Spot-welding method and spot-welded joint

Abstract

To provide a spot-welding method capable of efficiently preventing shoulder cracks in the spot welding of a galvanized steel, and a spot-welded joint in which the shoulder cracks are reduced by a new constitution.SOLUTION: A spot-welding method according to the present invention includes: a first step of performing spot welding on a set of two or more steels in which at least one of an upper sheet and a lower sheet is a galvanized steel; and a second step of forming a molten and solidified portion (7) that does not penetrate in a sheet thickness direction of the galvanized steel in a region overlapping with a shoulder portion (SP) of an impression portion (IP) formed on a surface of the galvanized steel. In addition, in a spot-welded joint (1) of the present invention, at least one of the upper plate and the lower plate is a galvanized sheet, and a melted and solidified portion (7) which does not penetrate in a sheet thickness direction of the galvanized steel is provided in a region overlapping with a shoulder portion (SP) of an impression portion (IP) formed on a surface of the galvanized steel.SELECTED DRAWING: Figure 2

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, in the case of high-strength steel sheets used for automobile bodies and parts, high-strength steel sheets whose surfaces are plated with metals such as zinc, particularly galvanized steel sheets, are used from the viewpoint of rust prevention.

[0004] On the other hand, the assembly of automobile bodies and the attachment of parts are mainly performed by spot welding. However, when spot welding is performed on a sheet assembly including a galvanized steel sheet, cracks may occur from the outer surface or inner surface of the steel sheet that is in contact with the spot welding electrode in the sheet thickness direction.

[0005] This type of cracking is said to be caused by liquid metal embrittlement (LME) (hereinafter, sometimes referred to as "LME cracking"). Specifically, when tensile stress due to electrode pressure and thermal expansion or contraction of the steel sheet is applied to the weld, the molten zinc-based coating metal penetrates into the grain boundaries of the steel sheet, reducing the grain boundary strength, which is said to cause this type of cracking.

[0006] LME cracks are broadly divided into internal cracks that occur on the mating surfaces of steel sheets and external cracks that occur on the outer surface of the steel sheet that comes into contact with the electrode. Fig. 1 is a cross-sectional schematic diagram of a spot-welded joint 1 formed by spot welding a sheet assembly including a galvanized steel sheet. In Fig. 1, the spot-welded joint 1 is composed of a steel sheet 21 that serves as the upper sheet, a steel sheet 22 that serves as the lower sheet, and a nugget 3 made of molten metal that is formed around the overlapping surface of these steel sheets 21, 22. In the spot-welded joint 1 shown in Fig. 1, the steel sheet 21 is a galvanized steel sheet.

[0007] 1 has a crack 4 just outside the corona bond that has developed from the overlapping surfaces of the steel plates 21. This crack 4 just outside the corona bond is an example of the internal crack mentioned above.

[0008] 1 has a sub-electrode crack 5 that has propagated from the outer surface of the steel sheet 21 at a flat portion PP of the indentation IP formed by the pressing of the pair of electrodes, and a shoulder crack 6 that has propagated from the outer surface of the steel sheet 21 at a step portion called a shoulder portion SP that has formed at the point on the steel sheet 21 where it comes into contact with the shoulder portion of the electrode. The sub-electrode crack 5 and shoulder crack 6 are examples of the above-mentioned outer crack.

[0009] Because such cracks can lead to a decrease in joint strength, various techniques for suppressing such cracks have been investigated. For example, Patent Document 1 discloses a technique for preventing external cracks by removing the plating coating on at least the inner region of the weld heat-affected zone on the surface that comes into contact with the electrode before spot welding. Patent Document 1 also discloses a technique for preventing internal cracks on the overlapping surfaces of steel sheets by removing the plating coating on at least both sides of the overlapping surfaces of the steel sheets in a ring-shaped region whose outer periphery is the outer edge of the heat-affected zone and whose inner periphery is 0.8 times the nugget diameter. It is claimed that these techniques disclosed in Patent Document 1 can easily prevent liquid metal embrittlement cracking during spot welding of plated steel sheets. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2016 / 159169 Summary of the Invention [Problem to be solved by the invention]

[0011] The technology disclosed in Patent Document 1 requires the removal of plating covering a specific area, and also aims to prevent all types of cracks, including external and internal cracks, so there is room for improvement in terms of efficiency.

