Spot-welding method and spot-welded joint

Abstract

To provide a spot-welding method and a spot-welded joint capable of easily and efficiently improving the toughness of a weld zone and obtaining excellent joint strength even when a high-strength steel sheet having a tensile strength equal to or higher than 1350 MPa is used.SOLUTION: A spot-welding method according to the present invention includes: a first step (S1) of spot-welding a set of two or more steel sheets, at least one of an upper sheet and a lower sheet of which is a non-plated or aluminum-plated high-strength steel sheet having a tensile strength equal to or higher than 1350 MPa, to form a nugget (3) on overlapping surfaces of the set of sheets; and a second step (S2) of irradiating, with a laser beam, an area overlapping with a shoulder portion (SP) of an indentation portion (IP) formed on the surface of the high-strength steel plate, thereby forming a molten solidified portion (5) that does not penetrate the high-strength steel plate in a plate thickness direction, thereby tempering an end portion (6) of the nugget (3).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spot welding method and a spot welded joint. [Background technology]

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of thin, high-strength steel sheets has been increasing in order to reduce the weight of vehicle bodies and parts and improve fuel efficiency.

[0003] Furthermore, high-strength steel sheets used for automobile bodies and parts include not only unplated high-strength steel sheets but also high-strength steel sheets whose surfaces are plated with metals such as aluminum from the viewpoint of rust prevention.

[0004] Meanwhile, automobile body assembly and component installation are primarily performed by spot welding due to cost and manufacturing efficiency considerations. The quality of welds formed by such spot welding is evaluated by tensile shear strength (TSS) and cross tensile strength (CTS), and it is known that TSS increases as the tensile strength of the base material increases. However, when high-strength steel plates with a tensile strength of 780 MPa or higher are used as the base material, the peel strength of CTS and other properties tends to decrease as the tensile strength of the base material increases. At 1350 MPa or higher, the tendency for peel strength to decrease becomes even more pronounced.

[0005] Generally, the reason for the decrease in CTS is thought to be that the toughness of the nugget edge is reduced due to the formation of a hard martensite structure caused by rapid cooling after nugget formation. When the toughness of the nugget edge is reduced in this way, cracks propagate from the nugget edge, making it more susceptible to brittle fracture.

[0006] Therefore, in order to prevent such brittle fracture, various methods have been studied for improving toughness by tempering the structure of the nugget and its surrounding area. One such method is a post-current application method, in which a nugget is formed by applying current, is cooled, and then current is applied again to temper the nugget and its surrounding area.

[0007] For example, Patent Document 1 discloses a method for welding overlapping portions of a plurality of steel plate members, the method comprising: a resistance spot welding step of forming, by resistance spot welding, a spot weld having a nugget, a heat-affected zone formed around the nugget, and a softest-softened zone having the lowest Vickers hardness in the heat-affected zone; and a tempering step of forming, between the center of the nugget and the softest-softened zone, a tempered region made of tempered martensite and having a Vickers hardness of 120% or less, where the Vickers hardness of the softest-softened zone is taken as 100%. The method disclosed in Patent Document 1 is said to be able to improve the peel strength of the spot welded portion.

[0008] Patent Document 2 also describes a method for manufacturing a steel sheet assembly including two or more overlapping steel sheets, including at least one steel sheet having a C content of 0.280% or more and 0.700% or less by mass, which includes a first current-carrying step in which a current is applied at a current value I1 (kA) while sandwiching and applying pressure to a pair of electrodes in the thickness direction of the sheets, and a time t of 20 ms to 200 ms after the first current-carrying step. c1 a first de-energizing step in which the current is de-energized, and after the first de-energizing step, a current value I2 (kA) that satisfies the formula (1): 0.60≦I2 / I1≦1.10 and a time t2 (ms) that satisfies the formula (2): 50≦t2≦1000 is applied, and after the second energizing step, a second energizing step in which the current is applied at a current value I2 (kA) that satisfies the formula (3): t c2 >3.5×10 -3 ×Ms 2 Time t that satisfies -3.3×Ms+1100 c2 After the elapse of 1000 ms, the tempering temperature at the current-applied position is 350°C or higher, and the formula (A): H = T × (logt HTand a tempering step of performing tempering under conditions where the tempering parameter H calculated by the formula (H = 17.7 - 5.8 × [C]) is 8000 or more and 18000 or less. The method disclosed in Patent Document 2 is said to be able to greatly improve joint strength compared to single-current resistance spot welding, even when using a sheet assembly including steel sheets with a relatively high carbon content. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2014 / 025063 [Patent Document 2] International Publication No. 2022 / 210749 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the conventional post-heating method has difficulty in applying to actual production sites because the range of conditions under which the desired effect can be obtained is narrow and it is susceptible to various disturbances that occur in actual production sites (i.e., it has low robustness). Note that disturbances that occur in actual production sites include, for example, the generation of dust, electrode wear, electrode misalignment, and gaps between steel sheets.

[0011] Therefore, there is a need for a practical and new method that can simply and efficiently improve the toughness of welds and obtain excellent joint strength, even when high-strength steel plates with a tensile strength of 1350 MPa or more are used as the base material.

[0012] The present invention has been made in view of the above circumstances, and has an object to provide a spot welding method that can simply and efficiently improve the toughness of a weld and obtain excellent joint strength, even when high-strength steel plates having a tensile strength of 1350 MPa or more are used. Another object of the present invention is to provide a spot-welded joint that has high toughness of a weld and excellent joint strength, even when high-strength steel plates having a tensile strength of 1350 MPa or more are used. [Means for solving the problem]

[0013] The present invention includes the following aspects.

[0014] (Aspect 1) A spot welding method for a plate assembly consisting of two or more steel plates, In the sheet combination, at least one of the upper sheet and the lower sheet is an unplated or aluminum-plated high-strength steel sheet having a tensile strength of 1350 MPa or more, a first step of spot welding the plate assembly to form a nugget on the overlapping surface of the plate assembly; A second step of irradiating a laser beam onto an area overlapping with a shoulder portion of the indentation formed on the surface of the high-strength steel plate to form a molten solidified portion that does not penetrate the high-strength steel plate in the thickness direction, thereby tempering the end of the nugget; A spot welding method comprising:

[0015] (Aspect 2) 2. The spot welding method according to claim 1, wherein the region to be irradiated with the laser is a ring-shaped region surrounding the flat portion of the indentation when viewed from above.

