Spot welding device, spot welding device control method, spot welded joint, and spot welded joint manufacturing method

Abstract

This spot welding device uses a pair of electrodes to sandwich a plurality of stacked steel sheets and apply electricity while applying a pressure, so as to be able to perform spot welding of the plurality of steel sheets, the spot welding device being characterized in that between the start and the end of electricity application, the speed at which the pair of electrodes approaches to each other is at least limited to 12.0 mm / s or less.

Description

[Technical Field]

[0001] The present invention relates to a spot welding apparatus, a control method for a spot welding apparatus, a spot welded joint, and a method for manufacturing a spot welded joint. This application claims priority based on Japanese Patent Application No. 2022-018764, filed on February 9, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] Spot welding devices that control the applied pressure are known, as described in Patent Documents 1 and 2, for example. In a method for manufacturing a spot-welded joint using a spot welding device, a sheet assembly is sandwiched between a pair of opposing electrodes, and the overlapping surfaces of the steel sheets are heated and melted by electrical heating using the electrodes, forming a molten and solidified portion (nugget) near the interface of the overlapping steel sheets, thereby joining the steel sheets (Fig. 1). In recent years, the expanded use of high-strength steel sheets has required improved joint strength, resulting in a demand for welding with a larger nugget diameter. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-96376 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-300738 Summary of the Invention [Problem to be solved by the invention]

[0004] However, increasing the nugget diameter increases the likelihood of localized overheating of the steel sheet at the nugget edge, leading to molten metal spattering (Figure 2). Spattered metal, known as "spatter" during spot welding, can reduce weld quality. The presence or absence of "spatter" can be confirmed by visually inspecting the welding phenomenon during the welding period. When spatter occurs, minute molten metal spatters from the weld (more specifically, from the gap between the steel sheets), making it easily visible. When spatter occurs, the amount of molten metal inside the steel sheet decreases, increasing the amount of electrode penetration into the steel sheet surface, resulting in a large indentation on the steel sheet surface. This indentation reduces the thickness of the steel sheet around the nugget, thereby reducing joint strength. Furthermore, large tensile stresses are generated in the indentation, which can lead to liquid metal embrittlement (LME) cracking when the steel sheet is galvanized.

[0005] If expulsion can be suppressed and indentation can be reduced, it is possible to suppress a decrease in joint strength and LME cracking, but it is difficult to completely prevent expulsion, taking into account various construction disturbances that occur during actual production, such as the gap between steel sheets (sheet gap), the inclination of the pressure electrode (strike angle), and welding points on the sheet edges.On the other hand, if indentation can be reduced even if expulsion occurs, it is possible to suppress a decrease in joint strength and LME cracking in spot-welded joints of galvanized steel sheets.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a spot welding apparatus, a control method for a spot welding apparatus, a spot-welded joint, and a method for manufacturing a spot-welded joint that are capable of reducing indentation even when the nugget diameter is increased, thereby increasing the strength of the welded joint and preventing LME cracking in spot-welded joints of galvanized steel sheets. [Means for solving the problem]

[0007] In a standard spot welding process, the sheet thickness increases due to thermal expansion inside the steel sheet when current is applied. This increases the electrode pressure reaction force during the early stage of current application, causing the electrode to displace away from the steel sheet (Fig. 3(a)). During the later stage of current application, the heated area inside the steel sheet expands, softening the steel sheet, causing the nugget to grow, and decreasing the electrode pressure reaction force. As a result, the electrodes approach each other and press the steel sheet together (Fig. 3(b)). Furthermore, toward the end of the later stage of welding, the molten metal expands in size (nugget diameter), causing the molten metal to splash from the nugget edge to the surrounding area. This causes the electrode pressure reaction force to suddenly decrease, causing the electrode to rapidly press the steel sheet, resulting in large indentations and significant expulsion (Fig. 3(c)). Therefore, the inventors hypothesized that if the electrode displacement speed (electrode approach speed) could be reduced at the time of expulsion, the nugget could be solidified before the electrode pressed firmly into the steel sheet surface, thereby reducing indentations and expulsion.

[0008] Most conventional methods for controlling electrode pressure have focused on increasing the responsiveness of the electrode pressure to ensure stability in the welding process. For example, if the responsiveness of the electrode pressure is extremely low, the electrode pressing action will be delayed when the steel sheet softens in the first half of the later welding period. This will result in an excessively low electrode pressure, reducing the contact area between the electrode tip and the steel sheet surface, and resulting in poor nugget formation due to localized heat generation on the steel sheet surface. For this reason, the conventional trend has been to increase the responsiveness of the electrode pressure to maintain a constant electrode pressure throughout the welding period.

[0009] If such conventional technology is applied to increase the responsiveness of the electrode pressure, the electrode pressure will be kept constant at a set value, but as mentioned above, when the steel sheet softens or expulsion occurs, the electrode will rapidly crush the surface of the steel sheet, causing the depression to expand and resulting in indentation. In other words, the idea devised by the inventor to reduce the electrode approach speed is completely different from conventional methods.

[0010] The present invention has been made to solve the above problems, and the gist of the present invention is as follows. (1) A spot welding device that can spot weld a plurality of overlapping steel sheets by clamping the overlapping steel sheets between a pair of electrodes, applying pressure to the overlapping steel sheets, and passing current therethrough, A spot welding device characterized in that the speed at which the pair of electrodes approach each other is limited to at least 12.0 mm / s or less from the start to the end of current flow. Here, when the pair of electrodes is composed of a movable electrode and a fixed electrode as in the embodiment described below, the "speed at which the pair of electrodes approaches each other" can be rephrased as the moving speed or displacement speed of the movable electrode. In particular, when the movable electrode is located above the fixed electrode, the "speed at which the pair of electrodes approaches each other" can be rephrased as the descending speed of the movable electrode. (2) The spot welding device according to (1), characterized in that the speed is limited to at least 12.0 mm / s or less during the period from the end of energization to the end of the pressure maintaining time. (3) A control method for a spot welding device that spot-welds a plurality of overlapping steel sheets by sandwiching the overlapping steel sheets between a pair of opposing electrodes, applying pressure to the overlapping steel sheets, and passing current therethrough, comprising the steps of: A method for controlling a spot welding apparatus, comprising limiting the speed at which the pair of electrodes approach each other to at least 12.0 mm / s or less during the period from the start of current flow to the end of current flow. (4) The method for controlling a spot welding device according to (3), characterized in that the speed is limited to at least 12.0 mm / s or less during the period from the end of energization to the end of the pressure maintaining time. (5) A spot-welded joint manufactured by spot welding a plurality of steel plates having a tensile strength of 980 MPa or more, wherein the nugget diameter formed by the spot welding is 5.0√t or more when the minimum thickness of the plurality of steel plates having a tensile strength of 980 MPa or more is t, The minimum thickness of the spot welded joint is 0.70 times or more the total thickness of the steel plates, A spot-welded joint characterized in that the area of the expulsion portion measured in an X-ray radiographic image of the weld is 30 to 160% of the area of the pressure-welded portion. (6) The spot welded joint according to (5), characterized in that the nugget diameter is 5.5√t or more. (7) The spot welded joint according to (5) or (6), characterized in that the spot welded joint is free of cracks. (8) The minimum thickness of the spot welded joint is 0.90 times or less the total thickness of the steel plates. or (6) 2. The spot welded joint according to claim 1 . (9)(5) or (6) A method for manufacturing a spot welded joint according to the present invention, A method for manufacturing a spot welded joint, characterized in that the speed at which a pair of electrodes provided in a spot welding device approach each other is limited to at least 12.0 mm / s or less from the start to the end of current flow. (10) A method for manufacturing a spot welded joint according to (9), characterized in that the speed is limited to at least 12.0 mm / s or less during the period from the end of energization to the end of the pressure maintenance time. [Effects of the Invention]

