How to determine welding conditions for resistance spot welding

Abstract

To provide a determination method of appropriate welding condition in a resistance spot weld of a plate set where surface treated steel plates each including a metal plating layer overlap each other.SOLUTION: A determination method of welding condition in a resistance spot weld determines, to be an appropriate spot welding condition, a spot welding condition that, among welding conditions of when spot welding is performed as a preliminary test, a nugget diameter dn is 1 / 4 to 2 / 3 inclusive of a target diameter, and a determination index being a value obtained by dividing a delivery diameter dc by a nugget diameter dn is 1.1 inclusive to 3 exclusive where the nugget diameter dn is a diameter of a molten section 5 in a direction along a plate surface of a plate set C, the delivery diameter dc is a diameter of an outer periphery of a delivery port 7, and the target diameter is a target value of the nugget diameter dn.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for determining welding conditions for resistance spot welding. [Background technology]

[0002] Resistance spot welding is a type of lap resistance welding. Resistance spot welding can be used to join overlapping steel plates. Resistance spot welding is a method in which a plate set of two or more overlapping steel plates is sandwiched, and pressure is applied to the top and bottom surfaces of the plate set with a pair of electrodes while a large welding current is passed between the electrodes for a short period of time to join the plate set.

[0003] In resistance spot welding, a phenomenon called "expansion" in which molten metal splashes may occur. Explosion is recognized as a defect or problem in spot welding. Explosion is likely to occur when the welding current value or welding pressure is inappropriate.

[0004] Non-Patent Document 1 considers that splashing may be suppressed when plating wetting and spreading over a wide area.

[0005] Patent Document 1 discloses a spot welding method for high-tensile zinc-plated steel sheets. In this spot welding method, when spot welding high-tensile zinc-plated steel sheets by multi-stage current conduction in three or more stages, the welding conditions such as current conduction time and welding current are adjusted so that an appropriate current range ΔI is 1.0 kA or more, preferably 2.0 kA or more, which can stably form a nugget having a desired nugget diameter or more and a molten residual thickness of 0.05 mm or more. In this spot welding method, it is preferable that the current conduction time of the first stage current conduction is 2 to 6 cycles, and that a cooling time of 1 to 3 cycles is provided between each current conduction from the second stage current conduction to the final stage current conduction. It is also preferable that the total current conduction time from the second stage current conduction to the final stage current conduction is set to 1 to 5 times the current conduction time of the first stage.

[0006] Patent Document 2 discloses a resistance spot welding method. In this resistance spot welding method, in the initial current flow of resistance spot welding, when "1.2≦outer dimension of alloy layer / outer dimension of weld nugget≦1.5", current flow between the electrodes is stopped, and after a predetermined interval has elapsed, main current flow is performed. As a result, the weld nugget is cooled during the interval, and even if the electrical resistance value in the region where the weld nugget is formed is reduced and main current flow is started thereafter, localized expansion of the weld nugget is suppressed, and spatter generation can be suppressed.

[0007] Patent Document 3 discloses a resistance spot welding method. In this resistance spot welding method, a constant current period is provided between the initial current and the main current, during which a constant current is passed between the electrodes, and when "1.2≦outer dimension of alloy layer / outer dimension of weld nugget≦1.5" is satisfied during this constant current period, the constant current period ends and the main current is started. This makes it possible to suppress the amount of heat generated during the constant current period, and to make the temperature distribution in the plate assembly uniform by heat conduction, and to transition to the main current in a state in which the ratio between the outer dimension of the alloy layer A and the outer dimension of the weld nugget N is appropriately obtained, and therefore it is said that it is possible to efficiently form the weld nugget while suppressing the generation of spatter during the main current.