[0012] The impact of cracks that occur in spot-welded joints on joint strength varies depending on the location of the cracks. Among the various types of cracks described above, shoulder cracks have a particularly large impact on joint strength. Therefore, there is a need for more efficient prevention of shoulder cracks, which have a large impact on joint strength.

[0013] The present invention has been made in view of the above circumstances, and aims to provide a spot welding method that can efficiently prevent shoulder cracks during spot welding of galvanized steel sheets. Another aim of the present invention is to provide a spot welded joint with a novel configuration that reduces shoulder cracks. [Means for solving the problem]

[0014] The present invention includes the following aspects.

[0015] (Aspect 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 assembly is a galvanized steel plate, a first step of spot welding the plate assembly; A second step of forming a molten solidified portion that does not penetrate the galvanized steel sheet in the thickness direction in an area overlapping with a shoulder portion of the indentation formed on the surface of the galvanized steel sheet; A spot welding method comprising:

[0016] (Aspect 2) The spot welding method according to the first aspect, wherein the means for forming the molten solidified portion is laser irradiation.

[0017] (Aspect 3) 3. The spot welding method according to claim 1, wherein the region where the molten solidification portion is formed is a ring-shaped region surrounding the flat portion of the indentation when viewed from above.

[0018] (Aspect 4) 3. The spot welding method according to claim 1, wherein the region where the molten solidified portion is formed is the entire area of ​​the indentation portion when viewed from above.

[0019] (Aspect 5) The spot welding method according to any one of the above-mentioned aspects 1 to 4, wherein the molten solidified portion is formed on both the upper plate and the lower plate.

[0020] (Aspect 6) A spot welded joint made of two or more steel plates, at least one of which is a galvanized steel plate, A spot welded joint characterized in that it has a molten solidified portion that does not penetrate the galvanized steel sheet in the thickness direction in a region overlapping with a shoulder portion of an indentation formed on the surface of the galvanized steel sheet.

[0021] (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.

[0022] (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.

[0023] (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. [Effects of the Invention]

[0024] According to the spot welding method of the present invention, shoulder cracks can be efficiently prevented in spot welding of galvanized steel sheets, and further, according to the present invention, a spot-welded joint with reduced shoulder cracks can be provided. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional schematic diagram of a spot-welded joint 1 formed by spot welding a sheet assembly including galvanized steel sheets. [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 schematic cross-sectional view illustrating a form in which the shoulder portion SP of the indentation IP and the molten solidified portion 7 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 of 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 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 extensive research focusing on post-processing steps after spot welding, and as a result, have discovered a new method for repairing LME cracks that have occurred in the shoulder of an indentation formed on the surface of a galvanized steel sheet by melting the area that overlaps with the shoulder of the indentation in the post-processing step after spot welding.

[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.

[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 sheet and the lower sheet of the sheet assembly is a galvanized steel sheet. The spot welding method of this embodiment includes a first step of spot welding the plate assembly, and a second step of forming a molten solidified portion that does not penetrate the zinc-plated steel sheet in the thickness direction in an area overlapping with the shoulder portion of the indentation portion formed on the surface of the zinc-plated steel sheet.

[0031] Here, 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 of this embodiment. The spot welded joint 1 shown in Fig. 2 is composed of a steel sheet 21 that serves as an upper sheet, a steel sheet 22 that serves as a lower sheet, and a nugget 3 made of molten metal that is formed around the overlapping surface of these steel sheets 21 and 22. In the spot welded joint 1 shown in Fig. 2, at least the steel sheet 21 is a galvanized steel sheet.

[0032] In the spot welded joint 1, an indentation IP is formed on each of the steel plates 21 and 22 by pressing in the pair of electrodes. Furthermore, the indentation IP has a flat portion PP formed on each of the steel plates 21 and 22 at a location that comes into contact with the top of the electrode, and a shoulder portion SP formed at a location that comes into contact with the shoulder of the electrode.

[0033] (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.

[0034] 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. 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).

[0035] 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 out The 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).

[0036] 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.

[0037] 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.

[0038] 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. 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 7. In the spot-welded joint 1 shown in FIG. 4(a), the molten solidified portion 7 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 7 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 7 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 7 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 of this embodiment, the configuration in which the molten solidified portion 7 is provided in the region overlapping with the shoulder portion SP includes at least the overlapping forms shown in (a) to (d) of FIG.

[0039] As described above, in the spot welding method of this embodiment, first, in the first step, spot welding is performed on a sheet pair consisting of steel sheet 21, which is the upper sheet, and steel sheet 22, which is the lower sheet. In this first step, a nugget 3 made of molten metal is formed around the overlapping surface of steel sheet 21 and steel sheet 22.