[0011] According to the above-described aspects of the present invention, even when the nugget diameter is increased, it is possible to reduce indentation, thereby increasing the strength of the welded joint and preventing LME cracking in spot-welded joints of galvanized steel sheets. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a side cross-sectional view showing the structure of the nugget and its surroundings. [Figure 2] FIG. 10 is a side cross-sectional view showing an example of indentation. [Figure 3] FIG. 10 is a cross-sectional side view showing the displacement of the electrode during the energization period. [Figure 4] 1 is a side view showing the appearance of a spot welding device according to an embodiment of the present invention. [Figure 5] 1 is a functional block diagram of a spot welding device according to an embodiment of the present invention; [Figure 6]4 is a flowchart showing a processing procedure performed by the spot welding device according to the present embodiment. [Figure 7] 10 is a graph showing a comparison of welding periods between the conventional technique and the present embodiment. [Figure 8] 1 is a cross-sectional view showing a schematic configuration of a spot-welded joint according to an embodiment of the present invention. [Figure 9] This is an example of an X-ray cross-sectional image. [Figure 10] 1 is an example of a planar X-ray image. DETAILED DESCRIPTION OF THE INVENTION

[0013] Next, an embodiment of the present invention will be described. In the following description, the definitions of "spot welding," "welding time," "initial pressure time," "pressure maintenance time," "nugget," "nugget diameter," and "indentation" are based on JIS Z3001-6:2013 "Welding Terminology - Part 6: Resistance Welding." Specifically, they are as follows: "Spot welding": Resistance welding in which overlapping base materials are clamped between the tips of properly shaped electrodes, and current and pressure are concentrated in a relatively small area to heat it locally, while simultaneously applying pressure with the electrode. "Weld time": The time that current flows in resistance welding. Also called weld time. "Initial pressure time (squeeze time)": In lap resistance welding, the time from when the electrode pressure command signal is issued to when the welding current starts to flow. Also called squeeze time. "Force maintenance time": The time during which the force is maintained at a predetermined value. The force rise and fall times are excluded. "Expulsion" or "Splash": In lap resistance welding, the phenomenon in which the base metal is locally overheated and melts and splashes, or the metal "Nugget": The molten and solidified part that occurs in the weld during lap resistance welding. "Nugget diameter": The diameter of the nugget measured at the joint interface by cross-sectional testing of a spot weld or projection weld. "Indentation": In lap resistance welding, a depression on the surface of the base material caused by the electrode tip and the disk electrode as a result of welding. Also called a dimple.

[0014] The inventors conducted a detailed investigation into the expulsion phenomenon and the associated indentation during spot welding and found that the speed at which a pair of electrodes approach each other during spot welding (hereinafter also referred to as the "electrode approach speed" or "approach speed") must be extremely low to prevent the expulsion phenomenon. Furthermore, limiting the upper limit of the electrode approach speed at the time of expulsion can reduce the indentation (depression) at the time of expulsion, thereby increasing the strength of the welded joint and preventing LME cracking in spot-welded joints of galvanized steel sheets. Meanwhile, conventional spot welding equipment requires high responsiveness in electrode movement to maintain a predetermined applied pressure while responding to rapid fluctuations in the distance between the steel sheets' surfaces due to factors such as expansion caused by heat generation and a reduction in the reaction force during application due to melting. For this reason, while some spot welding equipment can achieve very high electrode displacement speeds (the speed at which the pair of electrodes approach or separate), none have deliberately set the electrode displacement speed, including the electrode approach speed, to low speeds. The spot welding apparatus, the control method for the spot welding apparatus, the spot-welded joint, and the method for manufacturing the spot-welded joint according to the present embodiment have been made based on the above findings. According to the present embodiment, it is possible to prevent the occurrence of LME cracking, particularly in the shoulder portion (the end of the recessed portion on the surface of the weld) of the weld (the portion joined by the spot welding apparatus, including the nugget and the recessed steel plate portions above and below it), and to obtain a resistance spot-welded joint with sound joint strength.

[0015] <1. Overall configuration of spot welding equipment> First, the overall configuration of the spot welding apparatus 10 will be described with reference to FIGS. 4 and 5. FIG. 4 is a side view showing the appearance of the spot welding apparatus 10, and FIG. 5 is a functional block diagram of the spot welding apparatus 10. Note that the following example describes a servomotor-type spot welding apparatus, but the device driving the electrodes is not limited to a servomotor. Other examples include an air cylinder. Also, while FIG. 4 illustrates two steel sheets 100a and 100b as the sheet pair to be joined, more steel sheets may be joined. The type of steel sheets 100a and 100b is not particularly limited. For example, the steel sheet disposed on at least one surface of the sheet pair to be joined (in this example, either or both of the steel sheets 100a and 100b) may be a zinc-based plated high-strength steel sheet with a tensile strength of 980 MPa or more. In this case, LME cracking is a concern. However, as described below, this embodiment makes it possible to suppress the occurrence of LME cracking. The thickness of the steel sheets 100a and 100b is also not particularly limited, but may be, for example, approximately 1.2 to 2.3 mm. The total thickness of the overlapping steel sheets is not particularly limited, but may be, for example, approximately 2.4 to 5.0 mm or 2.4 to 4.5 mm. In this case, according to this embodiment, the total amount of recession (the total amount of recession on both the front and back surfaces of the spot-welded joint) can be 30% or less of the total thickness (original thickness) of the overlapping steel sheets. In other words, the minimum thickness of the spot-welded joint (the minimum value of the distance between the steel sheet surfaces at the indentation portion) can be 0.70 times or more of the total thickness (original thickness) of the overlapping steel sheets. Therefore, the total amount of recession can be reduced relative to the sheet thickness, thereby reducing the indentation. Furthermore, when the thickness of the thinner of the steel sheets 100a, 100b (or the thinnest steel sheet when three or more steel sheets are joined) is t (mm), the nugget diameter formed by spot welding can be 5.0√t (mm) or more or 5.5√t (mm) or more. In this way, in this embodiment, a relatively large nugget is formed, which is likely to cause expulsion and, as a result, indentation.However, in this embodiment, as described below, the speed at which the electrodes (the movable electrode 16a and the fixed electrode 16b) approach each other during spot welding (electrode approach speed) is intentionally limited to a specific value (12.0 mm / s) or less. This reduces the indentation even if expulsion occurs, and the total amount of indentation can be kept to 30% or less of the total thickness (original thickness) of the overlapping steel sheets. As a result, the strength of the welded joint can be increased and LME cracking can be prevented. Note that, instead of the thickness t (mm) of the thinnest steel sheet as described above, the smallest thickness t (mm) among steel sheets (e.g., high-strength steel sheets with a tensile strength of 980 MPa or more) that serve as strength members of the welded joint is defined as t (mm), and the nugget diameter is set to 5.0√t (mm) or more or 5.5√t (mm) or more. In this embodiment, the steel sheet used for spot welding is not limited to a flat steel sheet. Spot welding is possible as long as the portion sandwiched between the electrodes has a steel sheet shape. Therefore, the steel sheet shape means that there is a portion that can be sandwiched between the electrodes. Specifically, it may be a cold-worked steel sheet (for example, a steel material processed by bending or drawing a steel sheet using a roll or a press), or a steel material manufactured by hot stamping a steel sheet (press forming after hot heating) (sometimes called a hot-stamped body, etc.).