[0008] Patent Document 4 discloses a resistance spot welding method and a method for determining welding conditions for resistance spot welding. In this resistance spot welding method, at least one steel sheet is a high-strength zinc-based plated steel sheet having a tensile strength of 980 MPa or more and 1770 MPa or less and having a zinc-based plating layer on the surface of the joining surface side. This resistance spot welding method includes a pre-current step and a main current step in which current is passed at a higher current value than in the pre-current step after the pre-current step. The pre-current step is performed under welding conditions such that, when the pre-current step is completed, the thickness of the thinner steel sheet of the two adjacent steel sheets to be joined is t [mm], and a region where the zinc-based plating is exhaled with a diameter of 2√t [mm] or less is formed, and the main current step is performed under welding conditions such that a nugget with a diameter of 3√t [mm] or more is formed. It is said that this resistance spot welding method can form a weld with excellent delayed fracture resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2003-236676 A [Patent Document 2] Patent Publication No. 2021-79416 [Patent Document 3] Patent Publication No. 2021-79410 [Patent Document 4] JP 2018-171649 A [Non-patent literature]

[0010] [Non-Patent Document 1] Ji et al., Journal of Mechanical Science and Technology 28 (11) (2014) 4761-4769 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a method for determining appropriate welding conditions in resistance spot welding of a sheet assembly in which surface-treated steel sheets having a metal plating layer are overlapped. [Means for solving the problem]

[0012] In order to achieve the above object, a method for determining welding conditions for resistance spot welding according to the present invention is as follows.

[0013] [1] A sheet assembly including a first steel sheet having a metal plating layer and a second steel sheet overlapped opposite the metal plating layer is sandwiched between a pair of electrodes and pressurized while passing a current between the electrodes to form a molten part between the first steel sheet and the second steel sheet to join the sheet assembly; A method for determining welding conditions for resistance spot welding in which a component of the metal plating layer is extruded to form an extrusion portion between the first steel sheet and the second steel sheet in an outer region in a direction along the sheet surfaces of the sheet assembly as viewed from the fusion zone, the method comprising: In the fusion part, the diameter in the direction along the plate surface of the plate assembly is the nugget diameter (dn) [mm], the diameter of the outer periphery of the discharge part is the discharge diameter (dc) [mm], and the target value of the nugget diameter (dn) [mm] is the target diameter (D) [mm]. Among the welding conditions when spot welding was performed as a preliminary test, a method for determining welding conditions for resistance spot welding, the method determining, as appropriate spot welding conditions, welding conditions in which the nugget diameter (dn) is greater than or equal to one-quarter and less than or equal to one-half of the target diameter (D), and a judgment index (k) calculated by dividing the discharge diameter (dc) by the nugget diameter (dn) is greater than or equal to 1.1 and less than 3.

[0014] [2] A method for determining welding conditions for resistance spot welding as described in [1] above, wherein the target diameter (D) is four or more times the square root of the sheet thickness [mm] of the thinner steel sheet between the first steel sheet and the second steel sheet.

[0015] [3] The method for determining welding conditions for resistance spot welding according to the above [1] or [2], wherein the metal plating layer is mainly composed of aluminum or zinc.

[0016] [4] The method for determining welding conditions for resistance spot welding according to the above [3], wherein the metal plating layer contains aluminum and zinc in a total amount of 85% or more and 99.5% or less.

[0017] [5] The method for determining welding conditions for resistance spot welding according to the above [4], wherein the metal plating layer contains at least one of Zn, Mg, and Sn in a total amount of 0.5% to 15%.

[0018] [6] In the preliminary test, A plurality of joint samples having different welding current values ​​are prepared by setting a pressure between the electrodes as a first pressing force and a time for passing current between the electrodes as a first current passing time, and the nugget diameter (dn) of each joint sample is measured, thereby determining a weld lobe, which is a relationship between the welding current value and the nugget diameter (dn); Based on the weld lobe, a proper current range (ΔW) is determined from a welding current value at which the nugget diameter (dn) is equal to four times the square root of the sheet thickness of the thinner steel sheet of the first steel sheet and the second steel sheet, to a welding current value at which expulsion occurs; If the appropriate current range (ΔW) is less than a predetermined target value, a first judgment spot welding test is performed while setting the pressure between the electrodes to the first pressure force and a time for passing current between the electrodes to a second current passing time which is shorter than the first current passing time, and the judgment index is obtained; a second judgment spot welding test is performed based on the value of the judgment index in the first judgment spot welding test, the second judgment spot welding test is performed by setting the applied pressure between the electrodes to a second pressure different from the first pressure and setting a time for passing current between the electrodes to a second current passing time, and the judgment index is obtained; The method for determining welding conditions for resistance spot welding according to any one of [1] to [5] above, wherein if the value of the judgment index in the second judgment spot welding test is 1.1 or more and less than 3, the second pressure force is determined to be an appropriate spot welding condition.

