Spot welding method and spot welded joint

Abstract

To provide a spot welding method capable of more easily preventing an internal crack in spot welding including a galvanized steel plate, and a spot welded joint in which the internal crack is reduced by a new configuration.SOLUTION: The spot-welding method of the present invention includes a first step <S1> of spot-welding a sheet combination composed of two or more steel sheets including at least one galvanized steel sheet to form a nugget (3) on the overlapping surfaces of the two or more steel sheets, and a second step <S2> of forming a melted and solidified portion (7) so as to penetrate the two or more steel sheets in the sheet thickness direction and eliminate a corona bond outer end portion (81) located outside the nugget (3). Further, the spot welded joint of the present invention is a welded joint which has a melted and solidified portion (7) which is positioned at the outside of the nugget (3) and passes through the two or more steel sheets in the sheet thickness direction and in which the corona bond outer end (81) positioned at the outside of the nugget (3) is eliminated by the melted and solidified portion (7).SELECTED DRAWING: Figure 3

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 a metal 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] Such cracks are said to be caused by liquid metal embrittlement (LME) (hereinafter, sometimes referred to as "LME cracks"). Specifically, it is said that such cracks are caused by the penetration of molten zinc-based coating metal into the grain boundaries of the steel sheet due to the application of electrode pressure, tensile stress due to thermal expansion or contraction of the steel sheet, tensile stress due to disturbances, etc. to the weld, thereby reducing the grain boundary strength. Here, typical disturbances include, for example, the impact angle, which is the inclination of the electrode relative to the steel sheet surface, misalignment between the upper and lower electrodes, and initial positional misalignment between the steel sheet surface and the electrodes.

[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. The spot-welded joint 1 shown in Fig. 1(a) 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 and 22. In the spot-welded joint 1 shown in Fig. 1(a), the steel sheet 21 is a galvanized steel sheet.

[0007] 1(a), there is a crack 4 just outside the corona bond (pressure-welded portion) 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(a), there is a sub-electrode crack 5 that has developed from the outer surface of the steel sheet 21 at the flat portion PP of the indentation IP formed by the pressing of the pair of electrodes, and a shoulder crack 6 that has developed from the outer surface of the steel sheet 21 at the shoulder portion SP, which is a step formed at the point of contact with the shoulder portion of the electrode on the steel sheet 21. The sub-electrode crack 5 and shoulder crack 6 are examples of the above-mentioned outer crack.

[0009] The spot welded joint 1' shown in FIG. 1(b) is the same as the spot welded joint 1 shown in FIG. 1(a), except that the upper steel sheet 21' is a steel sheet other than a zinc-plated steel sheet, and the lower steel sheet 22' is a zinc-plated steel sheet. In the spot welded joint 1' shown in FIG. 1(b), a crack 4 has occurred just outside the corona bond in the upper non-galvanized steel sheet 21'. This crack was caused by the supply of zinc from the lower zinc-plated steel sheet to the upper sheet through the overlapping surface between the steel sheets. As described above, when a zinc-plated steel sheet is placed on either side of the overlapping surface, there is a risk of internal cracks occurring in at least one of the zinc-plated steel sheet and the non-galvanized steel sheet.

[0010] Because the presence of cracks as described above 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]

[0011] [Patent Document 1] International Publication No. 2016 / 159169 Summary of the Invention [Problem to be solved by the invention]

[0012] 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 further improvement in terms of simplicity.

[0013] The impact of cracks that occur in spot-welded joints on joint strength varies depending on their location. In particular, internal cracks occur just outside the corona bond (pressure weld) or inside the corona bond, and both are feared to have a significant impact on joint strength. In addition, internal cracks occur on the overlapping surface of the steel sheets, making them difficult to detect from the outside. Therefore, there is a demand for a simpler method to prevent such internal cracks.