[0016] The spot welding apparatus 10 spot-welds two or more overlapping steel sheets (e.g., steel sheets 100a and 100b shown in FIG. 4 ) and includes a control unit 11, a memory unit 12, a pressing force measurement unit 13, a servomotor 14 (electrode drive unit), a current supply unit 15, a movable electrode 16a, a fixed electrode 16b, and an input unit 17. The spot welding apparatus 10 includes a CPU, a ROM, a RAM, a pressing force measurement unit (e.g., a device that measures the pressing force using a built-in load cell and estimates the pressing force based on the load current of the servomotor), a servomotor, a current supply unit, an electrode and electrode drive mechanism, and an input unit (e.g., a keyboard, an input switch, etc.). The spot welding apparatus 10 uses this hardware configuration to implement the control unit 11, the memory unit 12, the pressing force measurement unit 13, the servomotor 14, the current supply unit 15, the movable electrode 16a, the fixed electrode 16b, and the input unit 17 described above.

[0017] The memory unit 12 stores data necessary for the processing performed by the spot welding apparatus 10. Examples of such data include data indicating the correlation between the torque of the servo motor and the electrode welding force, data indicating the correlation between the rotational speed and torque change rate of the servo motor 14 and the electrode displacement speed, and programs necessary for the operation of the control unit 11. Here, the data indicating the correlation between the rotational speed and torque change rate (time change rate) of the servo motor 14 and the electrode displacement speed will be described in detail. The electrode displacement speed is the displacement speed of the movable electrode 16a. Here, since the fixed electrode 16b is fixed and located above the fixed electrode 16b, the speed at which the movable electrode 16a and the fixed electrode 16b, which constitute a pair of electrodes, approach each other (electrode approach speed) can also be rephrased as the descending speed of the movable electrode 16a. As shown in the data, the rate of change of the torque of the servo motor 14 also affects the electrode displacement speed. The reason for this is as follows. If the rigidity of the movable electrode 16a and fixed electrode 16b guns (including the frame containing the pair of electrodes; the same applies below) is low, the guns may bend when a strong torque is applied to these electrodes. When expulsion occurs, the pressure reaction force from the steel sheets 100a and 100b decreases, releasing the gun's deflection and causing the gun to expand like a spring, i.e., the movable electrode 16a and fixed electrode 16b move at high speed. As a result, even if the rotation speed of the servo motor 14 is slowed, the electrode approach speed of the movable electrode 16a (the speed at which the movable electrode 16a and fixed electrode 16b approach each other) may exceed the upper limit of 12.0 mm / s. This causes the movable electrode 16a to suddenly press into the steel sheets 100a and 100b, resulting in indentation.

[0018] In other words, simple feedback control using only the rotational speed of the servo motor 14 may not accurately determine the displacement speed of the movable electrode 16a. Therefore, in this embodiment, in such cases, it is preferable to determine the displacement speed of the movable electrode 16a by taking into account not only the rotational speed of the servo motor 14 but also the rate of change of torque. For example, a rapid decrease in torque at the moment of expulsion is detected, and if the rate of torque decrease (time change rate) exceeds a predetermined value, the electrode displacement speed is reduced. Furthermore, to prevent rapid electrode displacement due to a decrease in gun rigidity, it is desirable to maintain a low servo motor torque. Therefore, in some cases, the direction of the electrode displacement speed may be upward. When the gun rigidity of the spot welding device 10 is high, it is not necessary to determine the displacement speed of the movable electrode 16a by taking into account the rate of change of torque (described above and below).

[0019] The upper limit (V D ) is within a range of 12.0 mm / s or less. Here, the downward direction of the descent speed is defined as positive. More specifically, the vertically downward direction is the positive direction in FIG. 4. Therefore, when the movable electrode 16a presses the steel plates 100a, 100b, the movable electrode 16a is displaced in the positive direction. In this case, the descent speed of the movable electrode 16a has a positive value. The control unit 11 compares the descent speed of the movable electrode 16a with the above-mentioned upper limit value and performs control as described below so that the descent speed of the movable electrode 16a does not exceed the above-mentioned upper limit value of 12.0 mm / s. In addition, the period from the start of energization to the end of energization also refers to the period from the start time of the welding time to the end time of the welding time.

[0020] In this embodiment, the upper limit can be arbitrarily set to a lower value by an input operation by the operator (using the input unit 17). Considering factors such as the gun rigidity, the required indentation size, the strength of the welded joint, and LME cracking resistance, as well as other factors (e.g., the size of expulsion), and taking into account the results of preliminary tests conducted as necessary, the upper limit may be limited to a speed lower than 12.0 mm / s. Furthermore, in this embodiment, the displacement speed of the movable electrode 16a may be set to different values in the early and later stages of current application. For example, the upper limit of the lowering speed in the early stage of current application may be set to at least 12.0 mm / s, and the upper limit of the lowering speed in the later stage of current application may be set to at least 8.0 mm / s. The early and later stages of current application are distinguished by the amount of heat input from the movable electrode 16a and the fixed electrode 16b to the steel sheets 100a and 100b (here, the current value is integrated over the current application period). For example, the period from the start of energization until one-third of the total heat input has been input may be defined as the early energization period, and the remaining period as the late energization period. The energization period and current value are preset as welding conditions, but the current value may also be changed sequentially by feedback control during energization. In this embodiment, the early energization period and the late energization period are determined based on the preset welding current value.

[0021] The pressure measuring unit 13 detects the torque and rotational speed of the servo motor 14 during at least the current-carrying period, and outputs the detected values to the control unit 11. Preferably, the torque and rotational speed of the servo motor 14 are detected not only during the current-carrying period but also during the welding period, including the initial pressure time, the current-carrying period, and the pressure-carrying period, and output the detected values to the control unit 11. The servo motor 14 drives (lowers) the movable electrode 16a, thereby pressing the steel sheets 100a, 100b with the movable electrode 16a and the fixed electrode 16b. The current supply unit 15 energizes the movable electrode 16a and the fixed electrode 16b under the control of the control unit 11. As a result, the overlapping surfaces of the steel sheets 100a, 100b are heated and melted via the movable electrode 16a and the fixed electrode 16b. The movable electrode 16a and the fixed electrode 16b sandwich two or more overlapping steel plates (for example, steel plates 100a and 100b shown in FIG. 4), and by passing a current through the overlapping surfaces, the overlapping surfaces are heated and melted. The input unit 17 can be operated by an operator, and the operator uses the input unit 17 to input various information (for example, the upper limit value of the displacement speed of the movable electrode 16a).