[0019] [7] A method for determining welding conditions for resistance spot welding as described in [6] above, in which, if the value of the judgment index in the second judgment spot welding test is not greater than 1.1 and less than 3, the second pressure is changed to another value and the next judgment spot welding test is repeated.

[0020] [8] The method for determining welding conditions for resistance spot welding according to the above [7], wherein the time during which current is passed between the electrodes in the judgment spot welding test is the second current passing time.

[0021] [9] Based on the value of the judgment index in the immediately preceding judgment spot welding test, a new pressure between the electrodes is determined and a next judgment spot welding test is performed to obtain a new judgment index; The method for determining welding conditions for resistance spot welding according to the above-mentioned [8], wherein if the finally obtained value of the judgment index is equal to or greater than 1.1 and less than 3, the finally determined pressure between the electrodes is determined to be the appropriate spot welding condition.

[0022]

[10] A method for determining welding conditions for resistance spot welding as described in [9] above, in which, if the finally determined judgment index value is not greater than 1.1 and less than 3, the last determined pressure between the electrodes is changed to another value and the judgment spot welding test is repeated.

[0023]

[11] A method for determining welding conditions for resistance spot welding described in any one of [6] to

[10] above, wherein, if the value of the judgment index in the first judgment spot welding test is less than 1.1, the second pressure is set to be greater than the first pressure.

[0024]

[12] A method for determining welding conditions for resistance spot welding described in any one of [6] to

[11] above, wherein the second pressure is set to be smaller than the first pressure when the judgment index value in the first judgment spot welding test is 3 or more.

[0025]

[13] A method for determining welding conditions for resistance spot welding as described in any one of [7] to

[10] above, wherein if the value of the judgment index in the second judgment spot welding test is less than 1.1, the second pressure force is changed to a larger value.

[0026]

[14] A method for determining welding conditions for resistance spot welding as described in any one of [7] to

[10] or

[13] above, in which, if the value of the judgment index in the second judgment spot welding test is 3 or more, the second pressure force is changed to a smaller value.

[0027]

[15] A method for determining welding conditions for resistance spot welding as described in

[10] or

[11] above, in which, if the finally determined value of the judgment index is less than 1.1, the last determined pressure between the electrodes is changed to a larger value.

[0028]

[16] A method for determining welding conditions for resistance spot welding as described in

[10] ,

[11] or

[15] above, wherein if the finally determined value of the judgment index is 3 or more, the last determined pressure between the electrodes is changed to a smaller value. Effect of the Invention

[0029] According to the present invention, a method for determining appropriate welding conditions in resistance spot welding of a sheet assembly in which surface-treated steel sheets having a metal plating layer are overlapped can be provided. [Brief description of the drawings]

[0030] [Figure 1] FIG. 2 is a diagram illustrating a welding method for resistance spot welding. [Diagram 2] 1A to 1C are diagrams illustrating the state of a welded portion by resistance spot welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] A method for determining welding conditions for resistance spot welding according to this embodiment will be described with reference to the drawings.

[0032] First, an overview of the method for determining welding conditions for resistance spot welding according to this embodiment will be described.

[0033] The method for determining the welding conditions for resistance spot welding according to this embodiment relates to resistance spot welding in which, as shown in FIG. 1 , a sheet set C including a first steel sheet 1 having a metal plating layer 10 and a second steel sheet 2 overlapping opposite the metal plating layer 10 is sandwiched between a pair of electrodes 3, 4, pressure is applied while an electric current is passed between the electrodes 3, 4, and a molten portion 5 is formed between the first steel sheet 1 and the second steel sheet 2 to join the sheet set C, and as shown in FIG. 2 , an extrusion portion 7 is formed by extruding components of the metal plating layer between the first steel sheet 1 and the second steel sheet 2 in an outer region in a direction along the sheet surfaces of the sheet set C as viewed from the molten portion 5.