[0014] The present invention has been made in view of the above circumstances, and aims to provide a spot welding method that can more easily prevent internal cracks in spot welding involving galvanized steel sheets. Another aim of the present invention is to provide a spot welded joint with reduced internal cracks through a novel configuration. [Means for solving the problem]

[0015] The present invention includes the following aspects.

[0016] (Aspect 1) A spot welding method for a plate assembly consisting of two or more steel plates, At least one steel plate constituting the plate assembly is a galvanized steel plate, a first step of spot welding the plate assembly to form a nugget on the overlapping surfaces of the two or more steel plates; a second step of forming a molten solidified portion that penetrates the two or more steel plates in the plate thickness direction and eliminates an outer end portion of the corona bond located outside the nugget; A spot welding method comprising:

[0017] (Aspect 2) The spot welding method according to the first aspect, wherein the means for forming the molten solidified portion is laser irradiation.

[0018] (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 nugget in a top view.

[0019] (Aspect 4) A spot welded joint made of two or more steel plates including galvanized steel plates, A nugget is formed on the overlapping surface of the two or more steel plates, A molten solidified portion is located outside the nugget and penetrates the two or more steel plates in the plate thickness direction, A spot welded joint, characterized in that the outer end of the corona bond located outside the nugget is lost due to the molten solidification portion.

[0020] (Aspect 5) A spot welded joint according to aspect 4, wherein the molten solidified portion is formed in a ring shape surrounding the nugget when viewed from above. [Effects of the Invention]

[0021] According to the spot welding method of the present invention, internal cracks in spot welds involving galvanized steel sheets can be more easily prevented, and the present invention can also provide a spot-welded joint with reduced internal cracks. [Brief explanation of the drawings]

[0022] [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 shows a first step of a spot welding method according to an embodiment of the present invention. <s1>FIG. 2 is a cross-sectional view illustrating the structure of the first embodiment. [Figure 3] FIG. 3 shows a second step of the spot welding method according to one embodiment of the present invention. <s2>FIG. 2 is a cross-sectional view illustrating the structure of the first embodiment. [Figure 4] FIG. 4 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 5] FIG. 5 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 6] FIG. 6 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

[0023] 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.

[0024] To achieve the above object, the present inventors conducted extensive research focusing on post-processing steps after spot welding. As a result, the present inventors discovered that in spot welding two or more steel sheets, including a galvanized steel sheet, by melting at least the outer end of the corona bond located outside the nugget after welding, it is possible to eliminate at least the initiation points of internal cracks that propagate from the outer end of the corona bond, thereby improving joint strength. Furthermore, the present inventors discovered that by melting the positions corresponding to the outer end of the corona bond so as to penetrate each steel sheet in the thickness direction, it is possible to more reliably and simply eliminate all or most of the internal cracks, thereby significantly improving joint strength.

[0025] The present invention has been completed based on these findings, and includes the following aspects of the spot welding method and spot-welded joint.

[0026] First, a preferred embodiment of the spot welding method of the present invention will be described in detail.

[0027] <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 steel sheets constituting the sheet assembly is a galvanized steel sheet.

[0028] Here, FIG. 2 shows the first step of the spot welding method of this embodiment. <s1>3 is a cross-sectional view illustrating the second step. <s2>2 and 3 are cross-sectional schematic diagrams for explaining the spot welding method for a plate assembly made up of two steel plates.

[0029] As shown in FIGS. 2 and 3, the spot welding method of this embodiment includes a first step <s1>As a result, spot welding is performed on a sheet pair consisting of a steel sheet 21 as an upper sheet and a steel sheet 22 as a lower sheet. In this embodiment, at least one of the steel sheets 21 and 22 is a galvanized steel sheet.

[0030] First step <s1>2, a nugget 3 made of molten metal is formed around the overlapping surface of the steel plate 21 and the steel plate 22. <s1>In this example, a corona bond 8, which is a pressure-welded portion between the steel sheets 21 and 22, is formed on the outside of the nugget 3, and a sheet separation SS, which is formed by the steel sheets 21 and 22 being separated from each other, is formed on the outside of the corona bond 8.