[0016] (Aspect 3) 2. The spot welding method according to claim 1, wherein the laser irradiation area is the entire area of ​​the indentation when viewed from above.

[0017] (Aspect 4) The spot welding method according to any one of the above aspects 1 to 3, wherein the laser is irradiated onto both the upper sheet and the lower sheet.

[0018] (Aspect 5) 5. The spot welding method according to any one of the above aspects 1 to 4, wherein the depth of the molten solidified portion in the plate thickness direction is 20.0% or more of the plate thickness of the high-strength steel plate.

[0019] (Aspect 6) A spot welded joint formed by two or more steel plates, at least one of which is an unplated or aluminum-plated high-strength steel plate having a tensile strength of 1350 MPa or more, A nugget is formed on the overlapping surface of the two or more steel plates, A spot welded joint, characterized in that a molten solidified portion that does not penetrate the high-strength steel plate in the thickness direction is formed in a region overlapping with a shoulder portion of the indentation portion formed on the surface of the high-strength steel plate, and the end of the nugget is tempered.

[0020] (Aspect 7) A spot welded joint according to aspect 6, wherein the molten solidified portion is formed in a ring shape surrounding the flat portion of the indentation when viewed from above.

[0021] (Aspect 8) A spot welded joint according to aspect 6, wherein the molten solidified portion is formed over the entire area of ​​the indentation portion when viewed from above.

[0022] (Aspect 9) The spot welded joint according to any one of the above-mentioned embodiments 6 to 8, wherein the molten solidified portion is formed on both the upper plate and the lower plate.

[0023] (Aspect 10) 10. The spot welded joint according to any one of aspects 6 to 9, wherein the depth of the molten solidified portion in the plate thickness direction is 20.0% or more of the plate thickness of the high-strength steel plate. [Effects of the Invention]

[0024] According to the spot welding method of the present invention, even when high-strength steel plates having a tensile strength of 1350 MPa or more are used, the toughness of the weld can be easily and efficiently improved, and excellent joint strength can be obtained. Furthermore, according to the present invention, even when high-strength steel plates having a tensile strength of 1350 MPa or more are used, it is possible to provide a spot-welded joint having high toughness of the weld and excellent joint strength. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic diagram showing each step of a spot welding method according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view and a top view of a spot-welded joint 1 produced by a spot welding method according to one embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional photograph for explaining a method for identifying the shoulder of an indentation in a spot-welded joint. [Figure 4] FIG. 4 is a cross-sectional schematic diagram illustrating a form in which the shoulder portion SP of the indentation IP and the molten solidified portion 5 in the spot-welded joint 1 overlap each other. [Figure 5] FIG. 5 is a schematic cross-sectional view and a top view of a spot welded joint 1 produced by a spot welding method according to another embodiment of the present invention. [Figure 6] FIG. 6 is an enlarged cross-sectional photograph of a spot-welded joint (Example 1) according to an example of the present invention, obtained by the spot welding method of the present invention. [Figure 7] FIG. 7 is an enlarged cross-sectional photograph of a spot-welded joint of a comparative example (Comparative Example 1) obtained by a conventional spot welding method. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of the spot welding method and spot welded joint of the present invention will be described in detail with reference to the drawings.

[0027] In order to achieve the above object, the present inventors have conducted intensive studies focusing on the post-process after spot welding. As a result, the present inventors have found that in the post-process after spot welding, a laser is irradiated onto the area overlapping with the shoulder of the indentation formed on the surface of the high-strength steel plate, and a molten solidified portion that does not penetrate the high-strength steel plate in the thickness direction is formed. The present inventors have found a practical and new method that can simply and efficiently temper the end of the nugget, that is, can simply and efficiently improve the toughness of the end of the nugget.

[0028] The present invention was completed based on these findings, and includes the following aspects of the spot welding method and spot-welded joint.

[0029] First, a preferred embodiment of the spot welding method of the present invention will be described in detail with reference to the drawings.

[0030] <Spot welding method> A spot welding method according to one embodiment of the present invention is a spot welding method for a sheet assembly consisting of two or more steel sheets, in which at least one of the upper and lower sheets of the sheet assembly is an unplated high-strength steel sheet having a tensile strength of 1350 MPa or more or an aluminum-plated high-strength steel sheet having a tensile strength of 1350 MPa or more.

[0031] The spot welding method of this embodiment includes a first step of spot welding the plate assembly to form a nugget on the overlapping surface of the plate assembly, and a second step of irradiating a laser onto an area overlapping with a shoulder of an indentation formed on the surface of the high-strength steel plate to form a molten solidified portion that does not penetrate in the thickness direction of the high-strength steel plate, thereby tempering the end of the nugget.

[0032] The spot welding method of this embodiment will be specifically described below with reference to the drawings. Here, Fig. 1 is a schematic diagram showing each step of the spot welding method according to one embodiment of the present invention, and Fig. 2 is a schematic diagram of a cross-sectional view and a top view of a spot welded joint 1 produced by the spot welding method according to one embodiment of the present invention.

[0033] As shown in Fig. 1, in the spot welding method of this embodiment, spot welding is performed on a sheet pair consisting of two steel sheets, a steel sheet 21 that serves as an upper sheet and a steel sheet 22 that serves as a lower sheet. In the embodiment shown in Fig. 1, at least steel sheet 21 of the sheet pair is an unplated high-strength steel sheet having a tensile strength of 1350 MPa or more or an aluminum-plated high-strength steel sheet having a tensile strength of 1350 MPa or more.