[0034] In addition, Fig. 1 illustrates an example in which the second steel sheet 2 also has a metal plating layer 20. In the example illustrated in Fig. 1, the first steel sheet 1 and the second steel sheet 2 are overlapped with the metal plating layer 10 and the metal plating layer 20 facing each other.

[0035] 2, the method of determining the welding conditions for resistance spot welding according to this embodiment is as follows: the diameter in the direction along the plate surface of the plate assembly C in the fusion zone 5 is the nugget diameter dn, the diameter of the outer periphery of the extrusion part is the extrusion diameter dc, and the target value of the nugget diameter dn is the target diameter D. Among the welding conditions when spot welding is performed as a preliminary test, the welding conditions under which the nugget diameter dn is 1 / 4 or more and 2 / 3 or less of the target diameter D, and the value obtained by dividing the extrusion diameter dc by the nugget diameter dn is 1.1 or more and less than 3, are set as appropriate spot welding conditions. The units of the nugget diameter (dn), the extrusion diameter (dc), and the target diameter (D) are [mm] unless otherwise specified.

[0036] That is, the judgment index k is defined by the following formula (1): In the method for determining welding conditions for resistance spot welding according to this embodiment, on the premise that the nugget diameter dn satisfies the following formula (2), the welding conditions for which the judgment index k satisfies the following formula (3) are set as appropriate spot welding conditions.

[0037] k = dc / dn (1)

[0038] D / 4≦dn≦2D / 3 (2)

[0039] 1.1≦k<3 (3)

[0040] According to the method for determining welding conditions for resistance spot welding according to this embodiment, appropriate welding conditions can be determined for resistance spot welding of a sheet assembly in which surface-treated steel sheets having a metal plating layer are overlapped.

[0041] The method for determining welding conditions for resistance spot welding according to this embodiment will be described in detail below.

[0042] First, a description will be given of the plate set C that is the target of resistance spot welding, the resistance spot welding, and the welded portion when the plate set C is welded.

[0043] The plate group C may include at least the first steel plate 1 and the second steel plate 2. The plate group C may be formed of three or more steel plates. The steel type of the steel plates (the first steel plate 1, the second steel plate 2, and the other steel plates) included in the plate group C is not particularly limited. The manufacturing method of these steel plates may be any method such as cold rolling or hot rolling, and the structure of the steel plates may be any method. In addition, there is no problem even if hot-pressed steel plates are used as the steel plates included in the plate group C. In addition, there is no problem with the thickness of the steel plates included in the plate group C. For example, steel plates having a thickness that can be used for general automobile bodies (thickness of about 0.5 to 4.0 mm) can of course be used.

[0044] The steel plates included in the plate set C may have different plate thicknesses. A steel plate other than the first steel plate 1 (for example, the second steel plate 2) may or may not have a metal plating layer.

[0045] The metal plating layer 10 is preferably mainly composed of aluminum (Al) or zinc (Zn). The metal plating layer 10 may contain at least one of iron (Fe), silicon (Si), zinc (Zn), magnesium (Mg), and tin (Sn).

[0046] The metal plating layer 10 preferably contains aluminum and zinc in a total amount of 85% to 99.5%. In the present embodiment, the term "%" simply refers to "% by mass."

[0047] The metal plating layer 10 preferably contains at least one of zinc, magnesium, and tin in a total amount of 0.5% to 15%.

[0048] The metal plating layer 20 is not essential. When the second steel sheet 2 has the metal plating layer 20, the metal plating layer 20 is the same as the metal plating layer 10 described above.

[0049] Resistance spot welding (resistance spot welding method) is a welding method that forms spot welds by utilizing the resistance heat generated in a plate assembly C by passing a large welding current through electrodes 3 and 4 that clamp and apply pressure to the plate assembly C.

[0050] In the example shown in FIG. 2, the welded portion includes a dot-like fusion portion 5 and a corona bond portion 6 formed in an outer region along the outer periphery of the fusion portion 5.

[0051] The point-like molten portion 5 is also called a nugget, and is a portion that melts and then solidifies at the contact portion (hereinafter sometimes simply referred to as between the steel plates) between the first steel plate 1 and the second steel plate 2 when a current (hereinafter sometimes referred to as a welding current) is passed through the sheet assembly C by the electrodes 3, 4 (hereinafter sometimes simply referred to as during welding). In this embodiment, the nugget diameter dn is the diameter of the molten portion 5 in the direction along the sheet surfaces of the sheet assembly C, i.e., the diameter of the outer circumferential shape of the molten portion 5.