[0031] The spot welding method of this embodiment includes a first step <s1>After that, the second step <s2>As a result, a process of forming the molten solidified portion 7 is performed so that the molten solidified portion 7 penetrates the steel plates 21 and 22 in the plate thickness direction and the outer end portion 81 of the corona bond located outside the nugget 3 disappears.

[0032] (Penetrates two or more steel plates in the thickness direction) Here, in this specification, the molten solidified portion "penetrates in the thickness direction of two or more steel plates" means that when the spot welded joint is placed on a horizontal plane so that the thickness directions of the steel plates are vertical, the molten solidified portion extends from the upper surface of the steel plate located at the top in the thickness direction and reaches the lower surface of the steel plate located at the bottom (i.e., a completely penetrated state). In addition, the molten solidified portion extends from the upper surface of the steel plate located at the top in the thickness direction and reaches a depth of 3t / 4 or more of the steel plate located at the bottom (i.e., a substantially penetrated state). Note that t means the thickness of the steel plate.

[0033] (Corona bond outer end) In this specification, the term "corona bond outer end" refers to the end of the two corona bond ends formed outside the nugget in a cross-sectional view of the spot-welded joint that is farther from the nugget. In a spot-welded joint, the sheet separation is formed starting from the corona bond outer end, so the corona bond outer end and the sheet separation tip are essentially in the same position. The sheet separation tip refers to the end of the sheet separation, formed by separating the base steel sheets from each other, that is closer to the nugget.

[0034] (Melted and solidified part) Furthermore, in this specification, the term "molten solidified portion" refers to a portion that is locally melted and solidified in the steel plate that constitutes the spot welded joint. The molten solidified portion can be clearly identified as a portion in which a characteristic solidification structure called a dendrite or dendritic crystal is formed in the cross section of the spot welded joint. The method for identifying the molten solidified portion will be described later.

[0035] As shown in FIGS. 2 and 3, the spot welding method of this embodiment includes a first step <s1>Even if an internal crack 4 occurs in the second process, which has a large impact on joint strength and is difficult to detect from the outside, <s2>By forming the molten solidified portion 7 so that at least the corona bond outer end 81 located outside the nugget 3 disappears, it is possible to melt and eliminate at least the portion that serves as the starting point of the internal crack 4 that propagates from the corona bond outer end 81. Furthermore, the spot welding method of this embodiment forms the molten solidified portion 7 so that it penetrates the steel sheets 21 and 22 in the plate thickness direction at a position corresponding to the corona bond outer end 81, which more reliably and easily eliminates all or most of the internal cracks 4, resulting in excellent joint strength. In other words, the spot welding method of this embodiment more easily prevents internal cracks in spot welding involving galvanized steel sheets.

[0036] (internal crack) In this specification, the term "internal crack" refers to a crack that originates at the overlapping surface of each steel plate and is of a certain size or larger that has a significant effect on joint strength, i.e., a crack whose length from the start point to the end point exceeds 100 μm.

[0037] (steel plate) In this embodiment, of the two or more steel sheets used for spot welding, at least one of the uppermost upper sheet or the lowermost 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. For example, when three steel sheets are used, at least one of the upper sheet or the lower sheet is a galvanized steel sheet, and the intermediate sheet between the upper sheet and the lower sheet may be either a galvanized steel sheet or a non-galvanized steel sheet.

[0038] The type of zinc coating in the zinc-coated steel sheet is not particularly limited, and examples thereof include hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating.

[0039] Furthermore, as the steel sheet other than the galvanized steel sheet that can be used for the spot welding of this embodiment, any steel sheet can be adopted depending on the desired joint strength, corrosion resistance, etc. Note that examples of the steel sheet other than the galvanized steel sheet include a steel sheet that is plated with a plating other than a zinc-based plating, an unplated steel sheet, and a chemical conversion treated steel sheet.