[0034] In the spot welding method of this embodiment, first, a plate combination consisting of a steel plate 21 serving as an upper plate and a steel plate 22 serving as a lower plate is sandwiched between a pair of electrodes, an upper electrode 41 and a lower electrode 42, and spot welding is performed, and a first step S1 is performed to form a nugget 3 made of molten metal around the overlapping surfaces of the steel plates 21 and 22. Next, a laser LB is irradiated to an area overlapping with a shoulder portion SP of an indentation portion IP formed on the surface of the upper steel plate 21, which is a high-strength steel plate, to partially melt the steel plate 21 in the plate thickness direction, and form a molten solidified portion 5 that does not penetrate the plate thickness direction of the steel plate 21. The heat at that time is transmitted to the end 6 of the nugget 3, and a second step S2 is performed to temper the end 6 of the nugget.

[0035] 1 and 2, the spot welded joint 1 obtained by the spot welding method of this embodiment has an indentation IP formed on the surface of each of the steel sheets 21 and 22 by pressing in the pair of electrodes. Furthermore, the indentation IP has a flat portion PP formed in each of the steel sheets 21 and 22 at a location that comes into contact with the top of the electrode, and a shoulder portion SP formed in each of the steel sheets 21 and 22 at a location that comes into contact with the shoulder of the electrode.

[0036] (How to identify the shoulder) In this specification, the "shoulder" of a spot-welded joint refers to a portion identified as follows from a photographic image of a cross section of the spot-welded joint cut in the thickness direction through the center of the nugget in a top view: Fig. 3 is a cross-sectional photograph for explaining a method for identifying the shoulder of an indentation in a spot-welded joint.

[0037] First, in a cross-sectional photograph of a spot-welded joint as shown in Figure 3, two points P1 and P2 are marked horizontally on the surface of the base metal part BP, which is made of an unwelded steel plate, at least 1 mm apart, and a horizontal reference line L1 is drawn connecting these points P1 and P2. Similarly, two points P3 and P4 are marked horizontally on the surface of the base metal part BP, which is located on the opposite side of the depression part DP of the indentation part IP, at least 1 mm apart, and a horizontal reference line L2 is drawn connecting these points P3 and P4. Next, shoulder vertices P5 and P6 of two shoulders SP located on both sides of the recess DP in the horizontal direction are determined. Here, the shoulder vertex is defined as a point (point P5 in FIG. 3) 30 μm away from the horizontal reference line in the direction in which the steel sheet surface recesses toward the center of the sheet thickness in the sheet thickness direction (hereinafter referred to as the "recess direction"), or a point (point P6 in FIG. 3) where the steel sheet surface is most convex on the opposite side to the recess direction with respect to the horizontal reference line. Note that, when there are multiple points that satisfy the above-mentioned condition for point P5, the point closest to the recess DP is adopted as point P5. A straight line L3 is drawn connecting the shoulder vertices P5 and P6 of the two shoulders SP, and the midpoint P7 between P5 and P6 on the straight line L3 is found. Next, a center line L4 is drawn that passes through this midpoint P7 and is perpendicular to the straight line L3, and an intersection P8 between this center line L4 and the surface of the flat portion PP of the indentation IP is found. Furthermore, point P9 is set on the surface of the flat portion PP of the indentation IP, 1 mm away from intersection P8 toward the outside of the indentation IP, and a line L5 is drawn connecting intersections P8 and P9. A line L6 is then drawn parallel to line L5 and 0.1 mm away from line L5 in the direction opposite to the indentation direction, and an intersection P10 between line L6 and the surface of the indentation DP is determined. This intersection P10 is designated as the inner position SP1 of one shoulder SP (the shoulder SP in the upper left part of the cross-sectional photograph in Figure 3). Furthermore, a straight line L7 is drawn that passes through one of the shoulder vertices P5 and is perpendicular to the horizontal reference line. Next, a predetermined distance W is drawn from the straight line L7 that is parallel to the straight line L7 and extends outward from the indentation IP. out Draw a straight line L8 at a distance W out The distance between the straight line L8 and the surface of the steel plate is set to 0.6 mm. Then, the intersection point P11 between the straight line L8 and the surface of the steel plate is determined. This point P11 is set to the outer position SP2 of one shoulder portion SP. The portion located between the inner position SP1 and the outer position SP2 obtained as described above becomes one shoulder SP of the spot-welded joint (the shoulder SP in the upper left part of the cross-sectional photograph in FIG. 3).

[0038] Using the same procedure as above, the other shoulder SP of the spot-welded joint (the shoulder SP in the upper right part of the cross-sectional photograph in FIG. 3) can also be identified. That is, first, a point P12 is located on the surface of the flat portion PP of the indentation IP, 1 mm away from the intersection point P8 toward the outside of the indentation IP, and a line L9 is drawn connecting the intersection points P8 and P12. Next, a line L10 is drawn parallel to the line L9 and 0.1 mm away from the line L9 in the direction opposite to the indentation direction, and an intersection point P13 between the line L10 and the surface of the indentation DP is determined. This intersection point P13 is designated as the inner position SP1 of the other shoulder SP (the shoulder SP in the upper right part of the cross-sectional photograph in FIG. 3). Furthermore, a line L11 is drawn that passes through the other shoulder apex P6 and is perpendicular to the horizontal reference line. Next, a line L11 is drawn that is parallel to the line L11 and is a predetermined distance W from the line L11 toward the outside of the indentation IP. out Draw a straight line L12 at a distance W outThe distance between the straight line L12 and the surface of the steel plate is set to 0.6 mm. Then, the intersection point P14 between the straight line L12 and the surface of the steel plate is determined. This point P14 is set to the outer position SP2 of one shoulder portion SP. The portion located between the inner position SP1 and the outer position SP2 obtained as described above becomes the other shoulder SP of the spot-welded joint (the shoulder SP in the upper right part of the cross-sectional photograph in FIG. 3).

[0039] Furthermore, although not shown, the shoulders SP at the lower left and right portions of the cross-sectional photograph in FIG. 3 can also be identified in the same manner as above.

[0040] In the above description, the recessed portion DP refers to the portion of the spot welded joint that is recessed from the steel plate surface in the recessed direction, and refers to the portion of the indentation portion IP that is composed of the flat portion PP and part of the shoulder portion SP.

[0041] In this specification, the term "area overlapping with the shoulder" refers to an area that overlaps with at least a part or all of the shoulder. Therefore, the area overlapping with the shoulder includes an area that completely overlaps with the shoulder, an area that overlaps with only a part of the shoulder, an area that includes the entire shoulder and the surrounding area of ​​the shoulder, an area that includes a part of the shoulder and a part of the surrounding area of ​​the shoulder, etc.

