excellent joint strength according to any one of claims 1 to 4, wherein the average bead hardness of the main bead is 15 or more lower than the average Vickers hardness of the tempered bead.   The laser welded joint having excellent joint strength according to any one of claims 1 to 5, wherein two or more sets of the compression field imparting beads and the tempered beads are included. It is a manufacturing method of the laser weld joint according to any one of claims 1 to 5,
(A) forming a closed-loop or closed-loop main bead having an average bead width of W;
(B) After the temperature of the main bead becomes Ms point −50 ° C. or lower, the outer toe is arranged at a distance of more than 1.5 W and 4.0 W or less inward from the outer toe of the main bead. And a closed loop or closed loop compression field applying bead,
(C) During the period from 0.3 to 3.5 sec from the formation start time of the compression field imparting bead, the outside of the main bead is adjusted so that the temperature of the main bead is 400 ° C. or higher and Ac1 point + 50 ° C. or lower. A laser excellent in joint strength characterized by comprising a step of forming a closed-loop or closed-loop tempered bead in which the inner toe is disposed at a distance greater than W and not more than 2.2 W inward from the toe. A method for manufacturing a welded joint. The bead and tempered bead are closed loop,
The angle formed by a line segment connecting the center of the bead and the start and end of the bead and a line segment connecting the center of the bead and the start and end of the tempered bead is 10 ° or more. A method of manufacturing a laser welded joint with excellent joint strength. A method for manufacturing a laser welded joint according to claim 6,
The joint according to claim 7 or 8, further comprising, after the step (c), one or more steps including the step (b) and the step (c) in order. A method for manufacturing a laser welded joint with excellent strength. When forming multiple welded joints on multiple steel plates,
Form multiple beads and then
The joint strength according to any one of claims 7 to 9, wherein a compression field imparting bead and a tempered bead are formed on the bead having a temperature Ms point of -50 ° C or lower. An excellent laser welded joint manufacturing method.

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