Method for spot welding metal plates having insulating coating and spot welded joint of metal plates having insulating coating

Abstract

To provide: a method for spot welding metal plates each having an insulating coating, capable of welding even a laminated portion in which three or more metal plates each having an insulating coating are stacked together; and a spot welded joint.SOLUTION: The present invention provides: a method for spot welding metal plates each having an insulating coating, which performs spot welding on a laminated portion formed by stacking three or more metal plates, each having an insulating coating on at least one surface, such that the insulating coating is disposed on an outer surface of at least one of two front and back outermost metal plates; and a spot-welded joint. The insulating coating on the metal sheet of the present invention is an insulating coating that provides an inter-electrode resistance of 150 mmΩ to 1000 mmΩ, inclusive, measured when the metal plate is clamped with a pressure of 2 kN between two electrodes having a tip curvature radius of 40 mm. In the present invention, spot welding is performed with a conductive through-hole that penetrates the laminated portion at a planned welding position for the spot welding to be performed on the laminated portion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spot welding method for metal plates having an insulating coating and a spot welded joint for metal plates having an insulating coating. [Background technology]

[0002] Insulating coatings such as paint films are widely used in metal structures to improve insulation, corrosion resistance, and appearance. When manufacturing metal structures using metal plates, resistance welding is often used as a joining method required for assembly. Examples of resistance welding include lap spot welding.

[0003] However, in metal structures, the cost of the post-process of applying a coating to the surface of stacked metal plates increases the price compared to assembling metal plates that already have a coating applied. Therefore, assembling structures using metal plates that have already been coated is desirable. Furthermore, most coatings are insulating. In such cases, joining methods using secondary materials such as rivets, bolts, nuts, or screws are often used because the insulating coating prevents current flow and resistance welding. However, joining methods using secondary materials have drawbacks, such as an increase in the weight of the joined structure and the secondary materials forming protrusions on the structure surface. Furthermore, the cost of the secondary materials increases the price of the structure. Therefore, methods have been developed for resistance welding even with metal bodies that have an insulating coating applied.

[0004] For example, Patent Document 1 proposes a welding method in which series spot welding is performed from one side of the overlapping portion of a steel sheet 41 having paint coatings 41a, 41b on both sides and a steel sheet 42 having paint coatings 42a, 42b on both sides, as shown in Figure 7. The series spot welding described in this document is specifically performed as follows. First, the overlapping portions of the coated steel sheets 41 and 42 are placed on the back bar 43, and then an uncoated steel sheet (mild steel sheet) 45 is placed on the electrode pressure surface. Then, electrodes 44a and 44b are brought into contact with each other, sandwiching the coated steel sheets 41 and 42 and the mild steel sheet 45 between the back bar 43 and the electrodes 44a and 44b, and applying pressure to them. When a welding current is applied to the electrodes 44a and 44b, the welding current first forms a welding current loop A1 and flows through the mild steel sheet 45, gradually heating it. As a result, the adjacent coating film 41a is destroyed. Once the destruction of paint coating 41a is complete, mild steel sheet 45 and steel sheet 41 are in a conductive state, so that the welding current passes through mild steel sheet 45 to reach steel sheet 41, forming welding current loop A2 at steel sheet 41. Welding current loop A2 then begins to heat steel sheet 41, and when the heating temperature reaches a predetermined temperature, paint coating 41b and then paint coating 42a on steel sheet 42 are successively heated and destroyed, establishing electrical continuity between steel sheets 41 and 42. As a result, welding current loop A3 is formed at steel sheet 42, and paint coating 42b is also destroyed in the same manner, ultimately forming welding current loop A4 between steel sheet 42 and back bar 43. The welding current passes through steel sheet 41 and reaches steel sheet 42, forming a welding current loop A3. This allows a weld nugget to be formed between steel sheets 41 and 42 directly below the electrodes. Therefore, the primary role of back bar 43 is to support the pressure applied to electrodes 44a and 44b and bring steel sheets 45, 41, and 42 into close contact. For this reason, the back bar does not necessarily have to be a good conductor. Furthermore, if the structure of the overlapping portion can support the pressure applied to electrodes 44a and 44b, back bar 43 is not necessarily required. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 56-151183 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the invention described in Patent Document 1 has the disadvantage that even the unpainted steel sheets 45 overlapping the steel sheets are welded. Furthermore, the series spot welding described in Patent Document 1 is a welding method in principle in which a current circuit is formed within the steel sheets. When attempting to weld three or more painted steel sheets, the Joule heat generated in this current circuit breaks down the paint coating while expanding the current path to the opposite side. Therefore, as the number of overlapping steel sheets increases, it becomes difficult to match the heat input between the steel sheet on the electrode side and the steel sheet positioned away from the electrode to achieve a uniform weld nugget diameter, which can easily lead to the problem of not being able to weld all of the painted steel sheets.

[0007] An object of the present invention is to provide a spot welding method for metal plates with an insulating coating, which is capable of welding stacked sections in which three or more metal plates with insulating coatings are stacked together, and a spot-welded joint for metal plates with an insulating coating. [Means for solving the problem]

[0008] [1] A spot welding method for metal plates having an insulating coating, comprising: stacking three or more metal plates having an insulating coating on at least one side thereof, the metal plates being the outermost two metal plates, such that the insulating coating is disposed on the outer side of at least one of the metal plates, the outermost two metal plates being the front and rear metal plates; sandwiching the stack between two electrodes from the stacking direction of the metal plates; and spot welding the stack by passing a current between the electrodes while applying pressure; the insulating coating of the metal plate is an insulating coating having an inter-electrode resistance value of 150 mmΩ or more and 1000 mmΩ or less when measured when one metal plate is sandwiched between the two electrodes having a tip curvature radius of 40 mm with a pressure of 2 kN, A spot welding method for metal plates having an insulating coating, comprising providing a conductive through hole that penetrates the laminated portion at a position where the spot welding is to be performed on the laminated portion, and then performing the spot welding.