[0042] In the spot welding method of this embodiment, in the second step S2, a laser LB is irradiated onto the region overlapping with the shoulder portion SP to form a molten solidified portion 5 that does not penetrate in the thickness direction of the steel plate 21. Therefore, in the spot welded joint 1 obtained by the spot welding method of this embodiment, a molten solidified portion 5 that does not penetrate in the thickness direction of the steel plate 21 is formed in the region overlapping with the shoulder portion SP.

[0043] Here, FIG. 4 is a cross-sectional schematic diagram illustrating an overlapping configuration of the shoulder portion SP of the indentation portion IP in the spot-welded joint 1 and the molten solidified portion 5. In the spot-welded joint 1 shown in FIG. 4(a), the molten solidified portion 5 is formed in a region that completely overlaps with the shoulder portion SP. In the spot-welded joint 1 shown in FIG. 4(b), the molten solidified portion 5 is formed in a region that overlaps with only a portion of the shoulder portion SP. Furthermore, in the spot-welded joint 1 shown in FIG. 4(c), the molten solidified portion 5 is formed in a region that includes the entire shoulder portion SP and a peripheral portion of the shoulder portion SP. Furthermore, in the spot-welded joint 1 shown in FIG. 4(d), the molten solidified portion 5 is formed in a region that includes a portion of the shoulder portion SP and a portion of the peripheral portion of the shoulder portion SP. In the spot welded joint 1 obtained by the spot welding method of this embodiment, the configuration having a molten solidified portion 5 in the area overlapping with the shoulder portion SP includes at least the overlapping forms of (a) to (d) in Figure 4.

[0044] In addition, in this specification, the molten solidified portion "does not penetrate the steel plate in the thickness direction" means the following state depending on the position in the spot welded joint. That is, in the base material, the molten solidified portion extends from one surface (i.e., the upper surface) of the first steel plate in the thickness direction and does not reach the other surface (i.e., the lower surface), or the molten solidified portion extends from one surface (i.e., the upper surface) of the first steel plate in the thickness direction and reaches the other surface (i.e., the lower surface), but the molten solidified portion is not formed on the second steel plate located below it. On the other hand, in the pressure weld or nugget, the molten solidified portion is formed between two steel plates (i.e., the steel plate on the laser irradiation side and the steel plate located below it) and does not exceed the imaginary line connecting the pressure weld end portions located on both sides of the nugget in the thickness direction.

[0045] Furthermore, the term "melted and solidified portion" refers to a portion that is locally melted and solidified in the steel plate that constitutes the spot welded joint.

[0046] As described above, the spot welding method of this embodiment includes a first step S1 in which spot welding is performed on a plate combination consisting of two steel plates, steel plate 21 and steel plate 22. After that, a laser LB is irradiated to an area overlapping with the shoulder portion SP of the indentation portion IP formed on the surface of steel plate 21, which is a high-strength steel plate, to partially melt the steel plate 21 in the plate thickness direction, and a molten solidified portion 5 that does not penetrate the plate thickness direction of the steel plate 21 is formed. This results in a second step S2 in which the end 6 of the nugget 3 is tempered. In this second step S2, the heat generated when forming the molten solidified portion 5 is transmitted to the end 6 of the nugget 3, thereby tempering the end 6 of the nugget 3.

[0047] In the spot welding method of this embodiment, in the second step S2, it is only necessary to irradiate the laser LB to the area overlapping with the shoulder portion SP of the indentation IP formed on the surface of the steel plate 21, which is a high-strength steel plate. Therefore, there are fewer restrictions on the conditions required for tempering the end portion 6 of the nugget 3 (for example, the position and temperature of the heating target, the steel plate surface shape, etc.), and tempering of the end portion 6 of the nugget 3 can be achieved easily and with high precision. Furthermore, the irradiation conditions for the laser LB in the second step S2 can also be any conditions that can form a molten solidified portion 5 that does not penetrate the steel plate 21 in the plate thickness direction. Therefore, the energy and time required to temper the end portion 6 of the nugget 3 can be reduced, and the end portion 6 of the nugget 3 can be tempered efficiently. Therefore, the spot welding method of this embodiment can easily and efficiently improve the toughness of the weld, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, and as a result, excellent joint strength can be obtained.

[0048] In this specification, the term "joint strength" refers to joint strength evaluated at least by cross tensile strength (CTS). CTS can be measured according to a cross tensile test based on JIS Z 3137:1999. Specific measurement conditions for CTS will be described in the examples below.

[0049] (steel plate) In this embodiment, the two or more steel sheets used for spot welding are not particularly limited as long as at least one of the upper and lower sheets is an unplated high-strength steel sheet having a tensile strength of 1350 MPa or more or an aluminum-plated high-strength steel sheet having a tensile strength of 1350 MPa or more. All of the two or more steel sheets used for spot welding may be such high-strength steel sheets, or only some of the two or more steel sheets may be such high-strength steel sheets.

[0050] There are no particular limitations on the steel plates other than the high-strength steel plates that can be used together with the high-strength steel plates, and any steel plate can be used depending on the desired joint strength, etc. Examples of such steel plates include unplated or aluminum-plated steel plates having a tensile strength of less than 1350 MPa, and high-strength steel plates having a plating other than aluminum and having a tensile strength of 1350 MPa or more.

[0051] Furthermore, the tensile strength (TS) of steel sheets other than the high-strength steel sheets that can be used together with the high-strength steel sheets is not particularly limited, but is preferably 1350 MPa or more. When such high-strength steel sheets are used, the risk of a decrease in joint strength, such as CTS, increases significantly. Therefore, the present invention is particularly advantageous when such high-strength steel sheets are used. The tensile strength of the steel sheets that can be used in this embodiment, including the high-strength steel sheets and other steel sheets, may be 1600 MPa or more, 1800 MPa or more, 2000 MPa or more, 2300 MPa or more, or 2500 MPa or more. For example, the steel sheet that can be used in this embodiment may be a high-strength steel sheet for hot stamping having a tensile strength of 1350 MPa or more. The upper limit of the tensile strength of the steel sheet is not particularly limited, but is, for example, 3000 MPa from the viewpoint of workability, etc.