[0040] In this embodiment, the tensile strength (TS) of at least one of the steel sheets used for spot welding, i.e., the galvanized steel sheet and the non-galvanized steel sheet that contacts the galvanized steel sheet via the overlapping surface, is preferably 780 MPa or higher. When such a high-strength galvanized steel sheet or non-galvanized steel sheet is used, the risk of LME cracking increases significantly. Therefore, the present invention is particularly advantageous when such a high-strength steel sheet is used. For example, in a sheet pair in which a galvanized steel sheet and a non-galvanized steel sheet are overlapped, if the tensile strength of the galvanized steel sheet is 780 MPa or higher and the tensile strength of the non-galvanized steel sheet is less than 780 MPa, there is a high risk of internal cracks occurring in the galvanized steel sheet. Furthermore, if the tensile strength of the galvanized steel sheet is less than 780 MPa and the tensile strength of the non-galvanized steel sheet is 780 MPa or higher, there is a high risk of internal cracks occurring in the non-galvanized steel sheet due to the supply of zinc from the galvanized steel sheet via the overlapping surface, as in the case of (b) in FIG. 1 described above. Furthermore, if the tensile strength of both the galvanized steel sheet and the non-galvanized steel sheet is 780 MPa or more, there is a high risk of internal cracks occurring in both steel sheets.

[0041] 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. The upper limit of the tensile strength of the steel sheet is not particularly limited, but is, for example, 3000 MPa or 2000 MPa from the viewpoint of workability, etc.

[0042] The tensile strength of a steel plate is determined by first measuring the Vickers hardness of the steel plate and then converting it. The Vickers hardness of a steel plate is measured using a method that conforms to JIS Z 2244:2009 "Vickers hardness test - Test method." When measuring the Vickers hardness of a steel plate, the measurement is carried out at a depth of 1t / 4 of the plate thickness t of the steel plate under a test load of 500g. From the Vickers hardness (HV) of the steel plate obtained in this way, the tensile strength (MPa) of the steel plate is calculated using the formula: tensile strength (MPa) = 3.3 x Vickers hardness (HV).

[0043] In this embodiment, the two or more steel plates used for spot welding may all be steel plates of the same strength, only some of the steel plates may be steel plates of the same strength, or all of the steel plates may be steel plates of different strengths.

[0044] 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) can be used depending on the application of the welded joint, etc. Furthermore, the thickness of each steel plate is also not particularly limited, and examples thereof include a thickness of 0.5 mm or more and 3.5 mm or less.

[0045] Hereinafter, each step in the spot welding method of this embodiment will be described in more detail with reference to the drawings.

[0046] [First step] As shown in FIG. 2, in the first step of this embodiment, <s1>is a process of spot welding a sheet pair consisting of a steel sheet 21 that will be the upper sheet and a steel sheet 22 that will be the lower sheet. In this embodiment, at least one of the steel sheets 21 and 22 is a galvanized steel sheet. 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, only the steel sheet 22 may be a galvanized steel sheet, or both the steel sheets 21 and 22 may be galvanized steel sheets.

[0047] First step <s1>First, a sheet set of overlapping steel sheets 21 and 22 is sandwiched between a pair of electrodes, an upper electrode 41 and a lower electrode 42. Then, while this pair of electrodes presses the sheet set in the sheet thickness direction, a current is passed in the sheet thickness direction at a predetermined value and for a predetermined time. As a result, the overlapping surfaces of 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 each of steel sheets 21 and 22 by the indentation of the upper electrode 41 and the lower electrode 42. The indentation IP has a flat portion formed at the location in contact with the top of the electrode and a shoulder portion which is a step portion formed at the location in contact with the shoulder of the electrode.

[0048] Furthermore, the first process <s1>In this example, a corona bond 8, which is the pressure-welded portion between the steel plates 21 and 22, is formed on the outside of the nugget 3, and a sheet separation SS is formed which extends outward from the outer end of the corona bond 8 (corona bond outer end 81) and separates the steel plates 21 and 22.