[0009] [2] A spot welding method for metal plates having an insulating coating as described in [1], wherein when the healthy part of the laminate is clamped between the two electrodes with a predetermined pressure, the contact area between the outer surface of the laminate and the electrodes is defined as an initial contact area, and at least a portion of the opening of the through hole is located within the initial contact area.

[0010] [3] The spot welding method for metal plates having an insulating coating described in [1] or [2], wherein the diameter of the opening of the through hole is 0.1 mm or more and less than the initial contact diameter, where the diameter of the contact surface between the outer surface of the laminate and the electrodes when the healthy part of the laminate is clamped between the two electrodes with a predetermined pressure.

[0011] [4] The spot welding method for metal plates having an insulating coating according to any one of [1] to [3], wherein the electrode has a tip with a radius of curvature of 15 mm or more and 100 mm or less.

[0012] [5] The spot welding method for metal plates having an insulating coating according to any one of [1] to [4], wherein the through holes are formed in a state where the metal plates are stacked.

[0013] [6] A spot welded joint of metal plates with insulating coatings, in which three or more metal plates with insulating coatings on at least one side are stacked together, with the insulating coating being disposed on the outside of at least one of the two outermost metal plates, the front and back, and a weld nugget is provided in the stacking section, connecting the metal plates in the stacking direction, The insulating coating of the metal plate is an insulating coating having an inter-electrode resistance value of 150 mmΩ or more and 1000 mmΩ or less when the metal plate is sandwiched between two electrodes of a spot welding machine, each having a tip curvature radius of 40 mm, with a pressure of 2 kN, A spot welded joint of metal plates having an insulating coating, which has one end and the other end connected to the outer surface of the outermost metal plate and the fusion boundary of the weld nugget formed on the inner side of the outermost metal plate in the stacking direction, and which leaves traces of a through hole. [Effects of the Invention]

[0014] According to the present invention, even in resistance welding of a laminated portion of metal plates having an insulating coating, if a conductive through hole is drilled in the laminated portion and the inner surface of the through hole serves as a current path in the penetration direction, thereby enabling direct contact between the electrode tip and the through hole, a current-carrying circuit can be easily formed. Furthermore, according to the present invention, even if the welding point position accuracy is low and the electrode tip does not come into direct contact with the through hole, by setting the inter-electrode resistance of the insulating coating within a predetermined range, current can be ensured from the electrode to the outermost metal plate via the insulating coating, and a current-carrying circuit can be formed via the outermost metal plate and the through hole. Furthermore, according to the present invention, the problem of difficulty in forming a weld nugget between adjacent metal sheets in lap spot welding, typically due to the outermost metal sheet being cooled by a water-cooled electrode, can be easily solved by combining through holes provided in the stacking direction of the metal sheets. That is, as long as sufficient current flow in the stacking direction is ensured by the through holes, a spot welded joint with generally uniform weld nuggets can be easily formed, even in multilayer stacks of metal sheets with insulating coatings, or on the outer layer side where cooling from the electrode may cause weld nugget growth. Here, "generally uniform" means that the diameter of the weld nugget between the outermost metal sheet and the adjacent metal sheet is at least 60% of the diameter of the weld nugget formed in the center of the stack. As described above, the present invention provides a spot welding method for metal plates with insulating coatings, which allows for welding of stacked sections of three or more sheets with through holes while maintaining the inter-electrode resistance value of the insulating coating within a predetermined range, and a spot-welded joint for metal plates with insulating coatings. [Brief explanation of the drawings]

[0015] [Figure 1] 1A to 1C are diagrams illustrating, in partial cross-sectional views, a method for spot welding metal plates having insulating coatings according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram showing a schematic partial cross section of only the upper electrode and the workpiece, including disturbance factors of electrode tilt, illustrating the difference in the influence on the contact state with the surface of the workpiece when the tip shape of the electrode in an embodiment of the present invention is flat and when it has a curvature. [Figure 3A]FIG. 1 is a diagram illustrating the results of a preliminary test conducted before the present invention was made to confirm the effect of forming an electric path by through holes that penetrate through a laminated portion of metal plates having an insulating coating. [Figure 3B] FIG. 3B is a diagram showing an example of a cross-sectional photograph of the spot weld obtained in the preliminary test of FIG. 3A. [Figure 4A] FIG. 1 is a diagram illustrating the results of a preliminary test conducted before the present invention was developed to confirm the conditions for forming a weld nugget after forming an electric path by a through hole that penetrates through a laminated portion of metal plates having insulating coatings. [Figure 4B] FIG. 4B is a diagram showing an example of a cross-sectional photograph of the spot weld obtained in the preliminary test of FIG. 4A. [Figure 4C] FIG. 4C is a view showing a surface photograph of the spot weld in FIG. 4B. [Figure 5A] FIG. 10 is a diagram showing an example of a cross-sectional photograph of a spot weld in an example in which the influence of the size of a through hole according to an embodiment of the present invention was investigated. [Figure 5B] FIG. 10 is a diagram showing another example of a cross-sectional photograph of a spot weld in an example in which the influence of the size of a through hole according to an embodiment of the present invention was investigated. [Figure 5C] FIG. 10 is a diagram showing yet another example of a cross-sectional photograph of a spot weld in an example in which the effect of the size of the through hole according to an embodiment of the present invention was investigated. [Figure 6A] FIG. 10 is a diagram showing an example of a cross-sectional photograph of a spot weld in an example in which the influence of the relative position on the indentation of a through hole according to an embodiment of the present invention was investigated. [Figure 6B] 10A and 10B are diagrams showing other examples of cross-sectional photographs of spot welds in examples in which the influence of the relative position on the indentation of a through hole according to an embodiment of the present invention was investigated. [Figure 6C] FIG. 10 is a diagram showing yet another example of a cross-sectional photograph of a spot weld in an example in which the influence of the relative position on the indentation of a through hole according to an embodiment of the present invention was investigated. [Figure 7] FIG. 1 is a diagram showing a partial cross-sectional view of a welded state of a coated steel plate in series spot welding according to the prior art. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the spot welding method for metal sheets having an insulating coating and the spot welded joint of metal sheets having an insulating coating of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0017] In a spot welding method for metal sheets having an insulating coating according to an embodiment of the present invention, as shown in Fig. 1, spot welding is performed on a stack 10 in which metal sheets 1, 2, 3, ..., n, each having an insulating coating on at least one side, are stacked in a number n ≥ 3. In this stack of metal sheets, the insulating coating is disposed on the outside of at least one of the two outermost metal sheets, the front and back.