[0052] The tensile strength of a steel plate can be determined by first measuring the Vickers hardness of the steel plate and then converting it. The Vickers hardness of a steel plate can be measured using a method that conforms to JIS Z 2244:2009 "Vickers hardness test - Test method." The Vickers hardness of a steel plate is measured at a depth of 1 / 4 of the plate thickness under a test load of 500 g. The tensile strength (MPa) of the steel plate obtained in this way is calculated using the formula: tensile strength (MPa) = 3.3 × Vickers hardness (HV).

[0053] In this embodiment, the Vickers hardness of the steel sheet used for spot welding is preferably 410 HV or more. The Vickers hardness of the steel sheet may be 480 HV or more, 540 HV or more, 600 HV or more, 690 HV or more, or 750 HV or more. The upper limit of the Vickers hardness of the steel sheet is not particularly limited, but is, for example, 900 HV from the viewpoint of workability, etc.

[0054] In this embodiment, the two or more steel plates used for spot welding may all be the same type of steel plate, only some of the steel plates may be the same type of steel plate, or all of the steel plates may be different types of steel plate.

[0055] Furthermore, the number of steel plates is not particularly limited as long as it is two or more, and any number (for example, two, three, four, five or more, etc.) can be used depending on the application of the welded joint, etc. Furthermore, the thickness of the steel plates is also not particularly limited, and examples thereof include a thickness of 0.5 mm or more and 3.5 mm or less.

[0056] Hereinafter, each step in the spot welding method of this embodiment will be described in detail with reference to the drawings.

[0057] [1st step] In the spot welding method of this embodiment, the first step S1 is a step of spot welding in which a sheet pair consisting of a steel sheet 21 serving as an upper sheet and a steel sheet 22 serving as a lower sheet is clamped between a pair of electrodes, an upper electrode 41 and a lower electrode 42, as shown in Fig. 1. In this first step S1, a nugget 3 made of molten metal is formed by spot welding around the overlapping surfaces of the steel sheets 21 and 22.

[0058] In this embodiment, at least the steel plate 21 is an unplated high-strength steel plate having a tensile strength of 1350 MPa or more or an aluminum-plated high-strength steel plate having a tensile strength of 1350 MPa or more, but is not limited to this. That is, in a plate set consisting of the steel plate 21 and the steel plate 22, only the steel plate 21 may be the high-strength steel plate, or both the steel plate 21 and the steel plate 22 may be the high-strength steel plate, or only the steel plate 22 may be the high-strength steel plate. It is sufficient that at least one of the upper plate or the lower plate is the high-strength steel plate.

[0059] In the first step S1, the stacked steel sheets 21 and 22 are sandwiched between a pair of electrodes, an upper electrode 41 and a lower electrode 42, and while the pair of electrodes press the stacked steel sheets in the thickness direction, a current is passed through the stack in the thickness direction at a predetermined current value and for a predetermined time. As a result, the stacked surfaces of the steel sheets 21 and 22 and their surrounding areas melt due to Joule heat caused by electrical resistance, and the resulting molten metal forms a nugget 3. At this time, an indentation IP is formed on the outer surfaces of the steel sheets 21 and 22 by the pressing of the pair of electrodes. The indentation IP has a flat portion PP formed at the location in contact with the top of the electrode and a shoulder portion SP formed at the location in contact with the shoulder of the electrode.

[0060] (Welding conditions) In the first step S1, a welding machine, electrodes, and welding conditions used in normal spot welding can be used as long as they can form a nugget 3 on the overlapping surfaces of the overlapping steel plates 21 and 22 and in the surrounding area.

[0061] For example, various power sources can be used for spot welding machines, such as inverter DC power supplies, inverter AC power supplies, single-phase AC power supplies, etc. The welding machine can be a spot welding robot type that combines a welding gun with an industrial robot, or a stationary type.

[0062] For example, the pair of electrodes used in the spot welding of the first step may be DR-type electrodes made of chromium copper with a tip diameter of 5 mm to 8 mm. Furthermore, the pressure applied by the pair of electrodes may be, for example, 250 kgf to 700 kgf (2.451 kN to 6.864 kN). Furthermore, the current value of the current may be, for example, 4 kA to 12 kA. Furthermore, the current application time may be, for example, 12 cycles to 60 cycles. Note that when the power supply frequency is 50 Hz, 1 cycle (1 cycle) is 1 / 50 seconds.

[0063] In the first step, the number of times of energization during spot welding is not particularly limited, and may be, for example, only one energization or two or more energizations.

[0064] In the first step, preliminary energization or post-energization may be performed before or after energization for forming the nugget, as long as the effects of the present invention are not impaired.

[0065] [Second process] In the spot welding method of this embodiment, the second step S2 is a step of irradiating a laser beam LB to an area overlapping with a shoulder portion SP of an indentation portion IP formed on the surface of an upper steel plate 21, which is a high-strength steel plate, as shown in Fig. 1. In this second step S2, the steel plate 21 is partially melted in the thickness direction by the irradiation of the laser beam LB, and a molten solidified portion 5 that does not penetrate the steel plate 21 in the thickness direction is formed. The heat generated when this molten solidified portion 5 is formed is transferred to an end portion 6 of the nugget 3, and the end portion 6 of the nugget 3 is tempered.

[0066] By performing this second step S2, the spot welding method of this embodiment can simply and efficiently improve the toughness of the weld, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, as described above, and as a result, excellent joint strength can be obtained.

[0067] The depth in the thickness direction of the molten solidified portion 5 formed in the second step S2 is not particularly limited as long as it does not penetrate the steel plate 21 on which the molten solidified portion 5 is formed and is within a range that can temper at least the end 6 of the nugget 3. However, from the viewpoint of more reliably tempering the end 6 of the nugget 3, the depth in the thickness direction of the molten solidified portion 5 is preferably 20.0% or more of the total thickness of the steel plate 21 on which the molten solidified portion 5 is formed (note that this total thickness is 100.0%), more preferably 50.0% or more, and even more preferably 70.0% or more. The depth in the thickness direction of the molten solidified portion 5 can be controlled by appropriately setting the output and speed of the laser LB, which is the means for forming the molten solidified portion 5.