[0049] (Welding conditions) First step <s1>In this case, the 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.

[0050] 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.

[0051] For example, the first step <s1>The pair of electrodes, the upper electrode 41 and the lower electrode 42, used in this spot welding may be DR-type electrodes made of chromium copper and having an electrode 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). The current value during energization may be, for example, 4 kA to 12 kA. The energization time may be, for example, 12 cycles to 60 cycles. When the power supply frequency is 50 Hz, 1 cycle (1 cycle) is 1 / 50 seconds.

[0052] First step <s1>In the above, the number of times of energization during spot welding may be only one, or two or more times.

[0053] In addition, the first process <s1>In this case, as long as the effect of the present invention is not impaired, preliminary energization or post-energization may be performed before or after energization to form the nugget.

[0054] [Second process] As shown in FIG. 3, in the second step of this embodiment, <s2>The second step is a step of forming the molten solidified portion 7 so that the outer end 81 of the corona bond, which is located outside the nugget 3, disappears while penetrating the steel plate 21 and the steel plate 22 in the plate thickness direction. <s2>In this example, laser LB is irradiated onto the surface of steel plate 21, which serves as the upper plate, at a position corresponding to outer end 81 of the corona bond in top view, thereby melting steel plate 21 and steel plate 22 in the plate thickness direction while eliminating outer end 81 of the corona bond. As a result, molten solidification portion 7 is formed which penetrates steel plate 21 and steel plate 22 in the plate thickness direction.

[0055] By forming such a molten solidified portion 7, as described above, the first step <s1>Even if an internal crack 4 has occurred, at least the starting point of the internal crack 4 that propagates from the outer end 81 of the corona bond can be eliminated by melting, and the entire internal crack 4 or most of the internal crack 4 can be eliminated more reliably and easily.

[0056] In addition, the second process <s2>The depth in the plate thickness direction of the molten solidified portion 7 formed by (i.e., the depth of the above-mentioned complete penetration state or substantially penetration state) can be controlled by appropriately setting various conditions of the melting means. For example, when laser irradiation is used as the melting means, the depth of the molten solidified portion 7 can be controlled by appropriately setting the laser output and speed.

[0057] Second process <s2>The melting means used in the method is a means capable of forming the above-mentioned molten solidified portion 7, that is, a means capable of melting each steel plate so as to penetrate the plate thickness direction. Examples of such melting means include a laser irradiator, an arc welder, and a plasma welder. Among them, it is preferable to use laser irradiation by a laser irradiator. When the means for forming the molten solidified portion is laser irradiation, the above-mentioned molten solidified portion 7 can be formed more accurately and simply.

[0058] When the molten solidified portion is formed by laser irradiation, the laser irradiation conditions 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.

[0059] In addition, the second process <s2>In the spot-welded joint 1 shown in FIG. 4, the region where the molten solidified portion 7 is formed is not particularly limited as long as it includes a portion corresponding to the outer end 81 of the corona bond located outside the nugget 3. For example, the region where the molten solidified portion 7 is formed may be a ring-shaped region surrounding the nugget 3 in a top view. FIG. 4 is a schematic diagram of a cross-sectional view and a top view of a spot-welded joint 1 manufactured by the spot welding method of the present embodiment. In the spot-welded joint 1 shown in FIG. 4, the molten solidified portion 7 is formed in a ring-shaped region surrounding the nugget 3 in a top view. This ring-shaped region corresponds to a region on the surface of the upper steel plate 21 that surrounds the flat portion of the indentation IP in a top view.

[0060] If the formation area of ​​the molten solidified portion 7 is made into such a ring-shaped area, it is possible to melt it so as to cover all or almost all of the area where internal cracks are likely to occur, thereby more reliably and simply reducing the above-mentioned internal cracks.