[0022] The control unit 11 controls each component of the spot welding device 10 and also performs the following processes. Specifically, the control unit 11 sets a welding period including an initial pressurizing time, a current application period, and a pressurizing force maintenance time. During the initial pressurizing time, the control unit 11 controls the servo motor 14 so that the movable electrode 16a and the fixed electrode 16b pressurize the steel sheets 100a and 100b with a constant pressurizing force. Specifically, the control unit 11 outputs drive start information to the servo motor 14. The servo motor 14 starts driving based on the drive start information. As a result, the movable electrode 16a and the fixed electrode 16b pressurize the steel sheets 100a and 100b. The pressurizing force measurement unit 13 detects the torque of the servo motor 14 at this time and outputs measurement information to the control unit 11. Based on the obtained measurement information and data stored in the memory unit 12 (data indicating the correlation between the servo motor torque and the pressurizing force applied by the electrodes), the control unit 11 controls the torque of the servo motor 14 so that the pressurizing force applied by the servo motor 14 is a predetermined value. That is, the control unit feeds back to the servo motor 14 the difference value between the measured pressure applied by the electrode measured by the pressure measuring unit 13 and the set pressure (set value of the pressure), and performs feedback control so that the measured pressure is maintained near the set pressure.

[0023] Furthermore, the pressure measuring unit 13 detects the rate of change of the rotation speed and torque of the servo motor 14 and outputs the measurement information to the control unit 11. The control unit 11 determines the displacement speed of the movable electrode 16a based on data stored in the storage unit (data indicating the correlation between the rate of change of the rotation speed and torque of the servo motor 14 and the displacement speed of the electrode). Next, the control unit 11 feedback-controls the servo motor 14 so that the determined displacement speed of the movable electrode 16a does not exceed a preset upper limit of the displacement speed (here, the electrode approach speed (lowering speed)). That is, if the determined descent speed of the movable electrode 16a exceeds the preset upper limit, the control unit 11 recalculates the rotation speed of the servo motor 14 so that the electrode descent speed is equal to or less than the upper limit. Then, the control unit 11 drives the servo motor 14 at the calculated rotation speed. On the other hand, if the determined descent speed of the electrode is equal to or less than the preset upper limit, the control unit 11 maintains the driving of the servo motor 14. If the descending speed of the movable electrode 16a were not limited to not exceed the upper limit, expulsion would occur, and the pressure reaction force from the steel sheets 100a, 100b would suddenly decrease. Then, the movable electrode 16a and the fixed electrode 16b would suddenly press into the steel sheets 100a, 100b. This would result in indentation.

[0024] As described above, the upper limit of the descent speed (electrode approach speed) is preset within a range of 12.0 mm / s or less. Preferably, it may be set to, for example, 10.0 mm / s or less, 8.0 mm / s or less, or 6.0 mm / s or less. If necessary, as described above, the upper limit may be set to different values between the early and later energization periods. However, in this case, it is preferable that the upper limit during the later energization period be lower than the upper limit during the early energization period. In this embodiment, the upper limit can be set arbitrarily within the above range by an input operation by the operator (using the input unit 17). This upper limit may be determined, for example, by conducting a preliminary test. Specifically, test pieces corresponding to the steel plates 100a and 100b are prepared and spot-welded. The descent speed of the movable electrode 16a at which expulsion occurs is identified, and this is set as the upper limit. The "presence or absence of expulsion" can be confirmed, for example, by visually observing the welding phenomenon during the energization period. When expulsion occurs, minute amounts of molten metal are scattered from the weld (more specifically, from the gap between the steel sheets), and this can be easily confirmed by visual inspection.If necessary, the upper limit may be determined based on test results such as the size of the indentation obtained in a preliminary test, the strength of the welded joint, and LME cracking resistance, rather than the presence or absence of expulsion.

[0025] The early energization period is a period in which good contact is formed between the steel sheets and between the steel sheet and the electrode, and it is desirable to set the displacement speed relatively high so that the electrode can respond quickly to the thermal expansion of the steel sheet. However, since expulsion can occur even in the early energization period, if the descent speed (electrode approach speed) of the movable electrode 16a exceeds a preset upper limit, the control unit 11 performs feedback control to keep the descent speed below that upper limit. Since expulsion is likely to occur in the later energization period, it is desirable to set the descent speed slower to prevent excessive indentation.

[0026] As described above, at the end of the later welding stage, the molten metal splashes from the nugget edge to the surrounding area due to the expansion of the molten diameter (nugget diameter), causing a sudden decrease in the electrode pressure reaction force. However, in this embodiment, even if the pressure reaction force of the steel sheets 100a, 100b suddenly decreases at the end of the later welding stage, the movable electrode 16a is slowly lowered (i.e., the control unit 11 feedback-controls the servo motor 14 so that the descent speed of the movable electrode 16a does not exceed a preset upper limit). Then, while the movable electrode 16a is slowly lowering, the nugget solidifies. Therefore, the indentation can be reduced. As a result, it is possible to suppress the occurrence of LME cracking, even if one or both of the steel sheets 100a, 100b are zinc-based plated high-strength steel sheets.

[0027] If the electrode driving device is an air cylinder rather than a servo motor 14, the air cylinder capacity may be set low to reduce the electrode displacement responsiveness and set the displacement speed low (i.e., 12.0 mm / s or less). If it is difficult to adjust the displacement speed of the movable electrode 16a to 12.0 mm / s or less even by adjusting the air cylinder capacity, a servo motor 14 may be used instead of an air cylinder. Even in control using a servo motor 14, the descent speed may be set to a specified value (i.e., a value below the above-mentioned upper limit of the descent speed) by setting the servo motor responsiveness low without sequentially calculating the descent speed. If necessary, spot welding devices using an air cylinder as the electrode driving device may be excluded.

[0028] During the pressure force maintenance time, the control unit 11 performs the above-mentioned feedback control of the constant pressure force. As a result, the movable electrode 16a and the fixed electrode 16b press the steel sheets 100a, 100b with a constant pressure force. This makes it possible to suppress LME cracking. Note that expulsion may occur even during the pressure force maintenance time after the current supply is terminated, causing the movable electrode 16a and the fixed electrode 16b to suddenly press the steel sheets 100a, 100b. Therefore, it is preferable that the control unit 11 perform the above-mentioned feedback control of the displacement speed of the movable electrode 16a (control to maintain the displacement speed of the movable electrode 16a at or below the upper limit) even during the pressure force maintenance time after the current supply is terminated.