[0068] In the second step S2, the irradiation conditions of the laser LB (for example, laser output, beam diameter, speed, etc.) are not particularly limited as long as they are conditions that can form the above-mentioned non-penetrating molten solidification portion 5 in the plate thickness direction and temper the end 6 of the nugget 3, and any conditions can be adopted depending on the type of steel plate, productivity, etc. Note that examples of the laser irradiation conditions include laser output, beam diameter, speed, etc.

[0069] Furthermore, in the second step S2, the region to be irradiated with the laser LB (i.e., the region to form the molten solidified portion 5) is not particularly limited as long as it is a region that overlaps with the shoulder portion SP of the indentation portion IP formed on the surface of the steel plate 21, which is a high-strength steel plate. For example, the region to be irradiated with the laser may be a ring-shaped region that surrounds the flat portion PP of the indentation portion IP in a top view, as shown as the molten solidified portion 5 in FIG. 2. That is, the molten solidified portion 5 may be formed in a ring shape that surrounds the flat portion PP of the indentation portion IP of the high-strength steel plate in a top view.

[0070] In this way, by making the area to be irradiated with laser a ring-shaped area that surrounds the flat portion PP of the impression portion IP when viewed from above, the energy and time required to temper the end portion 6 of the nugget 3 can be further reduced, and the end portion 6 of the nugget 3 can be tempered more efficiently.

[0071] Here, "viewed from above" means "viewing the object to be observed on the steel plate or welded joint (for example, an indentation, a shoulder, a molten and solidified portion, etc.) from above in the vertical direction."

[0072] The ring-shaped region to be irradiated with laser light is a region in which band-shaped regions of a predetermined width are connected in a circular ring shape, and the band width may be set appropriately within a range that allows the end of the nugget to be tempered. The means for performing the ring-shaped laser irradiation is not particularly limited. For example, the laser irradiation may be performed while moving the laser irradiation unit along a ring-shaped trajectory so as to surround the flat portion PP of the indentation portion IP in a top view. In this case, the width of the laser irradiation, i.e., the width of the ring-shaped region, can be controlled by appropriately setting the laser beam diameter, output, and speed. The width of the laser irradiation can also be controlled by performing the laser irradiation multiple times. This method allows the laser to be irradiated over a wider region.

[0073] In addition, in the second step S2, the region to be irradiated with the laser may be the entire region of the indentation IP when viewed from above. Here, FIG. 5 is a schematic diagram of a cross-sectional view and a top view of a spot-welded joint 1 manufactured by a spot welding method according to another embodiment of the present invention. In the spot-welded joint 1 shown in FIG. 5, when viewed from above, laser irradiation is performed on the entire region including the flat portion PP and shoulder portion SP of the indentation IP formed on the surface of a steel plate 21, which is a high-strength steel plate, thereby forming a molten solidified portion 5 that does not penetrate the steel plate 21 in the thickness direction throughout the entire region. Note that the configuration of the spot-welded joint 1 shown in FIG. 5 other than the region where the molten solidified portion 5 is formed is the same as that of the spot-welded joint 1 shown in FIG. 2 described above.

[0074] In this way, by irradiating the laser beam over the entire area of ​​the indentation IP when viewed from above, a wider area including the nugget 3 and its end 6 can be tempered, thereby more reliably obtaining excellent joint strength.

[0075] In the second step S2, the laser may be irradiated to both the upper and lower sheets. By irradiating the areas overlapping with the shoulder portions SP of both the upper and lower sheets with the laser, the end portion 6 of the nugget 3 can be more reliably tempered, and therefore, excellent joint strength can be more reliably obtained.

[0076] (Other processes) In the spot welding method of the present invention, any step that is performed in a normal spot welding method may be performed before or after each of Steps 1 and 2, as long as it does not impair the effects of the present invention. Examples of such any step include a sheet assembly forming step, a cooling step, and various surface treatment steps.

[0077] Next, a spot welded joint produced by the spot welding method of the present invention, that is, a spot welded joint according to yet another embodiment of the present invention, will be described in detail with reference to the drawings.

[0078] <Spot welded joints> 2 , which is one embodiment of the present invention, has a spot-welded joint 1 in which, as described above, at least steel sheet 21, which serves as the upper sheet, and steel sheet 22, which serves as the lower sheet, is an unplated high-strength steel sheet having a tensile strength of 1350 MPa or more, or an aluminum-plated high-strength steel sheet having a tensile strength of 1350 MPa or more. Furthermore, spot-welded joint 1 of this embodiment has a nugget 3 on the overlapping surface of steel sheets 21 and 22.

[0079] As shown in FIG. 2, the spot welded joint 1 of this embodiment has a unique configuration in which a molten solidified portion 5 that does not penetrate the steel plate 21 in the thickness direction is formed in the area overlapping with the shoulder portion SP of the indentation portion IP formed on the surface of the steel plate 21, which is a high-strength steel plate, and the end portion 6 of the nugget 3 is tempered.

[0080] As described above, in the spot welded joint 1 of this embodiment, the heat generated when forming the molten solidified portion 5 in the area overlapping with the shoulder portion SP of the indentation portion IP of the steel plate 21, which is a high-strength steel plate, is transferred to the end portion 6 of the nugget 3, thereby tempering the end portion 6 of the nugget 3. Therefore, even when a high-strength steel plate is used, the toughness of the weld is high, resulting in a welded joint with excellent joint strength.