[0052] The shape of the fusion zone 5 when viewed from above (from a perspective perpendicular to the plate surfaces of the plate assembly C) is usually circular. Therefore, as shown in Fig. 2, the width of the fusion zone 5 measured by observing the cross section of the welded portion may be regarded as the nugget diameter dn.

[0053] The corona bond portion 6 is a portion where the steel sheets are solid-state welded together.

[0054] The first steel plate 1 and the second steel plate 2 are joined at a fusion zone 5 or at a fusion zone 5 and a corona bond zone 6 .

[0055] The extrusion portion 7 is an outer region of the welded portion (molten portion 5 and corona bond portion 6) in the direction along the sheet surfaces of the sheet assembly C when the plating components of the metal plating layers 10, 20 melt during welding, flow out between the first steel sheet 1 and the second steel sheet 2, and then solidify. In this embodiment, the extrusion diameter dc is the diameter of the outer circumferential shape of the extrusion portion 7 in the direction along the sheet surfaces of the sheet assembly C. As shown in Fig. 2, the width of the outer circumferential shape of the extrusion portion 7 measured by observing the cross section of the welded portion may be regarded as the extrusion diameter dc.

[0056] A welding device that can be used for the resistance spot welding according to this embodiment may be any device that includes a pair of upper and lower electrodes 3, 4 and is capable of arbitrarily controlling the pressure between the electrodes 3, 4, i.e., the pressure applied to the plate set C during welding, and the welding current. The electrodes 3, 4 may be pressed against the plate surfaces of the plate set C perpendicularly or at an angle.

[0057] There are no particular limitations on the pressure mechanism (air cylinder, servo motor, etc.) and type (fixed type, robot gun, etc.) of the welding device, the electrode shape, etc. As the type of the tip of the electrodes 3 and 4, for example, DR type (dome radius type), R diameter (radius type), D type (dome type), etc. described in JIS C 9304:1999 may be adopted.

[0058] The welding current value, welding time, and welding pressure during welding may or may not be constant. For example, a control method may be used in which parameters such as resistance and voltage during welding are monitored and the welding current value and welding time are changed based on the fluctuations.

[0059] When the value of the judgment index k(dc / dn) is small, the electrical path (current diameter) during welding is small, the heat generated between the steel sheets is excessive, and splashing is likely to occur. When the value of the judgment index k is large, the electrical path (current diameter) is also large, and there are cases where sufficient heat generation to form the fusion zone 5 cannot be obtained.

[0060] The target diameter D is a target value of the nugget diameter dn, and is a guideline for the size of the fusion zone 5 to achieve an appropriate joint. The target diameter D should be four or more times the square root of the thickness [mm] of the thinner steel plate out of the first steel plate 1 and the second steel plate 2.

[0061] It is believed that the expulsion that occurs during resistance spot welding is greatly affected by the heat generation pattern of the welded portion at the beginning of current flow. This heat generation pattern correlates with the spreading of plating components such as the metal plating layer 10 between the steel sheets (the formation process of the extrusion part 7 and the size of the extrusion diameter dc). This spreading is also correlated with the electrical path (current flow diameter or resistance). Therefore, it is believed that by appropriately repeating heating and cooling during resistance spot welding, the spreading (the formation state of the extrusion part 7) becomes stable, and the joint becomes stable in correlation with this. As a result, by setting the welding conditions so that the ratio of the nugget diameter dn to the extrusion diameter dc becomes appropriate, that is, by making the judgment index k satisfy the formula (3), the formation of the fusion zone 5 can be stabilized.

[0062] If we define appropriate welding conditions as those that result in little expulsion, assuming that plate assembly C can be joined with a specified strength, then in the resistance spot welding of this embodiment, welding conditions that result in a wide appropriate range of welding current (range of welding current that can suppress the occurrence of expulsion) can be evaluated as being robust and excellent welding conditions. EXAMPLES

[0063] In the following, examples will be described with reference to comparative examples. In the following examples and comparative examples, preliminary welding tests were conducted on the resistance spot welding of the plate assembly shown in Table 1, and appropriate spot welding conditions were determined based on the preliminary test results. In the following, when Example 1 according to the present invention and Comparative Example 1 are collectively described, they will be referred to as this example, etc.