[0061] Furthermore, the second process <s2>In the case of forming the molten solidified portion 7, the melting means such as laser irradiation may be applied from the steel plate 21 side, from the steel plate 22 side, or from both the steel plate 21 side and the steel plate 22 side. Since the molten solidified portion 7 is formed by penetrating in the plate thickness direction, for example, even if the melting means is applied from the steel plate 22 side where no internal cracks have occurred, the internal cracks generated in the steel plate 21 can be reliably eliminated.

[0062] Here, in this specification, "top view" means that when a spot-welded joint is placed on a horizontal plane so that the thickness direction of each steel plate is in the up-down direction, the object to be observed of the steel plate or welded joint (for example, an indentation portion, a molten and solidified portion, etc.) is viewed from above in the vertical direction.

[0063] The means for forming the molten solidified portion 7 into a ring shape when viewed from above is not particularly limited. For example, a melting means such as laser irradiation may be moved so as to draw a predetermined ring shape, and a predetermined portion of the welded joint may be melted into a ring shape at once using a melting means having a ring shape.

[0064] 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 appropriately set depending on the size of the internal cracks to be eliminated. For example, when 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 for the formation of a wider molten solidified portion 7.

[0065] In this embodiment, the region where the molten solidified portion 7 is formed is not limited to such a ring-shaped region. For example, if the location of internal cracks is known in advance based on experience or structure, the molten solidified portion 7 may be formed to correspond to the location of the internal cracks. For example, the molten solidified portion 7 may be formed in one or more dot-shaped shapes when viewed from above, or may be formed in one or more linear shapes.

[0066] Furthermore, as described above, the width of the molten solidified portion 7 in a cross-sectional view may be appropriately set depending on the size of the internal cracks to be eliminated. Therefore, as shown in FIGS. 3 and 4, the molten solidified portion 7 may be formed so that not only the outer end 81 of the corona bond but also the entire corona bond 8 is eliminated, or it may be formed so that only a portion of the corona bond 8, including the outer end 81, is eliminated. Since the molten solidified portion 7 is formed in a region that includes at least the outer end 81 of the corona bond, it is formed so as to overlap with a portion of the leading end of the sheet separation SS. In either case, by melting the outer end 81 of the corona bond, at least the portion that serves as the starting point of the internal cracks 4 that propagate from the outer end 81 of the corona bond can be eliminated.

[0067] (Other processes) In the spot welding method of this embodiment, any step that is performed in a typical spot welding method may be performed before or after each of the first and second steps, as long as the effect of the present invention is not impaired. Examples of such any step include a sheet assembly forming step, a cooling step, and various surface treatment steps.

[0068] 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.

[0069] <Spot welded joints> 4, which is one embodiment of the present invention, is configured from two steel plates, with at least one of the upper steel plate 21 and the lower steel plate 22 being a galvanized steel plate, as described above. Furthermore, the spot welded joint 1 of this embodiment has a nugget 3 formed on the overlapping surface of the steel plates 21 and 22, and a molten solidified portion 7 located outside the nugget 3 and penetrating the steel plates 21 and 22 in the plate thickness direction. Furthermore, in the spot welded joint 1 of this embodiment, the outer end 81 of the corona bond located outside the nugget 3 is eliminated by the molten solidified portion 7.

[0070] In the spot welded joint 1 shown in Figure 4, not only the outer end 81 of the corona bond but the entire corona bond 8 has disappeared, but as long as at least the outer end 81 of the corona bond has disappeared, a portion of the corona bond 8, i.e., a portion closer to the nugget 3, may remain.

[0071] In addition, the spot welded joint 1 of this embodiment is formed by the first step <s1>In this example, since the sheet separation SS is formed starting from the outer end 81 of the corona bond, the tip of the sheet separation is also lost due to the molten solidified portion 7. Therefore, in the spot welded joint 1 of this embodiment, as shown in FIG. 4, the steel plates 21 and 22 extend outward from the molten solidified portion 7 in a spaced apart state in a cross-sectional view.