[0081] (Tempering determination method) Whether or not the end of the nugget has been tempered can be confirmed by measuring the Vickers hardness distribution in the cross section of the spot weld. Specifically, the Vickers hardness distribution is obtained at a position 1 / 4 of the plate thickness away from the overlapping surface of the steel plates on the cross section of the spot weld, and the tempering state can be determined by determining the tempering range from the obtained hardness distribution. The measurement range for Vickers hardness (HV) is 5 mm on either side of the nugget center, i.e., 10 mm from the left end to the right end of the measurement range, with the nugget center as the center of the measurement range. The hardness measurement pitch is 0.3 mm, and the measurement load is 500 gf. Next, in the hardness measurement distribution obtained, the hardness of a total of 10 points, including 5 points counted from the left end of the measurement range and 5 points counted from the right end of the measurement range, is averaged to determine the base material hardness.Then, the tempered reference hardness used to determine the tempering is calculated using the formula: tempered reference hardness (HV) = 0.95 × base material hardness (HV). Furthermore, in the hardness distribution obtained, the range where the Vickers hardness (HV) is equal to or less than the standard hardness for tempering is determined at both the left and right ends of the nugget. If this range is secured for 0.6 mm or more in the direction perpendicular to the plate thickness direction (i.e., the direction parallel to the overlapping surface) at least at one of the left and right ends of the nugget, the end of the nugget is judged to be tempered. In addition, in this judgment method, if the entire area within the nugget, i.e., the range from the left end to the right end of the nugget in the cross section of the spot weld, is below the tempered standard hardness, the range determined from the left end of the nugget and the range determined from the right end of the nugget will have the same value.

[0082] As described above, in the spot welded joint 1 of this embodiment, the steel plates that make up the spot welded joint 1 are not particularly limited in type or number, as long as at least one of the upper and lower plates is the high-strength steel plate described above, and the plates described in the above-mentioned spot welding method can be used. Note that the steel plates that make up the spot welded joint 1 are the same as those described in the above-mentioned spot welding method, and therefore a detailed description thereof will be omitted.

[0083] 2, the molten solidified portion 5 is formed in a ring shape surrounding the flat portion PP of the indentation portion IP of the steel plate 21 when viewed from above. When the molten solidified portion 5 is formed in a ring shape surrounding the flat portion PP of the indentation portion IP of the steel plate 21 when viewed from above, the energy and time required to temper the end portion 6 of the nugget 3 can be further reduced, and therefore, a spot welded joint having the above-mentioned excellent joint strength can be obtained more efficiently.

[0084] The means for forming the molten solidified portion 5 into such a ring shape is as explained in the spot welding method above.

[0085] 5, the molten solidified portion 5 may be formed over the entire area of ​​the indentation IP of the steel plate 21 when viewed from above. When the molten solidified portion 5 is formed over the entire area of ​​the indentation IP of the steel plate 21 when viewed from above, a wider area including the nugget 3 and its end 6 can be tempered, thereby more reliably achieving excellent joint strength.

[0086] In the spot welded joint 1 of the present embodiment, as described above, the molten solidified portion 5 may be formed on both the upper plate and the lower plate. By forming the molten solidified portion 5 in the region overlapping with the shoulder portion SP of both the upper plate and the lower plate, the end portion 6 of the nugget 3 can be tempered more reliably, and therefore excellent joint strength can be exhibited more reliably.

[0087] (Application example) As described above, the spot welding method of the present invention can simply and efficiently improve the toughness of the weld, even when using high-strength steel plates with a tensile strength of 1350 MPa or more, and as a result, can obtain excellent joint strength. Therefore, the method can be applied to the manufacture of various structural parts that require excellent joint strength, such as transportation machinery such as automobiles, industrial machinery, and buildings. In particular, the present invention can be particularly suitably used in the manufacture of automobile bodies and parts, which require high production efficiency and excellent joint strength.

[0088] The spot welding method and spot welded joint of the present invention are not limited to the above-described embodiments or the examples described below, and appropriate combinations, substitutions, modifications, etc. are possible within the scope of the object and intent of the present invention. In this specification, ordinal numbers such as "first" and "second" are used to distinguish items to which the ordinal numbers are assigned, and do not indicate the order, priority, importance, etc. of each item. [Example]

[0089] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0090] (Fabrication of spot welded joints) In order to verify the effect of the present invention, a spot-welded joint was produced as follows. First, a sheet assembly consisting of two overlapping unplated hot stamping steel sheets, each 1.6 mm thick and 2500 MPa in tensile strength, was spot welded using a stationary resistance spot welder powered by a single-phase AC power source with a power frequency of 50 Hz, to form a nugget on the overlapping surface of the two steel sheets (step 1). A chromium-copper DR-type electrode with a tip diameter of 6 mm and a shoulder radius of 40 mm was used. The electrode pressure was set to 500 kgf, and current application began after the pressure stabilized. Current application was performed once for 22 cycles at a current value of 7 kA. The time from the end of current application to the release of the electrodes, i.e., the hold time, was set to 10 cycles.

[0091] Next, a laser was applied to the area overlapping the shoulder of the indentation formed on each surface of the two hot stamping steel sheets after spot welding. This partially melted the area in the thickness direction, forming a molten solidification zone that did not penetrate the thickness direction of each steel sheet, and tempering the edge of the nugget (step 2). A fiber laser was used as the laser source, and laser irradiation was performed using a remote laser method. The laser was irradiated in a ring-shaped trajectory, surrounding the flat portion of the indentation when viewed from above. The laser output when irradiating both the upper and lower sheets was set to 1.4 kW to 2.0 kW, as shown in Table 1 below, and the focal position was set 5 mm away from the steel sheet surface. The moving speed of the laser irradiation zone was set to 3 m / min. In this manner, three types of spot-welded joints (Examples 1 to 3) of examples of the present invention were produced using different laser irradiation conditions.

[0092] A spot-welded joint of Comparative Example 1 was produced in the same manner as the above-described Inventive Example, except that laser irradiation was not performed, that is, the second step was not performed.

[0093] A spot-welded joint of Comparative Example 2 was produced in the same manner as the above-described Inventive Example, except that the second step was carried out with a laser output of 1.2 kW as shown in Table 1 below.

[0094] (Spot weld joint evaluation) Three spot-welded joints for each of the above-mentioned invention examples and comparative examples were produced under the same conditions, and the cross sections of the spot-welded joints were observed as follows to measure the depth (mm) of the molten solidified portion in the plate thickness direction and the depth ratio (%) to the plate thickness. Furthermore, for the spot-welded joints of the invention examples and comparative examples, the base metal hardness and the tempered range (mm) of the nugget end were measured according to the above-mentioned tempering determination method, and it was evaluated whether the nugget end was tempered or not.