[0018] In this embodiment, the insulating coating on the metal plate is an insulating coating that provides an inter-electrode resistance of 150 mmΩ or more and 1000 mmΩ or less when one metal plate is sandwiched between two electrodes with a tip curvature radius of 40 mm and a pressure of 2 kN.

[0019] In this embodiment, a conductive through hole 12 is provided at the intended welding position for spot welding on the laminated portion 10, and the laminated portion 10 is sandwiched between two electrodes from the stacking direction of the metal plate, and spot welding is performed by applying pressure and passing current between the electrodes. In the following description, the "metal plate having an insulating coating according to an embodiment of the present invention" may be simply referred to as the "metal plate according to this embodiment," and the "spot welding method for a metal plate having an insulating coating according to an embodiment of the present invention" may be simply referred to as the "spot welding method according to this embodiment."

[0020] A spot-welded joint of metal plates with an insulating coating according to an embodiment of the present invention includes three or more stacked metal plates 1, 2, 3, ..., n, each having an insulating coating on at least one side thereof. The insulating coating is disposed on the outer surface of at least one of the two outermost metal plates (the front and rear metal plates). The stacked metal plates have a weld nugget that connects the metal plates in the stacking direction. Furthermore, in this embodiment, a trace of a through hole 12 is left between the outer surface of the outermost metal plate and the fusion boundary of the weld nugget formed on the inner side of the outermost metal plate in the stacking direction, each having one end and the other end, connecting the two (see Figures 4B and 4C). The trace of the through hole 12 also remains as a depression extending in the thickness direction in the surface layer of the spot-welded joint. These traces indicate that the spot-welded joint of the metal sheets having the insulating coating according to this embodiment was produced by welding using the spot welding method according to this embodiment. Therefore, the spot-welded joint of the metal sheets having the insulating coating according to this embodiment will be described together with the spot welding method according to this embodiment. Here, the fusion boundary of the weld nugget is further explained. This boundary defines the boundary between the melted and solidified portion of the weld nugget and the unmelted portion of the base metal. Generally, in welding, the heat source is localized, and the position of the fusion boundary is determined by the balance between the strength of the heat source and heat loss due to heat conduction to the surrounding area. Furthermore, the fusion boundary is perpendicular to the direction of heat flow. In spot welding, heat transfer to the water-cooled electrode is also a major factor in heat loss. This is why spot welding does not melt the surface of the metal sheet on the electrode side. Furthermore, considering that the fusion boundary of the weld nugget formed in the metal sheet on the electrode side is perpendicular to the direction of heat flow toward the water-cooled electrode, it can be seen that it is approximately parallel to the surface of the metal sheet.

[0021] (Metal plates and number of layers) Typical examples of metal sheets according to this embodiment include ordinary steel sheets ranging from mild steel sheets to high-strength steel sheets, but are not limited to these. Stainless steel sheets, electromagnetic steel sheets, nickel sheets, titanium sheets, and other sheets that may be spot-welded are also applicable as long as they have an insulating coating formed on at least one side. The thickness of the metal sheet can be selected depending on the type of metal sheet, and is not particularly limited, but may be, for example, 0.1 mm or more and 2.0 mm or less.

[0022] The reason why the number n of metal sheets stacked in the laminate 10 in the spot welding method according to the present embodiment is set to three or more is that up to two metal sheets having an insulating coating can be welded using conventional techniques such as those exemplified in the background art without applying the spot welding method according to the present embodiment. In the present embodiment, there is no particular upper limit on the number n of stacked sheets. However, if it is necessary to drill through holes 12 in the laminate 10 in a stacked state, the number n of stacked sheets may be limited depending on the drilling method, such as the length of the drill bit in the case of drilling. Furthermore, the number n of stacked sheets may also be limited depending on the magnitude of the voltage drop between each metal sheet constituting the laminate 10 relative to the power supply capacity of the spot welding machine.

[0023] (Formation of through holes) There is no particular limitation on the means for drilling the through holes 12. For example, mechanical processing such as drilling, laser piercing, electrical discharge machining, etc. may be used. Among these, drilling is preferred because it is highly versatile and can be achieved inexpensively.