[0029] The problems associated with slowing the lowering speed described in the latter part of this paragraph have been widely known, and a lower limit has traditionally been set. Therefore, there is no need to specifically limit the lower limit of the lowering speed in the present invention. However, the lower limit of the lowering speed may be set based on conventional knowledge. For example, it may be set to 1.0 mm / s or more, 1.8 mm / s or more, or 2.5 mm / s or more. This is because a slow lowering speed prevents the electrode from following the fluctuations in the electrode pressure due to thermal contraction at the end of current application, which may increase expulsion during welding and result in internal defects in the weld metal. Furthermore, if the response of the electrode pressure is extremely slow, the electrode pressing operation may be delayed when the steel sheet softens in the first half of the latter period of current application. This may result in an excessively low electrode pressure, reducing the contact area between the electrode tip and the steel sheet surface and potentially resulting in poor nugget formation due to localized heating on the steel sheet surface.

[0030] <2. Processing by Spot Welding Equipment (Method for Controlling Spot Welding Equipment, Method for Manufacturing Spot Welded Joints)> Next, an example of the procedure for welding using the spot welding device 10 will be described with reference to the flowchart shown in FIG. 6. To form a nugget with a desired diameter, a welding period including an initial pressure application time, a current application period, and a pressure application maintenance time is determined in advance through a preliminary test. The current application pattern, pressure, and other factors are also determined in the preliminary test. In the preliminary test, the steel sheets to be joined are overlapped and spot-welded to form a nugget. The above-mentioned parameters are then determined so that the desired nugget diameter and other factors are obtained.

[0031] During the initial pressurizing time in step S10, the control unit 11 controls the servo motor 14 so that the movable electrode 16a and the fixed electrode 16b press the steel sheets 100a, 100b with a constant pressurizing force. Specifically, the control unit 11 outputs drive start information to the servo motor 14. The servo motor 14 starts driving based on the drive start information. As a result, the movable electrode 16a and the fixed electrode 16b press the steel sheets 100a, 100b. The pressurizing force measuring unit 13 detects the torque of the servo motor 14 at this time and outputs measurement information to the control unit 11. Based on the obtained measurement information and data stored in the memory unit 12 (data indicating the correlation between the torque of the servo motor and the pressurizing force applied by the electrodes), the control unit 11 controls the torque of the servo motor 14 so that the pressurizing force applied by the servo motor 14 is constant. That is, the control unit feeds back to the servo motor 14 the difference value between the measured pressurizing force measured by the pressurizing force measuring unit 13 and the set pressurizing force.

[0032] In step S20, the control unit 11 outputs current supply start information to the current supply unit 15. Based on the current supply start information, the current supply unit 15 starts supplying current to the movable electrode 16a and the fixed electrode 16b in accordance with a preset current supply pattern (a current supply pattern for obtaining a desired nugget, etc.). The current supply unit 15 outputs heat input amount information regarding the heat input amount (here, the current value is assumed to be integrated over the current supply period) to the control unit 11. The control unit 11 distinguishes between the early current supply period and the late current supply period based on the heat input amount information. Specifically, the control unit 11 may define the period from the start of current supply to the point where one-third of the total heat input amount has been input as the early current supply period, and the remaining period as the late current supply period. Of course, the method of dividing the period into the early current supply period and the late current supply period is not limited to this method.

[0033] In step S30, the pressure measuring unit 13 detects (senses) the torque of the servo motor 14 and outputs the measurement information to the control unit 11.

[0034] In step S40, the control unit 11 identifies the pressure applied by the servo motor 14 based on the obtained measurement information and the data (data indicating the correlation between the torque of the servo motor and the pressure applied by the electrodes) stored in the memory unit 12. Next, the control unit 11 compares the pressure applied by the servo motor 14 with a preset pressure (a set pressure, a pressure for obtaining a desired nugget or the like).

[0035] In step S50, the control unit 11 calculates the output value of the servo motor 14 so that the difference between the pressure applied by the servo motor 14 and the preset pressure becomes zero (i.e., so that the two match). That is, the control unit 11 controls the torque of the servo motor 14 so that the pressure applied by the movable electrode 16 a and the fixed electrode 16 b becomes constant.

[0036] In step S60, the control unit 11 compares the current application period with a predetermined time (the time required to obtain a desired nugget, etc.), and if it determines that the current application period has reached the predetermined time, the process proceeds to step S80; if it determines that the current application period has not reached the predetermined time, the process proceeds to step S70. The control unit 11 can also determine whether the amount of heat input to the movable electrode 16a and the fixed electrode 16b matches a preset total heat input amount. If the control unit 11 determines that the amount of heat input to the movable electrode 16a and the fixed electrode 16b matches the preset total heat input amount, the process proceeds to step S80; if it determines that the two do not match, the process proceeds to step S70.

[0037] In step S70, the pressing force measuring unit 13 detects the rate of change of the rotational speed and torque of the servo motor 14 and outputs the measurement information to the control unit 11. The control unit 11 determines the displacement speed of the movable electrode 16a based on data stored in the memory unit (data indicating the correlation between the rate of change of the rotational speed and torque of the servo motor 14 and the electrode displacement speed). Next, the control unit 11 feedback-controls the servo motor 14 so that the determined descent speed of the movable electrode 16a does not exceed a preset upper limit. That is, if the determined displacement speed of the movable electrode 16a exceeds the preset upper limit of the descent speed, the control unit 11 recalculates the rotational speed of the servo motor 14 so that the descent speed of the movable electrode 16a is equal to or less than the upper limit. Then, the control unit 11 drives the servo motor 14 at the calculated rotational speed. On the other hand, if the determined descent speed of the movable electrode 16a is equal to or less than the preset upper limit, the control unit 11 maintains the driving of the servo motor 14. The spot welding apparatus 10 then returns to step S30. If necessary, the distance between the movable electrode 16 a and the fixed electrode 16 b may be measured directly, and the displacement speed of the movable electrode 16 a may be calculated from the measured distance. In this case, it is not necessary to output the rotation speed and torque change rate of the servo motor 14 as measurement information to the control unit 11, and it is not necessary to determine the displacement speed of the movable electrode 16 a based on the data stored in the memory unit of the control unit 11.

[0038] In step S80, the control unit 11 ends the energization period. In step S90, processing for the pressure force maintenance time is performed. In this step, it is desirable to perform the above-mentioned constant pressure force feedback control and displacement speed (lowering speed) control (processing of steps S30 to S70). Specifically, the control unit 11 performs the processing of steps S30 to S50 in parallel with the processing of step S90. In step S90-1, the control unit 11 determines whether a predetermined time has elapsed. If it determines that the predetermined time has elapsed, the control unit 11 proceeds to step S90-2 and ends the processing for the pressure force maintenance time. If it determines that the predetermined time has not elapsed in step S90-1, the control unit 11 proceeds to step S90-3 and performs the same processing as step S70. Thereafter, the control unit 11 returns to step S90. As a result, the movable electrode 16a and the fixed electrode 16b press the steel sheets 100a, 100b with a constant pressure force. This makes it possible to suppress LME cracking.