[0040] Then, after this first step, as the second step, 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 zinc-plated steel plate, is partially melted in the thickness direction of the plate to form a molten solidified portion 7 that does not penetrate the steel plate 21 in the thickness direction.

[0041] Here, 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 galvanized 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 galvanized steel plate in the thickness direction and reaches the other surface (i.e., the lower surface), but no molten solidified portion is formed on the second steel plate located below it. On the other hand, in the pressure welded portion or nugget portion, the molten solidified portion is formed between two steel plates (i.e., the steel plate on the laser irradiated side and the steel plate located below it) and does not exceed the imaginary line connecting the pressure welded portion ends located on both sides of the nugget portion in the thickness direction.

[0042] Further, 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.

[0043] The spot welding method of this embodiment can repair shoulder cracks that have a significant impact on joint strength in the first step by melting the area overlapping the shoulder SP in the second step. Furthermore, the spot welding method of this embodiment can efficiently repair the shoulder cracks by melting the area overlapping the shoulder SP of the galvanized steel sheet 21 partially in the thickness direction, rather than melting the entire area in the thickness direction. In other words, the spot welding method of this embodiment can efficiently prevent shoulder cracks from occurring in spot welding of galvanized steel sheets.

[0044] In this specification, "cracks in the shoulder region" refers to cracks of a certain size or larger that have a significant effect on joint strength, that is, cracks whose length from the start point to the end point exceeds 100 μm.

[0045] (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 sheet or the lower sheet is a galvanized steel sheet. All of the two or more steel sheets used for spot welding may be galvanized steel sheets, or only some of the two or more steel sheets may be galvanized steel sheets.

[0046] Furthermore, there are no particular limitations on the steel sheet other than the zinc-plated steel sheet that is used together with the zinc-plated steel sheet, and any steel sheet can be used depending on the desired joint strength, etc. Examples of steel sheets other than the zinc-plated steel sheet include steel sheets plated with a material other than zinc, unplated steel sheets, and chemically treated steel sheets.

[0047] In this embodiment, the tensile strength (TS) of the steel sheets used for spot welding, i.e., the galvanized steel sheet and the non-galvanized steel sheet, is not particularly limited, but the tensile strength of the steel sheet is preferably 780 MPa or more. When such high-strength galvanized steel sheets are used, the risk of LME cracking increases significantly, so the present invention is particularly advantageous when such high-strength steel sheets are used. The tensile strength of the steel sheet may be 980 MPa or more, 1200 MPa or more, 1500 MPa or more, or 1800 MPa or more. Note that 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.

[0048] 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).

[0049] In this embodiment, the Vickers hardness of the steel sheet used for spot welding is preferably 230 Hv or more. The Vickers hardness of the steel sheet may be 300 Hv or more, 360 Hv or more, 450 Hv or more, or 540 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.

[0050] 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.

[0051] 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.

[0052] Hereinafter, each step in the spot welding method of this embodiment will be described in detail with reference to the drawings.

[0053] [1st step] In the spot welding method of this embodiment, the first step is a step of spot welding a sheet pair consisting of a steel sheet 21 that serves as an upper sheet and a steel sheet 22 that serves as a lower sheet. In this embodiment, at least the steel sheet 21 is a galvanized steel sheet, but this is not limited to this. That is, in the sheet pair consisting of the steel sheets 21 and 22, only the steel sheet 21 may be a galvanized steel sheet, or both the steel sheets 21 and 22 may be galvanized steel sheets, or only the steel sheet 22 may be a galvanized steel sheet. It is sufficient that at least one of the upper sheet or the lower sheet is a galvanized steel sheet.

[0054] In this first step, a pair of overlapping steel sheets 21 and 22 is clamped between a pair of electrodes, and while the pair of electrodes presses the sheet pair in the thickness direction, electricity is passed through the pair of electrodes in the thickness direction at a predetermined current value for a predetermined time. As a result, the overlapping 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. This 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.

[0055] (Welding conditions) In the first step, any welding machine, electrode, 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.

[0056] 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 of a spot welding robot type that combines a welding gun with an industrial robot, or a stationary type.

[0057] 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.

[0058] 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.

[0059] In the first step, preliminary energization or post-energization may be performed before or after energization for forming the nugget, as long as it does not impair the effects of the present invention.