[0024] Considering the contribution of the through hole 12 to the formation of a current path during the initial welding process, as described in detail below, it is preferable to form the through hole 12 in a stacked state of the metal plates 1, 2, 3, ..., n, i.e., in the state of the laminated portion 10. This reliably eliminates the possibility of misalignment between the plate-side ends of the inner circumferential surfaces of the through holes 12 of the upper and lower metal plates, or the trapping of cutting chips between the metal plates. When applying the present invention to press-formed components, drilling the through holes 12 for each component during press forming can achieve the same effect as drilling the through holes 12 in the laminated portion 10. This is because the positions of the through holes 12 in the press-formed components are determined within the die set, and the positions of the through holes 12 in the laminated portion 10 during welding are determined by aligning the component end faces. In this case, the through holes in each metal plate do not need to be perfectly coaxial. It is sufficient that the openings of adjacent through holes between the metal plates are positioned closely together. Specifically, welding is possible if a portion of the opening is within the initial contact area. The number of through holes to be formed may be one at each planned welding position.

[0025] (Conductive effect of through holes) The metal plates 1, 2, 3, ..., n according to this embodiment have an insulating coating on at least one surface of the plate, and spot welding is not possible with a laminated portion in which such metal plates are simply stacked. Therefore, in this embodiment, a conductive through hole 12 is provided through the laminated portion 10 at a predetermined welding position in the laminated portion 10 of the metal plates. When a voltage is applied while pressure is applied to the predetermined welding position by the upper and lower electrodes 20 during spot welding, the through hole 12 contributes to short-circuiting between the metal plates at the inner circumferential surface of the hole where the metal surfaces of the through hole 12 of the upper and lower metal plates are exposed.

[0026] If the through-hole 12 is formed by stacking metal plates, it is likely that burrs and other cutting debris from drilling the through-hole fill the gap between the metal plates around the hole, causing a short circuit. Even if the through-holes in the laminate are punched out using a press or other method and aligned with each other, as long as the metal plates are pressed together at the hole position due to the electrode pressure applied by the upper and lower electrodes 20, it is likely that a partial short circuit will occur between the inner surfaces of the through-holes 12 of adjacent metal plates. It is not necessary to perfectly align the holes in the punched metal plates when stacking them. When a healthy portion of the laminate is sandwiched between two electrodes with a predetermined pressure, the contact area between the outer surface of the laminate and the electrodes is defined as the initial contact area. It is sufficient for the holes in each metal plate to be located within this initial contact area. In either case, when current begins to flow, the short-circuited area generates Joule heat, burning the surrounding insulating coating, expanding the current path, and rapidly reducing the resistance between the electrodes (see Figure 3A).

[0027] As described above, when the electrode tip and the through hole are well aligned as shown in Figure 1, the inner surface of the through hole in the laminate becomes the current path in the penetration direction, and it is clear that a welding current circuit can be easily formed even if an insulating coating is arranged on the outside of the outermost metal plate in the laminate.

[0028] (insulating coating) The accuracy of the welding point position in general spot welding is not necessarily high, and there is a high probability that the welding point position will deviate from the opening of the above-mentioned through hole. Therefore, when spot welding metal plates with insulating coatings, a stable welding current circuit cannot be formed using only the conductive through hole. Therefore, the insulating coating of the metal plate in this embodiment is an insulating coating that has an inter-electrode resistance of 150 mmΩ or more and 1000 mmΩ or less when measured when one metal plate is sandwiched between two electrodes with a tip curvature radius of 40 mm and a pressure force of 2 kN. Such insulating coatings typically include thin-film coatings for metal sheets, such as clear coatings for stainless steel sheets and insulating coatings for electromagnetic steel sheets, but are not limited to these.

[0029] The interelectrode resistance value was measured by clamping a metal plate having an insulating coating between electrodes with a tip curvature radius of 40 mm, applying a pressure of 2 kN while passing a current of 1 A using a low-current power supply different from the welding power supply, and measuring the interelectrode resistance value. This interelectrode resistance value depends on the pressure, and tends to decrease as the pressure increases. Therefore, in this embodiment, for convenience, the interelectrode resistance value at a pressure of 2 kN is measured multiple times at one location, taking into account variations in the thickness of the insulating coating, and similar measurements are also made at 10 locations, and the average of these measurements is used to define the resistance value. When the interelectrode resistance value of a cold-rolled steel sheet SPCC (JIS G 3141) having a thickness of 1.6 mm was measured by the measurement method used here, it was approximately 1 mΩ.

[0030] The reason why the inter-electrode resistance of the insulating coating is set to 150 mmΩ or more is that if the inter-electrode resistance is less than this, there is not much difference between the welding of metal plates without an insulating coating, and a weld nugget will be formed on the center axis of the electrode regardless of the position of the conductive through-hole, etc. In this embodiment, good welding is possible even if the inter-electrode resistance is, for example, 300 mmΩ or more, or 500 mmΩ or more. On the other hand, the upper limit of the inter-electrode resistance value of the insulating coating is set to 1000 mmΩ or less because, if this limit is exceeded, if the electrode contact point position deviates from the opening of the through-hole, a no-current error will occur and the welding sequence will stop, or conversely, insulation will break down, causing the metal plate surface to melt and become unsuitable for welding.