[0039] In step S100, control unit 11 opens movable electrode 16a and fixed electrode 16b. Through the above processing, spot welding device 10 ends the welding process. The energization conditions during the energization period are not particularly limited, but may be a current value of 5 to 12 kA, a welding force of 3 to 8 kN, and an energization period of 15 to 100 ms. Specific values may be determined in a preliminary test as described above. The above-described method is one example, and the above-described method may be modified as long as the spot welding method limits the upper limit of the electrode approach speed.

[0040] <3. Comparison of this embodiment with the prior art> Next, a comparison between this embodiment and the prior art will be described with reference to Figures 7(a) and (b). Figure 7(a) is a graph showing the electrode position, current value, and voltage value at each point in the welding period in the prior art (i.e., a technology that does not limit the upper limit of the lowering speed). Figure 7(b) is a graph showing the electrode position, current value, and voltage value at each point in the welding period in this embodiment. Note that the electrode position indicates the position of the upper electrode (when the upper electrode is the movable electrode 16a and the lower electrode is the fixed electrode 16b as shown in Figure 4). Position 0 is the position where the electrodes are in (direct) contact with each other (without multiple steel plates). In both Figures 7(a) and (b), welding was performed under conditions where the welding current was increased to increase the nugget diameter (targeting 5.5√t) (t: thickness of the thinnest steel plate included in the plate assembly).

[0041] As shown in Figure 7(a), in the prior art, the upper electrode rises in the early stage of current application, but expulsion occurs at the end of the later stage of current application, causing the upper electrode to rapidly descend. The final electrode plunge depth relative to the original plate thickness also increases. In contrast, in this embodiment, the descent speed is slower when expulsion occurs, and the final electrode plunge depth is also smaller.

[0042] As described above, at the end of the later welding stage, the molten metal splashes from the nugget edge to the surrounding area due to the expansion of the molten diameter (nugget diameter), causing a sudden decrease in the electrode pressure reaction force. However, in this embodiment, even if the pressure reaction force of the steel sheets 100a, 100b suddenly decreases at the end of the later welding stage, the movable electrode 16a is slowly lowered. Then, while the movable electrode 16a is slowly lowering, the nugget solidifies. Therefore, the indentation can be reduced. As a result, it is possible to suppress the occurrence of LME cracking even if one or both of the steel sheets 100a, 100b are zinc-based plated high-strength steel sheets.

[0043] <4. Spot welded joints> Next, the configuration of a spot welded joint 100 according to this embodiment will be described with reference to Figures 8 and 9. Figure 8 is a cross-sectional view of the spot welded joint 100 cut along a plane that passes through the center of the nugget 120 and is parallel to the thickness direction of the spot welded joint 100. Figure 10 is an X-ray image of the weld taken from the surface of the steel plate using X-ray transmission. The spot welded joint 100 includes steel plates 100a and 100b and a weld 110. Although Figure 8 shows two steel plates 100a and 100b spot welded together, three or more steel plates may also be spot welded together.

[0044] The steel plates 100a and 100b are the same as the steel plates described above. At least one of the steel plates 100a and 100b may be a steel plate with a tensile strength of 980 MPa or more. Both may be steel plates with a tensile strength of 980 MPa or more. In particular, when spot welding multiple steel plates with a tensile strength of 980 MPa or more together, a larger nugget diameter is often required to achieve higher joint strength. Therefore, the following describes a spot-welded joint with a larger nugget diameter that is manufactured by spot welding multiple steel plates with a tensile strength of 980 MPa or more together.

[0045] The welded portion 110 is a portion joined by the spot welding device 10, and includes a nugget 120 and recessed steel plate portions (recesses) 110a, 110b above and below the nugget 120. Furthermore, an expulsion portion 200 is formed near the nugget 120. Here, the steel plates 100a, 100b each have a tensile strength of 980 MPa or more. Furthermore, when three or more steel plates are spot welded, the tensile strength of at least two of the steel plates is 980 MPa or more.

[0046] The nugget diameter (the diameter of the nugget 200 in the cross section shown in FIG. 8 , passing through the center of the nugget 200 and extending in a direction perpendicular to the thickness direction of the weld 110; see FIG. 9 ) is 5.0√t (mm) or more, preferably 5.5√t (mm) or more, where t (mm) is the smallest thickness of the steel plates 100a, 100b (in the case of spot-welding three or more steel plates, the smallest thickness among the steel plates having a tensile strength of 980 MPa or more). Here, t (mm) is not the thickness of the thinnest steel plate, but is the smallest thickness (mm) among the steel plates that serve as strength members of the welded joint (for example, high-strength steel plates having a tensile strength of 980 MPa or more), and the nugget diameter is 5.0√t (mm) or more or 5.5√t (mm) or more. In this way, the spot-welded joint 100 according to this embodiment includes a relatively large nugget 120. When such a large nugget 120 is formed in the weld 110, expulsion 200 is likely to occur. The weld diameter can also be defined as a length similar to the nugget diameter. The weld diameter is the diameter of the welded portion 121. The welded portion 121 is formed around the nugget. In the welded portion 121, the solid metal constituting the steel plates 100a and 100b undergoes plastic flow and is welded to each other. The weld diameter is the length of a line segment passing through the center of the nugget and connecting the ends of the welded portion 121 (the ends are formed in the gap) (see FIG. 9). In order to increase the nugget diameter, spot welding can be performed with a large current or large heat input. For example, a spot welding method for obtaining a nugget diameter of 5.0√t (mm) or more or 5.5√t (mm) or more can be easily obtained by conducting preliminary tests.

[0047] Furthermore, the minimum thickness of the spot-welded joint 100 (the minimum thickness of the weld 110) (specifically, the length of the line segment connecting the bottoms of the recesses 110a, 110b) is 0.70 times or more the total thickness of the steel plates 100a, 100b (the total thickness of these steel plates when three or more steel plates are spot-welded). The minimum thickness of the spot-welded joint 100 may be 0.73 times or more, 0.75 times or more, 0.78 times or more, 0.81 times or more, or 0.84 times or more the total thickness of the steel plates 100a, 100b (the total thickness of these steel plates when three or more steel plates are spot-welded). Here, the minimum thickness of the weld 110 is the minimum thickness of the front and back surfaces of the weld 110 in the direction perpendicular to the contact surface of the steel plate 100b. The specific measurement method is as follows. That is, the minimum thickness of the spot-welded joint 100 (the minimum thickness of the welded joint 110) is determined by pinching the front and back surfaces of the indentation portion of the welded joint 110 with a caliper gauge and measuring the smallest thickness. For example, in the case of welding two steel plates, the minimum distance from the front surface of the indentation portion of the upper steel plate to the back surface of the indentation portion of the lower steel plate is determined as the minimum thickness of the welded joint. In the case of welding three or more steel plates, the minimum distance between the front and back surfaces of the indentation portions is determined as the minimum thickness of the welded joint. In this embodiment, since the indentation is kept small, the minimum thickness of the spot-welded joint 100 can be sufficiently large even if the nugget diameter is 5.0√t or larger. In other words, the depth of the recesses 110a, 110b is shallow. Note that the upper limit of the minimum thickness of the spot-welded joint 100 is not particularly limited, but from the viewpoint of ensuring the joining strength between the steel plates 100a, 100b, it may be 0.95 times or less, 0.90 times or less, or 0.88 times or less the sum of the thicknesses of the steel plates 100a, 100b.