[0064] [Table 1]

[0065] The melting points of the base materials of steel sheets 1 to 3 shown in Table 1 are in the range of 1400 to 1570°C. Steel sheets 1 to 3 are plated steel sheets having a metal plating layer, and the melting points of the plating of the plating layer of these steel sheets are in the range of 400 to 700°C, respectively.

[0066] The plating layers of steel sheets 1 to 3 contain aluminum and zinc as main components in a total amount of 95%.

[0067] The tensile strengths shown in Table 1 were determined by preparing JIS No. 5 tensile test pieces for steel plates 1 to 3 in a direction parallel to the rolling direction and conducting tensile tests in accordance with the provisions of JIS Z 2241:2011.

[0068] In the present embodiment, an inverter DC resistance spot welding machine was used as the welding device. DR-type chromium copper electrodes were used as the welding electrodes. The same set of electrodes was used. Only main current was passed, and the current value (welding current) during current was kept constant. The pressure applied by the electrodes to the sheet assembly was the pressure applied at the end of current passing. The pressure applied by the electrodes to the sheet assembly was kept constant throughout the process of holding the pressure during current passing and at the end of current passing.

[0069] In this embodiment, the resistance spot welding was performed at room temperature, with the electrodes constantly water-cooled.

[0070] In this embodiment and the like, the target diameter of the nugget diameter dn was set to four times the square root of the sheet thickness of the thinner steel sheet among the steel sheets constituting the sheet assembly shown in Table 1.

[0071] Table 2 shows the experimental conditions and evaluation results of the resistance spot welding in Comparative Example 1. Table 3 shows the experimental conditions and evaluation results of the resistance spot welding in Example 1.

[0072] [Table 2]

[0073] [Table 3]

[0074] The nuggets evaluated in resistance spot welding in Example 1 and Comparative Example 1 are the nuggets formed between the steel plates listed under "Steel Plates" in the "Sheet Combination" section in Tables 2 and 3. That is, in Tables 2 and 3, under Conditions 1-2 and 2-2, steel plates 1, 2, and 3 of sheet combination No. B are overlapped and resistance spot welded, and the nugget diameter dn of the nugget formed between steel plate 2 and steel plate 3 is evaluated. Under the other conditions, the nugget diameter dn of the nugget formed between steel plate 1 and steel plate 2 is evaluated.

[0075] In Tables 2 and 3, "pressure" refers to the pressure between the electrodes, i.e., the pressure N applied to the sheet assembly by the electrodes. "Current application time" refers to the time s (seconds) that electricity is applied to the sheet assembly via the electrodes to perform resistance spot welding.

[0076] In Tables 2 and 3, "ΔW" indicates the appropriate current range ΔW. The appropriate current range ΔW is the value (kA) obtained by subtracting the welding current value (C1) from the welding current value (C2) at which expulsion occurs during welding, after confirming the welding current value (C2) at which expulsion occurs from the welding current value (C1) at which the nugget diameter dn is at least one-fourth of the target diameter D. This is the appropriate range of welding current at which resistance spot welding can be performed without expulsion. The other items in Tables 2 and 3 will be described later.

[0077] The first preliminary test (the first test in the column of appropriate current range evaluation in Tables 2 and 3) of Example 1 and Comparative Example 1 will be described.

[0078] In the first preliminary test, 25 types of joint samples (welded joints) were manufactured (i.e., 25 experiments were performed) while increasing the welding current value by 0.2 kA from 3.0 kA to 8.0 kA at the specified welding pressure listed in the first test in the column for the appropriate current range evaluation in Table 2 or Table 3.