[0031] By setting the inter-electrode resistance of the insulating coating within this range, even if the accuracy of the electrode position is low and there is no direct contact between the electrode tip and the through-hole, it is possible to ensure electrical conduction from the electrode to the outermost metal plate via the insulating coating, and to form an electrical circuit via the outermost metal plate and the through-hole. Note that if neither of the two outermost metal plates has an insulating coating on the outside, electrical conduction from the electrode to the outermost metal plate is easily ensured even if the accuracy of the electrode position is low and there is no direct contact between the electrode tip and the through-hole, and therefore this is not included in the scope of the present invention.

[0032] (Initial contact diameter and indentation diameter) In spot welding, a laminated portion is sandwiched between two electrodes in the lamination direction of the metal sheets, and welding is performed by applying pressure between the electrodes and passing an electric current. Generally, after welding, an indentation is formed on the outer surface of the laminated portion as a pressed mark of the electrode. In this embodiment, the size of this indentation may be referred to as the indentation diameter, which is the diameter in the plane direction of the outer surface of the laminated portion. Furthermore, in this embodiment, the diameter of the contact portion (contact surface) between the outer surface of the laminated portion and the electrode at the initial stage of the start of spot welding may be referred to as the initial contact diameter, in contrast to the indentation diameter that can be confirmed after spot welding. The indentation is formed when the metal sheets (laminated portion) soften and the electrode is pressed into the metal sheets (laminated portion), and the electrode is also pressed around the initial contact diameter. Therefore, the indentation diameter is always larger than the initial contact diameter. In this embodiment, the initial contact diameter can be measured using carbon paper or pressure-sensitive paper for copying. Specifically, for example, pressure-sensitive paper is sandwiched between the outer surface of a laminated portion (also referred to as a healthy laminated portion) without through holes and an electrode, and a pressure (predetermined pressure) to be used as a welding condition is applied to the electrode. The initial contact diameter can be measured as the diameter of the colored portion that appears on the pressure-sensitive paper. There are several types of pressure-sensitive paper, but one with a maximum measurable pressure of 50 MPa can be used, for example.

[0033] (Through-hole size) The opening of the through hole 12 used in the spot welding method according to this embodiment preferably has a diameter of 0.1 mm or more but less than the initial contact diameter. From the standpoint of work efficiency, it is preferable that the diameters of the through holes formed in each metal plate constituting the laminated portion are the same. This is because through holes 12 with a diameter of less than 0.1 mm are difficult to process industrially, and even if they can be processed, expensive equipment is required. When drilling through holes 12, the diameter of the through holes 12 is preferably 0.8 mm or more. This is because commercially available and readily available drills generally have a diameter of 0.8 mm or more, and drills thinner than this, even if available, are too thin to be practical in terms of durability and workability. On the other hand, the upper limit of the diameter of the through hole 12 is preferably less than the initial contact diameter because if the diameter is greater than or equal to the initial contact diameter, the electrode tip will fall into the through hole 12, potentially preventing the formation of a weld nugget on the upper and lower surfaces of the laminate 10. Furthermore, the upper limit of the diameter of the through hole 12 is preferably 2.0 mm or less to prevent the cavity in the weld nugget from becoming too large, resulting in an excessively large indentation (see FIG. 5C ). To keep the cavity remaining in the weld nugget relatively small, the upper limit of the diameter of the through hole 12 is more preferably 1.4 mm or less. The diameter of the opening of the through hole may be 60% or less, or 40% or less, of the initial contact diameter. It is preferable to prevent the electrode from penetrating into the through hole, and the diameter of the opening of the through hole may be smaller than the outer diameter of the electrode. Furthermore, in the spot-welded joint according to this embodiment, the diameter of the trace that can be identified as the opening of the through hole before welding is preferably 0.1 mm or more and less than the diameter of the indentation remaining on the outer surface of the weld.

[0034] (Preferable opening position of the through hole) The contact area between the outer surface of the laminate and the electrode at the beginning of spot welding is sometimes referred to as the initial contact area. In the spot welding method according to this embodiment, the opening of the through hole 12 is preferably located within the initial contact area between the outer surface of the laminate 10 and the electrode 20. This ensures that the opening of the through hole 12 is completely blocked by the tip of the spot welding electrode 20, which is advantageous not only for ensuring a current path at the beginning of welding but also for preventing the ejection of molten metal during welding. In this embodiment, the insulating coating has an interelectrode resistance of 150 mmΩ to 1000 mmΩ measured when a metal sheet is clamped between the two electrodes with a pressure of 2 kN. This allows welding even if a portion of the opening of the through hole 12 is outside the initial contact area. In this case, the current flowing through the insulating coating directly below the electrode and into the metal sheet reaches the inner wall surface of the through hole 12. As long as the metal sheets are in close contact at the through hole 12, the current path required for spot welding is formed between the electrodes. As a result, a weld nugget can be formed depending on the magnitude of the welding current. In the spot-welded joint according to this embodiment, the opening of the through hole after welding is preferably located within the region of the electrode impression remaining on the outer surface of the welded portion.