[0048] Furthermore, in the X-ray transmission image of the weld 110, the area of the expulsion portion 200 is 30 to 160% of the area of the pressure-welded portion 121. In other words, the expulsion area ratio is 30 to 160%. Here, the expulsion area ratio is calculated as follows. First, a planar X-ray transmission image is acquired. FIG. 10 shows an example of a planar X-ray transmission image. The dark, approximately circular portion is the nugget 120. The pressure-welded portion 121 is formed around the periphery of the nugget 120, and the expulsion portion 200 extends from the pressure-welded portion 121. Next, a straight line passing through the center of the nugget and minimizing the nugget diameter is determined from this X-ray transmission image. Next, the spot-welded joint is cut, machined, and polished so that the plane containing the determined straight line and the plate thickness direction becomes the observation surface. The cut surface is observed, and the pressure-welded diameter, which is the diameter of the pressure-welded portion 121, is measured. Next, from the measured pressure-contact diameter, the area of the pressure-contact portion 121 (=π×(pressure-contact diameter / 2) 2 ) is obtained. The nugget diameter is also determined by observing the cross section. Meanwhile, the area of the expulsion portion 200 is obtained based on the X-ray image. The area of the expulsion portion is determined by finding the area of the region surrounded by the expulsion and the pressure-welded portion from the X-ray image and subtracting the above-mentioned pressure-welded portion area from that area. Next, the area of the expanse portion 200 is divided by the area of the pressure-welded portion 121, and the resulting value is defined as the expanse area ratio. Thus, in this embodiment, the area of the expanse portion 200 is limited to 160% or less of the area of the pressure-welded portion 121. If the area of the expanse portion 200 exceeds 160% of the area of the pressure-welded portion 121 (i.e., if the expanse area ratio exceeds 160%), the thickness of the welded portion 110 becomes insufficient, and sufficient joint strength cannot be obtained. If necessary, the expanse area ratio of the spot-welded joint may be set to 140% or less, 120% or less, 100% or less, 80% or less, 70% or less, 60% or less, or 55% or less. If necessary, the expanse area ratio may be set to 35% or more, 40% or more, 45% or more, or 50% or more.

[0049] Furthermore, it is preferable that the spot welded joint 100 is free of cracks. Here, "free of cracks" means that there are no cracks or voids in an X-ray inspection and a visual inspection of a cross section (including a line passing through the center of the nugget and in the plate thickness direction). Thus, in this embodiment, even though a relatively large nugget 120 is formed, the occurrence of cracks can be suppressed. [Example]

[0050] Next, an example of this embodiment will be described. In this example, in order to confirm the effect of this embodiment, a pair of steel plates was spot-welded under the conditions shown in Table 1. The steel plates were high-strength steel plate A (CR1180 steel, plate thickness 1.6 mm) with a tensile strength of 1180 MPa (1180 to 1330 MPa), Hot-dip galvanized steel sheet B (GA980 steel, sheet thickness 1.6 mm) with a tensile strength of 980 MPa (980-1130 MPa), Hot-dip galvanized steel sheet C (GA270 steel, sheet thickness 0.6 mm) with a tensile strength of 270 MPa (270-330 MPa), Hot-dip galvanized steel sheet D (GI980 steel, thickness 1.2 mm) with a tensile strength of 980 MPa (980-1130 MPa), A hot-stamped product E (thickness: 2.0 mm) made of hot-dip aluminized steel sheet with a tensile strength of 1.5 GPa (1500 to 1650 MPa) was used. The combinations of these steel sheets are shown in Table 1. For example, in the A / A combination, one pair of steel sheets A was spot-welded.

[0051] [Table 1]

[0052] For the initial pressure time and pressure force maintenance time, the set of steel sheets was pressed for 0.5 seconds and 0.2 seconds, respectively, at the pressure shown in Table 1. In addition, except for Comparative Example 6, the electrode approach speed of the movable electrode was controlled to 12.0 mm / s or less even during the pressure force maintenance time.

[0053] The current pattern for the current application period included not only the main current application but also a pre-current application + main current application. The conditions for the main current application are shown in Table 1. The welding current for the pre-current application was 9kA, the welding time was 40ms, and the pressure was the same as that for the main current application. The conditions for the pre-current application were 9kA and 40ms.

[0054] In these energization conditions, "welding current" refers to the current value passed between the electrodes during the energization period. The current value here is a value aimed at achieving a nugget diameter of 5.0√t (t: the smallest thickness (mm) of the steel sheets to be spot-welded) (5.5√t in Examples 7 and 8, and 4.0√t in Example 9 and Comparative Example 5). "Pressure force" refers to the set pressure applied to the pair of steel sheets during the welding period. "Welding time" refers to the length of the energization period. "Gap" refers to the gap between the steel sheets at the steel sheet overlap surface (the portion to be spot-welded).

[0055] The upper limit of electrode approach speed V in Table 1 D is the upper limit set for the descending speed of the movable electrode, and the upper limits for the current application period and the pressure force maintenance time are listed. Examples with numbers in parentheses are examples where no upper limit is set, and the maximum actual value of the electrode approach speed is listed in parentheses.

[0056] The "presence or absence of expulsion" was confirmed by visually observing the welding phenomenon during the energization period. When expulsion occurs, minute molten metal particles are scattered from the weld (more specifically, from the gap between the steel sheets), making it easily visible. The "expulsion area ratio" was measured using the method described above. A value of 30 to 160% is considered acceptable. The "nugget diameter" was determined from a photograph of the weld cross section. The "nugget diameter" section indicates the X value obtained by dividing the nugget diameter by √t (X = nugget diameter / √t). A nugget diameter of 5.0√t or greater satisfies the criteria for a welded joint according to the present invention. The "weld thickness to original plate thickness ratio" is the value obtained by dividing the minimum thickness of the weld (mm) by the total thickness of the steel sheets before welding, and is a parameter equivalent to indentation. A value of 0.70 or greater, which indicates minimal strength loss, is considered acceptable. Peel strength is the maximum load required to fracture a test piece, measured using a cross-tensile test according to JIS Z 3137:1999. In this example, a passing level is 4.0 kN or more. A higher peel strength is preferable, for example, 5.0 kN or more is more preferable. The presence or absence of "LME cracking" was determined by cutting the spot welded joint to an appropriate size and observing the cross section cut across the nugget using an optical microscope.