[0079] Further, for each joint sample, the welded portion was cut in the thickness direction of the plate assembly. Then, the cross section was mirror-polished and observed with an optical microscope (magnification 10 times) to measure the nugget diameter dn. Then, the relationship between the measured nugget diameter dn and the corresponding welding current value (i.e., the weld lobe) was obtained. Then, the welding current value (C1) at which the nugget diameter dn becomes 1 / 4 or more of the target diameter D and the welding current value (C2) until the occurrence of expulsion during welding were derived from the weld lobe. The welding current value (C1) and the welding current value (C2) may be obtained, for example, by deriving a logarithmic approximation function from the weld lobe. Then, the appropriate current range ΔW was obtained from the welding current value (C1) and the welding current value (C2). In this embodiment, when the appropriate current range ΔW is equal to or greater than the target value of 1.0 kA, it is judged as good (OK), and when it is less than 1.0 kA, it is judged as bad (NG).

[0080] The second and subsequent preliminary tests of Comparative Example 1 (the second and subsequent tests in the column for evaluation of the appropriate current range in Table 2) will be described.

[0081] In Comparative Example 1, while changing the welding conditions (pressure) until the optimum current range ΔW reached the target value of 1.0 kA or more, 25 types of joint samples were manufactured with different welding current values ​​for each welding condition (pressure) as in the first preliminary test, and a weld lobe was obtained for each preliminary test to determine the optimum current range ΔW. Although a detailed explanation is omitted, in Comparative Example 1, it was necessary to perform up to six preliminary tests.

[0082] For example, in condition No. 1-1 shown in Table 2, the optimum current range ΔW was 0.8 kA under the conditions of a pressure of 1500 N and a current flow time of 0.42 seconds, so in the second and subsequent tests, the pressure was increased to 2000 N, 2500 N, and so on, and weld lobes were obtained for each, and welding conditions (pressure) under which the optimum current range ΔW was 1.0 kA or more were determined. Under these conditions, it was necessary to perform the first preliminary test to the third preliminary test, and the total number of experiments (the number of joint samples manufactured) reached 75 (75 pieces). Under the other conditions in Comparative Example 1, the total number of experiments was also required from 75 to 150.

[0083] In Example 1, as shown in Table 3, when the appropriate current range ΔW obtained in the first preliminary test was less than the target value of 1.0 kA (NG), a first judgment spot welding test (the first midway stop test in Table 3, an example of a judgment spot welding test) was first conducted as a preliminary test.

[0084] In the first judgment spot welding test, the pressure between the electrodes was set to the same pressure (hereinafter referred to as the first pressure) as in the first preliminary test, the current time between the electrodes was set to a second current time (0.08 s in this embodiment) shorter than the current time in the first preliminary test (hereinafter referred to as the first current time), and the welding current was set to 6.0 kA, and one joint sample was manufactured for each condition by performing a resistance spot welding test. Then, in the same manner as in the first preliminary test, the judgment index was obtained, and the cross section of the joint sample was observed with an optical microscope to measure the nugget diameter dn and the discharge diameter dc to obtain the judgment index.

[0085] Next, a second judgment spot welding test (the second midway stop test in Table 3, an example of a judgment spot welding test) was conducted based on the judgment index obtained in the first judgment spot welding test. In the second judgment spot welding test, the pressure between the electrodes was set to a second pressure force different from the first pressure force, and the time for which electricity was passed between the electrodes was set to a second current passing time, and the rest of the test was the same as in the first judgment spot welding test, and a joint sample was produced for each condition.

[0086] The second pressurizing force was determined as follows: When the judgment index value in the first judgment spot welding test was less than 1.1, the second pressurizing force was set to be larger than the first pressurizing force.

[0087] Furthermore, when the judgment index value in the first judgment spot welding test was 3 or more, the second applied pressure was set to be smaller than the first applied pressure.

[0088] If the judgment index value of the joint sample manufactured in the second judgment spot welding test was 1.1 or more and less than 3, the second pressure force was determined to be the appropriate spot welding condition (Conditions No. 2-1, 2-4 to 2-8). In Table 3, if the judgment index value was 1.1 or more and less than 3, it was evaluated as appropriate (OK), and if it was not 1.1 or more and less than 3, it was evaluated as poor (NG).