[0035] (Traces of weld nuggets and through holes) In this embodiment, when a laminated metal plate having a through hole at the intended welding position is sandwiched between electrodes and welding current begins, the current concentrates on the inner wall of the through hole, which has the lowest electrical resistance, and the inner wall begins to melt. This molten portion destroys the insulating coating near the inner wall of the through hole and spreads in the circumferential direction where the inner diameter of the through hole expands, thereby widening the current path. As a result, the molten portion continues to expand due to resistance heating, except near the surface layer of the laminated metal plate, as described below. This molten portion becomes a weld nugget when welding is completed. In contrast, in this embodiment, the surface of the metal plate on the electrode side of the laminate is strongly cooled by the water-cooled electrode and therefore does not melt. Furthermore, near the metal plate surface, heat transfer to the electrode prevails over resistance heating, preventing the molten zone from spreading from the inner wall of the through hole to the surrounding area. The depth to which the inner wall of the through hole, which does not expand in diameter, remains from the outer surface of the laminate depends on the balance between the amount of resistance heat generated, which is determined by the welding current, and the heat loss due to heat transfer to the electrode, which is determined by the welding pressure. When the welding current is stopped, the surface of the laminate is rapidly cooled by heat transfer to the electrode. The inner wall of the through hole near the surface, where the molten zone did not spread to the periphery, remains as a trace of the through hole. Cooling also begins inside the laminate due to heat transfer to the periphery, and the solidified structure grows from the fusion boundary toward the center of the fusion zone, forming a weld nugget and completing the weld. As described above, in this embodiment, the trace of the through hole and the weld nugget are spatially connected. That is, in the spot-welded joint according to this embodiment, a trace of the through hole remains, which has one end on the outer surface of the outermost metal sheet and the other end on the fusion boundary of the weld nugget formed on the inner side of the outermost metal sheet in the stacking direction, connecting the two.

[0036] (Preferred electrode tip shape) 2 is a partial cross-sectional view of the upper electrode and workpieces, illustrating the difference in the influence of a flat tip shape and a curved tip shape on the contact state with the surface of the workpiece (laminated portion of metal plates) for the electrode according to this embodiment, including the disturbance factor of electrode tilt. Specifically, in terms of electrode tip shape, (A1) and (A2) in FIG. 2 show a flat electrode 20a with a flat electrode tip, while (B1) and (B2) in FIG. 2 show an R-shaped electrode 20b with a curved electrode tip. Furthermore, in terms of the facing angle between the electrode and the workpiece surface, (A1) and (B1) in FIG. 2 show a case where the electrode and the workpiece surface are opposed at a right angle, while (A2) and (B2) in FIG. 2 show a case where the electrode and the workpiece surface are opposed at an inclination of 2°. When the electrode is a flat electrode 20a, except for the case where the electrode tip is ideally parallel to the surface of the laminate 10 as shown in (A1) in Figure 2, when the electrode is tilted by 2°, as shown in (A2) in Figure 2, local contact between the electrode 20a and the workpieces occurs, which tends to result in unstable welding. Therefore, the electrode according to this embodiment is preferably an R-shaped electrode 20b having a tip with a curvature radius of 15 mm or more and 100 mm or less. (B1) and (B2) in Fig. 2 show the contact state between the tip of the electrode 20b and the laminate 10, taking an example of an electrode with a tip curvature radius of 100 mm. As shown in (B1) and (B2) in Fig. 2, even if the electrode 20b is tilted by about 2°, local contact between the electrode 20b and the workpiece is unlikely to occur, which is preferable for continuing stable welding. On the other hand, if the radius of curvature of the electrode tip is reduced to less than 15 mm, the electrode tip will be pressed into the laminated portion, and there is a concern that the indentation will become too deep, although this is not shown in the drawing.

[0037] (Preliminary Tests Leading to the Invention) Here, the results of the preliminary test leading to the present invention will be described with reference to FIGS. 3A, 3B, and 4A to 4C. FIGS. 3A and 3B relate to the results of a test confirming the effectiveness of forming an electric path through the conductive through-hole 12 penetrating the laminated portion 10. FIGS. 4A to 4C also relate to the results of a weld nugget formation confirmation test conducted based on the results of the electric path formation confirmation test. In this confirmation test, a single-phase AC power supply was used for welding, but welding can also be performed using a DC power supply. In addition, eight identical steel plates were stacked in this preliminary test, but it goes without saying that the overlapping metal plates do not have to be identical.

[0038] The test materials were cold-rolled steel sheets with a thickness of 0.7 mm and a transparent insulating coating, and eight sheets were stacked to form the laminate 10. Furthermore, conductive through holes 12 were drilled with a 1.0 mm diameter drill at the intended welding positions in the laminate 10. An electrode was placed against the opening of the through hole 12 drilled in the laminate 10, and spot welding was performed under the following welding conditions. Since no jig was used to precisely align the opening of the through hole 12 with the electrode, the electrode did not necessarily come into contact with the opening. Single-phase AC power frequency: 50Hz Upper electrode (φ13mm): DR type tip φ4mm R20mm Lower electrode (φ16mm): DR type tip φ6mm R40mm Squeeze time: 50 cycles Welding time: 8 cycles Welding current value (single-phase AC effective value): 1 kA (hereinafter simply referred to as effective value) -Pressure holding time after power supply is stopped: 10 cycles Pressure: 3.4kN

[0039] Figure 3A shows the measurement results of the interelectrode resistance and welding current after the start of current flow. Figure 3A shows that the interelectrode resistance rapidly decreases within one cycle after the start of welding, confirming that welding current can be established through the conductive through hole 12. Figure 3B shows a cross-sectional photograph of the area around the through hole 12 at that time. Figure 3B also shows that the welding current was low at this time, and although the inner surface of the through hole 12 was partially melted, a large weld nugget was not formed. It can also be seen that the opening of the remaining through hole is not in the center of the indentation, but is slightly off-center.