[0095] Furthermore, the cross tensile strength (CTS) of the spot-welded joints of the invention examples and comparative examples was measured as follows.

[0096] (Cross-sectional observation of spot welded joint) First, the spot-welded joint is cut in the thickness direction through the center of the nugget when viewed from above to reveal a cross section. This cross section is then embedded in a cold-setting resin. The specimen observation surface after resin embedding is roughly polished using waterproof abrasive paper with grit sizes of 80, 400, 800, and 1500, and then finely polished using a 3 μm diamond spray. The polished specimen observation surface is corroded to the extent that the fusion boundary can be distinguished, making the nugget visible. The etchant used to corrode the specimen observation surface is, for example, picric acid. The specimen observation surface after corrosion is then photographed using a magnifying observation tool such as a microscope to confirm the state of the molten solidification portion. The depth (mm) of the molten solidification portion in the thickness direction and the depth ratio (%) of the molten solidification portion relative to the thickness of the steel plate on which the molten solidification portion was formed are then determined. Here, the depth of the molten solidified portion in the thickness direction is determined as the shortest distance from the surface of the base material in the unformed region of the indentation, i.e., the region outside the indentation, to the tip of the molten solidified portion. Specifically, it is determined as the distance between the horizontal reference line on the surface of the base material (horizontal reference lines L1 and L2 in FIG. 3) and a straight line passing through the tip of the molten solidified portion and parallel to the horizontal reference line.

[0097] FIG. 6 shows an enlarged cross-sectional photograph of the spot-welded joint of Example 1, which is an example of the present invention, and FIG. 7 shows an enlarged cross-sectional photograph of the spot-welded joint of Comparative Example 1.

[0098] (Measurement of cross tensile strength (CTS) of spot welded joints) The CTS of spot-welded joints was measured in accordance with the cross tension test based on JIS Z 3137:1999. The CTS measurement was carried out at a speed of 10 mm / min. The CTS measurement was carried out on two samples, and the average value of the measurement results was used.

[0099] The various measurement results and evaluation results of the spot-welded joints of the invention examples and comparative examples are shown in Table 1 below.

[0100] In Table 1, the tempering evaluation is indicated by a circle (◯) when the tempered range of the nugget end is 0.6 mm or more at at least one of the left and right ends of the nugget, indicating that the nugget end is tempered. On the other hand, when the tempered range of the nugget end is less than 0.6 mm at both the left and right ends of the nugget, the nugget end is indicated by a cross (×) when the nugget end is not tempered.

[0101] In Table 1, the underlined CTS measurement values ​​indicate unfavorable results.

[0102] [Table 1]

[0103] In the spot-welded joint of the example of the present invention, a molten solidified portion that does not penetrate through the steel plates in the plate thickness direction is formed in the region overlapping with the shoulder of the indentation as shown in Figure 6, and the edge of the nugget is tempered, so that it was found to have a high CTS and excellent joint strength as shown in Table 1. In other words, it was found that the example of the present invention can simply and efficiently improve the toughness of the weld and obtain excellent joint strength, even when a high-strength steel plate is used as the base metal.

[0104] On the other hand, in the spot-welded joint of Comparative Example 1, in which the second step was not performed, no molten solidification portion was formed other than the nugget as shown in FIG. 7, and the weld portion was not tempered. Therefore, as shown in Table 1, the CTS was low and sufficient joint strength was not obtained.

[0105] In addition, in the spot welded joint of Comparative Example 2, in which the second step was performed at a low laser output, although a molten solidified portion was formed, the depth was shallow, and the end of the nugget was not tempered by the formation of the molten solidified portion. As a result, as shown in Table 1, the CTS was low and sufficient joint strength was not obtained. [Explanation of symbols]

[0106] 1 Spot welded joints 21 (Top) Steel Plate 22 (Lower) Steel Plate 3. Nuggets 41 Upper electrode 42 Lower electrode 5 Melting and solidification area 6 End IP impression area PP flat part SP shoulder

Claims

1. A spot welding method for a plate assembly consisting of two or more steel plates, At least one of the upper plate and the lower plate of the plate set is an unplated or aluminum-plated high-strength steel plate having a tensile strength of 1350 MPa or more, a first step of spot welding the plate assembly to form a nugget on an overlapping surface of the plate assembly; A second step of irradiating a laser beam onto an area overlapping with a shoulder portion of an indentation formed on the surface of the high-strength steel plate to form a molten solidified portion that does not penetrate the high-strength steel plate in the thickness direction, thereby tempering the end of the nugget; A spot welding method comprising:

2. 2. The spot welding method according to claim 1, wherein the region to be irradiated with the laser is a ring-shaped region surrounding the flat portion of the indentation when viewed from above.

3. The spot welding method according to claim 1 , wherein the laser irradiation area is the entire area of ​​the indentation when viewed from above.

4. The spot welding method according to claim 1 , wherein the laser irradiation is performed on both the upper plate and the lower plate.

5. The spot welding method according to claim 1, wherein a depth of the molten solidified portion in the plate thickness direction is 20.0% or more of a thickness of the high-strength steel plate.

6. A spot welded joint formed by two or more steel plates, at least one of which is an unplated or aluminum-plated high-strength steel plate having a tensile strength of 1350 MPa or more, A nugget is provided on the overlapping surfaces of the two or more steel plates, A spot welded joint, characterized in that a molten solidified portion that does not penetrate the high-strength steel plate in the thickness direction is formed in a region overlapping with a shoulder portion of an indentation portion formed on the surface of the high-strength steel plate, and the end of the nugget is tempered.

7. The spot welded joint according to claim 6, wherein the molten solidified portion is formed in a ring shape surrounding the flat portion of the indentation portion when viewed from above.

8. The spot welded joint according to claim 6, wherein the molten solidified portion is formed over the entire area of ​​the indentation portion when viewed from above.

9. The spot welded joint according to claim 6, wherein the molten solidified portion is formed on both the upper plate and the lower plate.

10. The spot welded joint according to claim 6, wherein the depth of the molten solidified portion in the plate thickness direction is 20.0% or more of the plate thickness of the high-strength steel plate.

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