[0060] [Second process] In the spot welding method of this embodiment, the second step is a step of forming a molten solidified portion 7 that does not penetrate the galvanized steel sheet in the thickness direction in a region that overlaps with the shoulder portion SP of the indentation portion IP formed on the surface of the galvanized steel sheet (i.e., steel sheet 21). In this second step, the region that overlaps with the shoulder portion SP of the indentation portion IP formed on the surface of the steel sheet 21, which is a galvanized steel sheet, is partially melted in the thickness direction using any melting means, and a molten solidified portion 7 that does not penetrate the steel sheet 21 in the thickness direction is formed.

[0061] By melting the region overlapping with the shoulder portion SP in the second step, even if a shoulder crack that has a significant impact on joint strength occurs in the first step, the shoulder crack can be repaired. Furthermore, in the second step, the region overlapping with the shoulder portion SP of the galvanized steel sheet 21 is melted partially in the thickness direction rather than melted entirely in the thickness direction, which allows the above-mentioned shoulder crack to be repaired efficiently.

[0062] The depth in the thickness direction of the molten solidified portion 7 formed in the second step is not particularly limited as long as it does not penetrate the steel plate 21 on which the molten solidified portion 7 is formed, and may be appropriately set within a range that allows shoulder cracks to be repaired. From the viewpoint of more reliably repairing shoulder cracks, the depth in the thickness direction of the molten solidified portion 7 is preferably 5% or more of the total thickness of the steel plate 21 on which the molten solidified portion 7 is formed, more preferably 20% or more, and even more preferably 50% or more. The depth of the molten solidified portion 7 can be controlled by appropriately setting various conditions of the melting means. For example, when laser irradiation, which will be described later, is used as the melting means, the depth of the molten solidified portion can be controlled by appropriately setting the laser output and speed.

[0063] The melting means used in the second step is not particularly limited as long as it can partially melt the area overlapping with the shoulder portion SP of the steel plate 21 in the plate thickness direction. Examples of such melting means include a laser irradiator, an arc welder, and a plasma welder. Among them, the melting means is preferably laser irradiation by a laser irradiator. When the means for forming the molten solidified portion is laser irradiation, the molten solidified portion can be formed more accurately and efficiently.

[0064] When the molten solidified portion is formed by laser irradiation, the laser irradiation conditions (for example, laser output, beam diameter, speed, etc.) are not particularly limited as long as they can melt the steel plate, and any conditions can be adopted according to the type of steel plate, productivity, etc. Examples of the laser irradiation conditions include laser output, beam diameter, speed, etc.

[0065] In the second step, the region where the molten solidified portion 7 is formed 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 galvanized steel sheet (i.e., steel sheet 21). For example, the region where the molten solidified portion 7 is formed may be a ring-shaped region that surrounds the flat portion PP of the indentation portion IP when viewed from above, as shown in FIG. 2. That is, the molten solidified portion 7 may be formed in a ring shape that surrounds the flat portion PP of the indentation portion IP when viewed from above.

[0066] In this way, when the region where the molten solidified portion 7 is formed is a ring-shaped region surrounding the flat portion PP of the indentation portion IP when viewed from above, that is, when the molten solidified portion 7 is formed in a ring shape surrounding the flat portion PP of the indentation portion IP when viewed from above, the location where the galvanized steel sheet is melted is limited to a predetermined region where shoulder cracking occurs, and the above-mentioned shoulder cracking can be prevented more efficiently.

[0067] 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."

[0068] In addition, the ring-shaped region forming the molten solidified portion 7 is a region in which band-shaped regions having a predetermined width are connected in an annular shape, and the width of the band can be set appropriately within a range in which the shoulder crack can be repaired. The means for forming the molten solidified portion 7 in such a ring shape is not particularly limited. For example, when the above-mentioned laser irradiation is used as the melting means, the width of the molten solidified portion 7 can be controlled by appropriately setting the laser beam diameter, output, and speed. In addition, the width of the molten solidified portion 7 can be controlled by performing laser irradiation multiple times, and this method allows the formation of a wider molten solidified portion 7.

[0069] In the second step, the region where the molten solidified portion 7 is formed may be the entire area 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, a molten solidified portion 7 that does not penetrate the steel plate 21 in the thickness direction is formed over the entire area, including the flat portion PP and shoulder portion SP, of the indentation IP formed on the surface of the steel plate 21, which is a galvanized steel plate. The configuration other than the region where the molten solidified portion 7 is formed is the same as that of the spot-welded joint 1 shown in FIG. 2 described above.