[0040] In contrast, when spot welding was performed under the above welding conditions, with only the welding current changed to 5 kA in order to form a larger weld nugget, significant expulsion occurred and a normal spot weld could not be formed. Therefore, an additional test was conducted to confirm whether a normal spot weld could be formed by using the welding conditions confirmed in Figures 3A and 3B as pre-current conditions to burn off the insulating coating around the through hole 12 to ensure a current path, and then conducting the welding current to form the weld nugget. In the additional test, the metal plate, laminate 10, and through hole 12 conditions were the same, but the current up to Figure 3A was treated as a pre-current to ensure current flow through the through hole 12. The welding conditions were as follows. A two-cycle cooling period was allowed after each pre-current and the two welding currents. Single-phase AC power frequency: 50Hz Upper electrode (φ13mm): DR type tip φ4mm R20mm Lower electrode (φ16mm): DR type tip φ6mm R40mm Squeeze time: 50 cycles Pre-energization: 8 cycles, 1kA (effective value) Welding current: 3 cycles, 5kA (effective value) repeated twice -Pressure holding time after power supply is stopped: 10 cycles Pressure: 3.4kN In this test, the welding current was divided into two pulsation current flows. This is to avoid the occurrence of expulsion caused by current flowing in one breath. If a sufficient current path can be secured with the preliminary current flow, there is no need to use pulsation current.

[0041] Figure 4A shows that the use of pre-current suppresses interelectrode resistance during welding, preventing expulsion and enabling smooth welding. Figure 4B, a cross-sectional photograph of the weld, confirms that weld nuggets (fused and solidified zones) of similar diameter have formed around the original through-holes 12 on each metal plate, penetrating almost the entire thickness of the laminate 10, leaving approximately half the thickness of the metal plate directly below the electrode 20. A relatively large cavity is also observed within the weld nuggets. This large cavity is thought to be a remnant of the cavity in the through-hole, which originally corresponded to the metal void. A small cavity is also observed near the fusion boundary. This is thought to be due to gases produced by decomposition of the insulating organic coating on the metal plate surface. In addition, from the cross-sectional photograph of the weld in Figure 4B and the surface photograph of the indentation in Figure 4C, traces of through holes 12 about half the thickness of the plate can be clearly seen in the metal plate directly below electrode 20, but it can also be seen that molten metal has partially flowed in from the inside, so that they do not form noticeable depressions.

[0042] (Preferred welding cycle) The above preliminary test results demonstrate that the spot welding method according to this embodiment preferably employs a welding cycle in which a pre-current is applied followed by a welding current. That is, it is preferable to apply a pre-current before a welding current to heat or partially melt the inner surface of the through hole 12, thereby burning off the insulating coating around the through hole and ensuring a current path. When a welding current to form a weld nugget is applied after a pre-current, the pre-current burns off the insulating coating around the through hole, ensuring a current path. This advantageously suppresses the occurrence of expulsion that occurs when a pre-current is not applied. Note that, in this embodiment, a welding cycle combining a pre-current and a welding current is not necessary unless a large weld nugget is formed, such as in applications where high joint strength is not required. A welding cycle using a low-current, long-duration welding current may also be used. For example, even in a weld as shown in FIG. 3B, a certain level of weld joint strength can be ensured. [Example]

[0043] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0044] (1. Diameter of the through hole and the formation of the welded joint) Figures 5A to 5C show cross-sectional photographs of welds formed when the diameter of the through hole 12 was varied. The common and individual welding conditions for these were as follows. The conditions for the metal plates and laminated portion 10 were the same as those for the preliminary test shown in Figures 4A to 4C, except for the diameter of the through hole. Two cooling cycles were allowed between the preliminary current and the welding current. The interelectrode resistance of the steel plates used was 480 mΩ on average per sheet. The through hole diameters listed below are the diameters of the through holes at the start of welding. The entire opening of the through hole at the start of welding in Figures 5A to 5C was positioned within the initial contact area. The opening diameter of the through hole at the start of welding in Figures 5A to 5C was set to be less than the initial contact diameter. Single-phase AC power frequency: 50Hz Upper electrode (φ13mm): DR type tip φ4mm R20mm Lower electrode (φ16mm): DR type tip φ6mm R40mm Squeeze time: 50 cycles Pre-energization: 3 cycles of up-slope energization + 7 cycles of specified current Welding current: 3 cycles, specified current -Pressure holding time after power supply is stopped: 10 cycles Pressure: 3.4kN Figure 5A: Through hole diameter 0.8 mm, preliminary current 1.0 kA, welding current 2.3 kA Figure 5B: Through-hole diameter 1.4 mm, preliminary current 1.0 kA, welding current 3.6 kA Figure 5C: Through hole diameter 2.0 mm, preliminary current 1.6 kA, welding current 4.4 kA

[0045] 5A to 5C, the welding was completed without any problems. The current values ​​for the pre-energization and welding were increased as the diameter of the through hole was increased so as to prevent a decrease in the current density on the inner surface of the through hole.

[0046] Comparing the cross-sectional photographs in Figures 5A to 5C, it can be seen that as the diameter of the through hole increases in the order of Figures 5A, 5B, and 5C, the cavity in the weld nugget becomes larger according to the volume of the original through hole. Similarly, it can be seen that the indentation depth, which is the depth of the depression in the indentation, also increases. In all of Figures 5A to 5C, as in Figure 4B, the trace of the through hole 12, approximately half the thickness of each metal plate directly below the electrode 20, can be clearly seen. In the welded joints in Figures 5A to 5C, the diameter of the opening of the through hole after welding was less than the indentation diameter, and the entire opening was located within the indentation area.