[0072] (Method for identifying melted and solidified areas) The melted and solidified portion 7 exhibits a characteristic solidification structure called dendrite or dendritic crystal. Therefore, the melted and solidified portion 7 can be identified by specifying the range of the solidification structure according to the following procedure. First, the spot-welded joint is cut in the thickness direction through the center of the nugget when viewed from above to expose a cross section. This cross section is then embedded in a cold-setting resin. After embedding in the resin, the specimen observation surface 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. After this polishing, the specimen observation surface is corroded to the extent that the fusion boundary can be discerned. The corrosive solution used to corrode the specimen observation surface is, for example, picric acid. The specimen surface after corrosion is then photographed at a magnification of 25x or more using a magnification observation tool such as a microscope. In the photographed image, the solidification structure shows a characteristic pattern, making it possible to identify the area of ​​the solidification structure.

[0073] 3 and 4 , the spot welded joint 1 of this embodiment has a molten solidified portion 7 formed such that the corona bond outer end 81 disappears. Therefore, even if internal cracks 4 occur on the overlapping surfaces of the steel plates 21 and 22 during welding, at least the portion that serves as the starting point of the internal cracks 4 that propagate from the corona bond outer end 81 disappears. Furthermore, the spot welded joint 1 of this embodiment has a molten solidified portion 7 formed at a position corresponding to the corona bond outer end 81 so as to penetrate the steel plates 21 and 22 in the plate thickness direction, and all or most of the internal cracks 4 disappear, thereby providing excellent joint strength. In other words, the spot welded joint 1 of this embodiment is a spot welded joint with reduced internal cracks 4 and is able to provide excellent joint strength.

[0074] In the spot welded joint 1 of this embodiment, even if the internal cracks 4 are not completely eliminated by the molten solidified portion 7, as described above, at least the portion that serves as the starting point of the internal cracks 4 that propagate from the outer corona bond end portion 81 has been eliminated, making it difficult for the cracks to propagate and providing excellent joint strength. However, in the spot welded joint 1 of this embodiment, it is desirable that the internal cracks 4 be completely eliminated by the molten solidified portion 7 in order to more reliably obtain even better joint strength.

[0075] 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. Note that the steel sheets that make up the spot welded joint 1 are as described in the spot welding method above, and therefore a detailed description thereof will be omitted.

[0076] Furthermore, in the spot welded joint 1 of this embodiment, the molten solidified portion 7 is formed in a ring shape surrounding the nugget 3 when viewed from above, as shown in FIG. Forming the molten solidified portion 7 in such a ring shape allows it to melt so as to cover all or almost all of the area where internal cracks 4 are likely to occur, thereby more reliably and simply reducing the above-mentioned internal cracks 4. Furthermore, since the ring-shaped molten solidified portion 7 can be visually confirmed from the surface of at least one of the steel plates 21 and 22, there is also the advantage that it can be determined from its appearance that the spot welded joint 1 is a spot welded joint in which internal cracks 4 are reduced.

[0077] A spot welded joint having a ring-shaped molten solidified portion 7 is cut at a position passing through the center of the ring-shaped molten solidified portion 7 when viewed from above, and when the cut surface is observed, if the molten solidified portion 7 is formed in at least one cross section, the effect of the present invention can be exerted.

[0078] The means and form of forming the molten solidified portion 7 are the same as those explained in the spot welding method above, and therefore a detailed explanation will be omitted.

[0079] (Application example) As described above, the spot welding method of the present invention can more easily prevent internal cracks in spot welds involving galvanized steel sheets, and can produce spot welded joints with reduced internal cracks, making it applicable to a variety of structural parts that require excellent joint strength, such as automobiles and other transportation machinery, industrial machinery, and buildings. In particular, the present invention can be particularly well-suited for use in the manufacture of automobile bodies and parts.