[0070] In this way, if the area where the molten solidified portion 7 is formed is the entire area of ​​the indentation IP when viewed from above, that is, if the molten solidified portion 7 is formed over the entire area of ​​the indentation IP when viewed from above, not only the shoulder cracks but also the cracks directly below the electrode can be efficiently repaired. This makes it possible to more reliably prevent a decrease in the joint strength of the spot welded joint.

[0071] In the second step, the molten solidified portion 7 may be formed on both the upper plate and the lower plate. By forming the molten solidified portion 7 on both the upper plate and the lower plate, shoulder cracks that may occur in the spot welded joint can be more reliably prevented.

[0072] (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.

[0073] 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.

[0074] <Spot welded joints> 2, which is one embodiment of the present invention, is a spot welded joint formed of two steel plates, with at least steel plate 21 of upper and lower steel plates 21 and 22 being a galvanized steel plate, as described above. The spot welded joint 1 of this embodiment has a unique configuration in which a molten solidified portion 7 that does not penetrate the galvanized steel plate in the thickness direction is provided in a region that overlaps with a shoulder portion SP of an indentation IP formed on the surface of steel plate 21, which is a galvanized steel plate.

[0075] The spot welded joint 1 of this embodiment has such a molten solidified portion 7 in the region overlapping with the shoulder portion SP of the indentation IP of the steel plate 21, which is a galvanized steel plate, thereby repairing the shoulder cracks and consequently reducing the occurrence of shoulder cracks. This makes the spot welded joint 1 of this embodiment a welded joint with excellent joint strength.

[0076] In addition, even if the shoulder crack is not completely eliminated by the molten solidified portion, the decrease in joint strength can be suppressed. However, from the viewpoint of more reliably preventing the decrease in joint strength, the shoulder crack is completely eliminated by the molten solidified portion.

[0077] In the spot welded joint 1 of this embodiment, the steel sheets 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 sheets is a zinc-plated steel sheet, and the same sheets as described in the above-mentioned spot welding method can be used. Note that the steel sheets 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.

[0078] In the spot welded joint 1 of this embodiment, the molten solidified portion 7 is formed in a ring shape surrounding the flat portion PP of the indentation portion IP when viewed from above, as shown in Fig. 2. When the molten solidified portion 7 is formed in a ring shape surrounding the flat portion PP of the indentation portion IP when viewed from above, the location where the galvanized steel sheet is melted is limited to a predetermined area where shoulder cracking occurs, so that a spot welded joint with reduced shoulder cracking can be obtained more efficiently.

[0079] The means for forming the molten solidified portion 7 into such a ring shape is as explained in the spot welding method above.

[0080] The molten solidified portion 7 may be formed over the entire area of ​​the indentation IP when viewed from above, as in the spot-welded joint 1 shown in Fig. 5. By forming the molten solidified portion 7 over the entire area of ​​the indentation IP when viewed from above, not only shoulder cracks but also cracks directly below the electrode can be reduced. This more reliably prevents a decrease in the joint strength of the spot-welded joint.

[0081] In the spot welded joint 1 of the present embodiment, as described above, the molten solidified portion 7 may be formed on both the upper plate and the lower plate. By forming the molten solidified portion 7 on both the upper plate and the lower plate, shoulder cracking in the spot welded joint can be more reliably reduced.

[0082] (Application example) As described above, the spot welding method of the present invention can efficiently prevent shoulder cracks in spot welding of galvanized steel sheets and can obtain spot-welded joints with reduced shoulder cracks, making it applicable to various structural parts that require excellent joint strength, such as transportation machinery such as automobiles, industrial machinery, and buildings. In particular, because the present invention can efficiently prevent shoulder cracks in spot welding of galvanized steel sheets, it can be particularly well suited for use in the manufacture of automobile bodies and parts, which require high production efficiency and excellent joint strength.

[0083] 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]

[0084] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0085] (Fabrication of spot welded joints) To verify the effectiveness of the present invention, spot welding was performed on a sheet assembly consisting of two overlapping zinc-plated steel sheets with a tensile strength of 980 MP using a stationary resistance spot welder with a single-phase AC power supply at a power frequency of 50 Hz (step 1). A chromium-copper DR-type electrode with an electrode tip diameter of 6 mm and a shoulder radius of 40 mm was used. The electrode pressure was set to 400 kgf, and current application began after the pressure stabilized. A multiple current application pattern was used. Specifically, current was applied for 40 cycles at a current value of 7 kA, followed by current application for 40 cycles at a current value of 7 kA, and then current was applied for 40 cycles at a current value of 10 kA. The non-current application interval between each current application was set to 2 cycles. The time from the end of current application to the release of the electrodes, i.e., the holding time, was set to 10 cycles.