[0047] (2. Weld nugget formation status depending on the positional relationship between the through hole and the initial contact area or indentation area) Figures 6A to 6C show cross-sectional photographs of welds formed when welding was performed at a location intentionally outside the through hole 12. The common and individual welding conditions for these are as follows. At the start of welding in Figures 6A to 6C, the entire opening of the through hole in Figure 6A was positioned within the initial contact area. At the start of welding in Figure 6B, the through hole was formed on the boundary between the initial contact area and its outer periphery, with a portion of the opening positioned outside the initial contact area. At the start of welding in Figure 6C, the entire opening of the through hole was positioned outside the initial contact area and adjacent to the boundary of the initial contact area. Note that the opening diameter of the through hole at the start of welding in Figures 5A to 5C was less than the initial contact diameter. The conditions for the metal plate, laminate 10, and through hole (1 mm diameter) 12 were the same as those in the preliminary test in Figures 4A to 4C. Two cooling cycles were allowed between the preliminary current and the two welding currents. Single-phase AC power frequency: 50Hz Upper electrode (φ13mm): DR type tip φ4mm R20mm Lower electrode (φ16mm): DR type tip φ6mm R40mm Squeeze time: 50 cycles Pre-energization: Up-slope energization 3 cycles + 1kA 7 cycles Welding current: 3 cycles, 2.8kA -Pressure holding time after power supply is stopped: 10 cycles Pressure: 3.4kN Figure 6A: The through-hole is located in the center of the indentation (the entire opening is within the indentation area). Figure 6B: The through-hole is located directly above the indentation boundary (part of the opening is outside the indentation area). Figure 6C: The through-hole is located adjacent to the outer boundary of the indentation (the entire opening is outside the indentation area).

[0048] In Figure 6A, there is almost no misalignment between the axis of the through hole and the electrode, resulting in the formation of a sufficient weld nugget. In Figure 6B, the misalignment between the axis of the through hole and the electrode is 2.1 mm, and the through hole is located directly above the indentation boundary. Although the weld nugget is insufficient outside the indentation, there are no unmelted areas and no molten metal is seen escaping from the through hole. In contrast, in Figure 6C, the misalignment between the axis of the through hole and the electrode is 2.4 mm, and the through hole is located adjacent to the outer boundary of the indentation. Therefore, the inner wall of the through hole is more molten near the center of the electrode than away from the electrode. Molten metal is also seen escaping from the through hole. Therefore, even if the metal sheets are in close contact at the through hole location, if the through hole is located outside the indentation, it is necessary to abandon melting the entire inner wall of the through hole and instead melt only a portion of the inner wall on the electrode side to prevent molten metal from escaping from the through hole. [Explanation of symbols]

[0049] 1, 2, 3, . . . , n Metal plate with insulating coating 10. Metal plate stack 12 Conductive through holes 20 Electrodes (spot welding electrodes) 20a electrode (flat electrode) 20b electrode (R-type electrode) 41, 42 Steel plate (painted steel plate) 41a, 41b, 42a, 42b Paint coating 43 Back Bar 44a, 44b electrode 45 Unpainted steel plate (mild steel plate) A1, A2, A3, A4 Welding Current Loop

Claims

1. A spot welding method for metal plates having an insulating coating, comprising: stacking three or more metal plates having an insulating coating on at least one side thereof, the stack being formed by stacking the metal plates so that the insulating coating is disposed on the outer side of at least one of the metal plates, the two outermost metal plates being the front and rear metal plates, and spot welding the stack by sandwiching the stack between two electrodes in the stacking direction of the metal plates and passing a current between the electrodes while applying pressure; the insulating coating of the metal plate is an insulating coating having an inter-electrode resistance value of 150 mmΩ or more and 1000 mmΩ or less when measured when one metal plate is sandwiched between the two electrodes having a tip curvature radius of 40 mm with a pressure of 2 kN, A spot welding method for metal plates having an insulating coating, comprising providing a conductive through hole that penetrates the laminated portion at a position where the spot welding is to be performed on the laminated portion, and then performing the spot welding.

2. 2. The spot welding method for metal plates having an insulating coating according to claim 1, wherein when the healthy portion of the laminate is sandwiched between the two electrodes with a predetermined pressure, the contact area between the outer surface of the laminate and the electrodes is defined as an initial contact area, and at least a portion of the opening of the through hole is located within the initial contact area.

3. 3. The spot welding method for metal plates having an insulating coating according to claim 1 or 2, wherein the diameter of the opening of the through hole is 0.1 mm or more and less than the initial contact diameter, where the diameter of the contact surface between the outer surface of the laminate and the electrodes when the healthy part of the laminate is clamped between the two electrodes with a predetermined pressure.

4. 3. The spot welding method for metal plates having an insulating coating according to claim 1, wherein the electrode has a tip with a radius of curvature of 15 mm or more and 100 mm or less.

5. 3. The spot welding method for metal plates having an insulating coating according to claim 1, wherein the through hole is formed in a state where the metal plates are stacked.

6. A spot welded joint of metal plates with insulating coatings, comprising three or more metal plates having an insulating coating on at least one side thereof, stacked in a state in which the insulating coating is disposed on the outside of at least one of the metal plates, the front and rear two metal plates being the outermost layers, and a weld nugget connecting the metal plates in the stacking direction, the insulating coating of the metal plate is an insulating coating having an inter-electrode resistance of 150 mmΩ or more and 1000 mmΩ or less when the metal plate is sandwiched between two electrodes of a spot welding machine, each having a tip curvature radius of 40 mm, with a pressure of 2 kN; A spot welded joint of metal plates having an insulating coating, which has one end and the other end connected to the outer surface of the outermost metal plate and the fusion boundary of the weld nugget formed on the inner side of the outermost metal plate in the stacking direction, and which leaves traces of a through hole.

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