[0080] 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 between items to which the ordinal numbers are assigned, and do not indicate the order, priority, importance, etc. of each item. [Example]

[0081] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0082] (Fabrication of spot welded joints) To verify the effects of the present invention, spot welding was performed (first step) 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. 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. Current application was performed at a current value of 7.5 kA for a current application time of 20 cycles. The time from the end of current application to the release of the electrodes, i.e., the holding time, was set to 5 cycles.

[0083] Next, a laser was irradiated onto the surface of the upper sheet of the two spot-welded galvanized steel sheets at a position corresponding to the outer edge of the corona bond in a top view. This melted the two galvanized steel sheets in the thickness direction while eliminating at least the outer edge of the corona bond. In this way, a molten solidification zone 7 penetrating the two galvanized steel sheets in the thickness direction was formed (second step). A fiber laser was used as the laser source, and irradiation was performed using a remote laser method. Laser irradiation was performed on a ring-shaped area surrounding the nugget in a top view, as shown in Figure 4. Specifically, ring-shaped irradiation was performed along a trajectory with a diameter of 8.5 mm. The laser output was set to 3 kW, and the focal position was set 5 mm away from the steel sheet surface. The moving speed of the laser irradiation area was set to 1.5 m / min.

[0084] In this manner, a spot-welded joint according to an example of the present invention was produced.

[0085] Furthermore, spot-welded joints of comparative examples were produced in the same manner as the above-described inventive examples, except that the second step was not carried out by a conventional spot welding method.

[0086] 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 internal cracks was examined according to the following procedure.

[0087] 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 internal cracks. The number of samples out of 10 in which internal cracks were observed was recorded. The criteria for determining the presence or absence of internal cracks were as follows: a crack length connecting the start and end points of the internal crack was determined to be "cracked" if it exceeded 100 μm, and a crack length of 100 μm or less was determined to be "crack-free."

[0088] The observation results are shown in the following Table 1. An enlarged cross-sectional photograph of the spot-welded joint of the example of the present invention taken during this observation is shown in Fig. 5, and an enlarged cross-sectional photograph of the spot-welded joint of the comparative example is shown in Fig. 6.

[0089] [Table 1]

[0090] As shown in Table 1 and FIG. 5, in the spot-welded joints of the present invention, no internal cracks were observed immediately outside the corona bond in any of the 10 samples.

[0091] On the other hand, in the spot-welded joints of the comparative example in which the second step was not performed, internal cracks were observed immediately outside the corona bond as shown in FIG. 6 in all 10 samples. [Explanation of symbols]

[0092] 1 Spot welded joints 21 (Top) Steel Plate 22 (Lower) Steel Plate 3. Nuggets 4 Internal cracks 5. Cracks directly below the electrode 6 Shoulder crack 7 Melting and solidification area 8 Coronabond 81 Corona bond outer edge IP impression area LB laser SP shoulder SS seat separation

Claims

1. A spot welding method for a plate assembly consisting of two or more steel plates, At least one steel plate constituting the plate assembly is a galvanized steel plate, a first step of spot welding the plate assembly to form a nugget on the overlapping surfaces of the two or more steel plates; a second step of forming a molten solidification portion that penetrates the two or more steel plates in a plate thickness direction and eliminates an outer end portion of the corona bond located outside the nugget; 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 or 2, wherein the region where the molten solidification portion is formed is a ring-shaped region surrounding the nugget in a top view.

4. A spot welded joint made of two or more steel plates including a galvanized steel plate, A nugget is provided on the overlapping surfaces of the two or more steel plates, A molten solidified portion is located outside the nugget and penetrates the two or more steel plates in the plate thickness direction, A spot welded joint, characterized in that the outer end of the corona bond located outside the nugget is lost due to the molten solidification portion.

5. The spot welded joint according to claim 4, wherein the molten solidified portion is formed in a ring shape surrounding the nugget when viewed from above.

Images