[0057] First, let us consider invention examples 1 to 15. In all invention examples, the "expulsion area ratio," "ratio of weld thickness to original plate thickness," and "peel strength" were at acceptable levels, and no LME cracking was observed. Note that invention example 1 is an example corresponding to the upper limit of the electrode approach speed. invention example 2 is an example corresponding to the vicinity of the (preferable) lower limit of the electrode approach speed. invention example 3 is an example in which the applied pressure was measured during the applied pressure maintenance time. invention example 4 is an example in which expulsion did not occur because the plate gap was crushed by pre-energization. Furthermore, indentation was small because the upper limit of the electrode approach speed was low. invention example 5 is an example of low applied pressure conditions. invention example 6 is an example of high applied pressure conditions.

[0058] Inventive Examples 7 and 8, the nugget diameter exceeded 5.5√t. In Inventive Example 7, three steel plates were spot welded. Even when a large nugget was formed, indentation was small and no LME cracking was observed. Inventive Example 9, the nugget diameter was approximately 4.0√t. Even when the nugget diameter was small, the target peel strength of 4.0 kN or more was achieved. However, the peel strength was slightly lower than that of the other inventive examples, being less than 5.0 kN. In Inventive Example 10, expulsion occurred during the pressure maintenance time. In Inventive Example 10, the electrode approach speed of the movable electrode was controlled to 12.0 mm / s or less even during the pressure maintenance time, resulting in small indentation and no LME cracking. In Inventive Examples 11 and 12, the steel plate thickness was changed. Good results were obtained even with the plate thickness used in Inventive Examples 11 and 12. Inventive Example 13, the gun rigidity was low. In the spot welding device according to this embodiment, the electrode approach speed of the movable electrode is controlled based on data showing the correlation between the rate of change (time rate of change) of the rotation speed and torque of the servo motor 14 and the electrode approach speed of the electrode (i.e., control is performed taking into consideration the deflection of the gun). As a result, indentation was reduced and no LME cracking was observed. Inventive Examples 14 and 15 are examples in which large nuggets were formed. In these examples as well, indentation was reduced and no LME cracking was observed.

[0059] Next, comparative examples 1 to 8 will be examined. In comparative example 1, pressure was applied using an air cylinder without setting an upper limit for the electrode approach speed. The maximum electrode approach speed of the movable electrode was 21.7 mm / s. Therefore, the "ratio of weld thickness to original plate thickness" was small (in other words, indentation was large). Furthermore, because the nugget shrank when expulsion occurred, the "peel strength" was also particularly low. LME cracking was also observed. In comparative example 2, the upper limit for the electrode approach speed exceeded 12.0 mm / s, resulting in LME cracking. The "ratio of weld thickness to original plate thickness" was also at an unacceptable level. In comparative example 3, the upper limit for the electrode approach speed during the current application period exceeded 12.0 mm / s. Because preliminary current was applied, expulsion could not be confirmed visually. However, expulsion was observed by X-ray observation. Furthermore, because the electrode approach speed increased in the later stages of current application, the "ratio of weld thickness to original plate thickness" was small (in other words, indentation was large). Furthermore, LME cracking also occurred.

[0060] Comparative Example 4 is an example in which no upper limit was set for the electrode approach speed. No LME cracking was observed, but the "ratio of weld thickness to original plate thickness" became small (in other words, indentation became large), and the peel strength was at an unacceptable level. Comparative Example 5 is an example in which a nugget diameter of 4√t was targeted. In this example, the electrode approach speed during the current application period also exceeded 12.0 mm / s. As a result, LME cracking was observed, the "ratio of weld thickness to original plate thickness" became small (in other words, indentation became large), and the peel strength was also at an unacceptable level. Comparative Example 6 is an example in which expulsion occurred during the pressure maintenance time. In Comparative Example 6, the upper limit for the electrode approach speed was high, so the movable electrode descended at a fast speed along with the occurrence of expulsion. As a result, the "ratio of weld thickness to original plate thickness" became small (in other words, indentation became large), and the peel strength was at an unacceptable level. In Comparative Example 7, the upper limit for the electrode approach speed exceeded 12.0 mm / s. As a result, the "ratio of weld thickness to original plate thickness" became smaller (in other words, the indentation became larger), and LME cracking was observed. In Comparative Example 8, the upper limit of the electrode approach speed also exceeded 12.0 mm / s. As a result, the "ratio of weld thickness to original plate thickness" became smaller (in other words, the indentation became larger), the peel strength became unacceptable, and LME cracking was also observed.

[0061] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0062] 10 Spot welding equipment 11 Control section 12 Storage section 13 Pressure measurement unit 14 Servo motor 15 Electrical part 16a Movable electrode 16b Fixed electrode 17 Input section

Claims

1. A spot welding device that can spot weld a plurality of overlapping steel sheets by clamping the overlapping steel sheets between a pair of electrodes and applying pressure to the overlapping steel sheets while passing current through the electrodes, limiting the speed at which the pair of electrodes approach each other to 1.0 mm / s or more and 12.0 mm / s or less from the start to the end of current flow; The spot welding device has a control unit and an electrode drive device, The control unit drives the electrode driving device so that the speed at which the pair of electrodes approach each other is equal to or less than an upper limit value. A spot welding device characterized by:

2. 2. The spot welding device according to claim 1, wherein the speed is limited to at least 12.0 mm / s or less during the period from the end of energization to the end of the pressurizing force maintaining time.

3. A control method for a spot welding device that spot-welds a plurality of overlapping steel sheets by sandwiching the overlapping steel sheets between a pair of opposing electrodes, applying pressure to the overlapping steel sheets, and passing current therethrough, comprising: limiting the speed at which the pair of electrodes approach each other to 1.0 mm / s or more and 12.0 mm / s or less from the start to the end of current flow; driving an electrode driving device so that the speed at which the pair of electrodes approach each other is equal to or less than an upper limit value; A method for controlling a spot welding apparatus, comprising:

4. 4. The method for controlling a spot welding device according to claim 3, wherein the speed is limited to at least 12.0 mm / s or less during the period from the start of energization to the end of the pressing force maintenance time.

5. A spot welded joint manufactured by overlapping and spot welding a plurality of steel plates having a tensile strength of 980 MPa or more, When the minimum thickness of the plurality of steel plates having a tensile strength of 980 MPa or more is t, the nugget diameter formed by the spot welding is 5.0√t or more, The minimum thickness of the spot welded joint is 0.70 times or more the total thickness of the plurality of steel plates, A spot welded joint characterized in that the area of the expulsion portion measured in an X-ray radiographic image of the weld is 30 to 160% of the area of the pressure-welded portion.

6. The spot welded joint according to claim 5, wherein the nugget diameter is 5.5√t or more.

7. 7. The spot welded joint according to claim 5, wherein the spot welded joint is free of cracks.

8. 7. The spot-welded joint according to claim 5, wherein the minimum thickness of the spot-welded joint is 0.90 times or less the total thickness of the plurality of steel plates.

9. The method for manufacturing a spot welded joint according to claim 5 or 6, A method for manufacturing a spot welded joint, comprising limiting the speed at which a pair of electrodes provided in a spot welding device approach each other to at least 12.0 mm / s or less from the start to the end of current flow.

10. The method for manufacturing a spot welded joint according to claim 9, wherein the speed is limited to at least 12.0 mm / s or less during the period from the end of energization to the end of the pressurizing force maintenance time.

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