[0086] Next, a laser was irradiated onto the shoulder of the indentation formed on each surface of the two galvanized steel sheets after spot welding, partially melting the shoulder in the sheet thickness direction to form a molten solidified portion that did not penetrate the sheet thickness direction of each galvanized steel sheet (step 2). A fiber laser was used as the laser light source, and irradiation was performed using a remote laser method. The laser irradiation was performed on a ring-shaped area surrounding the flat portion of the indentation when viewed from above. Specifically, ring-shaped irradiation was performed with a trajectory of 9.5 mm in diameter, followed by ring-shaped irradiation with a trajectory of 10.3 mm in diameter. The laser output was set to 2 kW, and the focal position was set at a position 5 mm away from the steel sheet surface. The moving speed of the laser irradiation area was set to 3 m / min. In this way, a spot welded joint according to the present invention was obtained.

[0087] Furthermore, a spot-welded joint of a comparative example was obtained by a conventional spot welding method, that is, in the same manner as the above-described inventive example, except that the second step was not carried out.

[0088] Ten spot-welded joints of each of the above-mentioned invention examples and comparative examples were produced under the same conditions, and the occurrence of shoulder cracks was examined according to the following procedure.

[0089] First, the spot-welded joint was cut in the thickness direction through the center of the nugget in a top view to expose the cross section. This cross section was then embedded in a cold-setting resin. The specimen observation surface after resin embedding was roughly polished using waterproof abrasive paper with grit sizes of 80, 400, 800, and 1500, followed by precision polishing using a 3 μm diamond spray. The polished specimen observation surface was then corroded to the extent that the fusion boundary could be discerned, making the nugget visible. The etching solution used to corrode the specimen observation surface was, for example, picric acid. The corroded specimen observation surface was then photographed using a magnifying observation tool such as a microscope to confirm the presence or absence of shoulder cracks. The number of samples out of 10 in which shoulder cracks were observed was recorded. The presence or absence of cracks was determined based on the crack length (connecting the start and end points of the crack) of more than 100 μm, and the absence of cracks (no cracks) of less than 100 μm.

[0090] The observation results are shown in the following Table 1. Fig. 6 shows an enlarged cross-sectional photograph of the spot-welded joint of the example of the present invention, and Fig. 7 shows an enlarged cross-sectional photograph of the spot-welded joint of the comparative example, taken during this observation.

[0091] [Table 1]

[0092] As shown in Table 1 and FIG. 6, in the spot-welded joints of the invention examples, no shoulder cracks were observed in any of the 10 samples. On the other hand, in the spot-welded joints of the comparative example in which the second step was not performed, shoulder cracks as shown in FIG. 7 were observed in all 10 samples. [Explanation of symbols]

[0093] 1 Spot welded joints 21 (Top) Steel Plate 22 (Lower) Steel Plate 3. Nuggets 4 Cracks just outside the corona bond 5. Cracks directly below the electrode 6 Shoulder crack 7 Melting and solidification area 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 a galvanized steel plate, a first step of spot welding the plate assembly; A second step of forming a molten solidified portion that does not penetrate the galvanized steel sheet in the thickness direction in an area overlapping with a shoulder portion of the indentation formed on the surface of the galvanized steel sheet; A spot welding method comprising:

2. 2. The spot welding method according to claim 1, wherein the means for forming the molten solidified portion is laser irradiation.

3. The spot welding method according to claim 1, wherein the region where the molten solidification portion is formed is a ring-shaped region surrounding the flat portion of the indentation portion when viewed from above.

4. The spot welding method according to claim 1, wherein the region where the molten solidified portion is formed is the entire area of ​​the indentation portion when viewed from above.

5. The spot welding method according to claim 1 , wherein the molten solidified portion is formed on both the upper plate and the lower plate.

6. A spot welded joint made of two or more steel plates, at least one of which is a galvanized steel plate, A spot welded joint, characterized in that it has a molten solidified portion that does not penetrate the galvanized steel sheet in the thickness direction in a region overlapping with a shoulder portion of an indentation formed on the surface of the galvanized steel sheet.

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.

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