[0089] To give a specific example for condition No. 2-1, in the first preliminary test (first time in the column for evaluating the appropriate current range), the appropriate current range ΔW was 0.8kA and unacceptable under welding conditions of a pressure of 1500N and current flow time of 0.42s, so a stop test (first time) was performed as the first judgment spot welding test with the same pressure to obtain a judgment index. However, since the judgment index of the first judgment spot welding test was 1.0 (NG), the pressure was increased to 2500N and a stop test (second time) was performed as the second judgment spot welding test, and the judgment index value became 1.2 (OK). As a result, a pressure of 2500N was determined to be the appropriate spot welding condition for condition No. 2-1.

[0090] If the judgment index value of the joint sample produced in the second judgment spot welding test was not 1.1 or more and less than 3, a judgment spot welding test similar to the second judgment spot welding test was repeated until the judgment index value was 1.1 or more and less than 3.

[0091] In this judgment spot welding test, a new pressure between the electrodes was determined based on the value of the judgment index in the immediately preceding judgment spot welding test (for example, the second test for the third test, or the third test for the fourth test), and a welding test was performed to obtain a new judgment index.

[0092] If the final judgment index value was 1.1 or more and less than 3 (the third test of the midway stop test under condition No. 2-3), the final determined pressure between the electrodes was determined to be the appropriate spot welding condition.

[0093] If the final judgment index value was not greater than 1.1 and less than 3 (the third test of the mid-way stop test under condition No. 2-2), the final determined pressure between the electrodes was changed to another value and the judgment spot welding test (the fourth test of the mid-way stop test under condition No. 2-2) was repeated.

[0094] Specifically, when the judgment index value in the second judgment spot welding test was not equal to or greater than 1.1 and less than 3, the second pressure force was changed to another value as follows.

[0095] When the judgment index value in the second judgment spot welding test was less than 1.1, such as in the second mid-stop test under condition 2-3, the second pressure force in the third mid-stop test was changed to a value greater than the second (immediately preceding) value.

[0096] In addition, when the value of the judgment index in the second judgment spot welding test and the subsequent judgment spot welding test is 3 or more, such as in the second and third mid-stop tests under condition 2-2, the second pressure force in the third and fourth mid-stop tests was changed to a value smaller than the value in the third and fourth tests (immediately before).

[0097] That is, in the judgment spot welding test, if the finally determined judgment index value was less than 1.1, the last determined pressure between the electrodes was changed to a larger value, and if the finally determined judgment index value was 3 or more, the last determined (immediately preceding) pressure between the electrodes was changed to a smaller value.

[0098] The current flow time in these judgment spot welding tests was the second current flow time.

[0099] For example, in the case of condition No. 2-2, the judgment index value was 2.8 (OK) in the fourth midway stop test as a judgment spot welding test. Therefore, in condition No. 2-2, the pressure force of 3600 N, which was the pressure force in the fourth midway stop test, was determined to be the appropriate spot welding condition.

[0100] In addition, in all cases of Conditions No. 2-1 to 2-8, the nugget diameter dn of the joint sample produced in the final midway stop test was greater than or equal to one-quarter and less than or equal to two-thirds of the target diameter D.

[0101] In Example 1, the appropriate spot welding conditions (applying force) determined as described above were used to perform a confirmation test (the last one in the column for appropriate current range evaluation in Table 3), and a weld lobe was obtained in the same manner as in the first preliminary test. Furthermore, an appropriate current range ΔW was obtained from the weld lobe in the same manner as in the first preliminary test, and it was confirmed that the appropriate spot welding conditions determined as described above were actually appropriate (1.0 kA or more).

[0102] In Example 1, the total number of experiments (number of joint samples manufactured) was sufficient at most 54 times (54 pieces) even at the maximum (Condition No. 2-2).

[0103] That is, in Example 1, it was possible to determine the appropriate welding conditions in which the appropriate current range ΔW was 1.0 kA or more with a small number of trials.

[0104] In this manner, a method for determining appropriate welding conditions in resistance spot welding of a sheet assembly in which surface-treated steel sheets having a metal plating layer are overlapped can be provided.

[0105] In addition, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, provided no contradiction arises. Furthermore, the embodiments disclosed in this specification are illustrative, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope that does not deviate from the purpose of the present invention. [Industrial Applicability]

[0106] The present invention can be applied to a method for determining welding conditions for resistance spot welding. [Explanation of symbols]

[0107] 1: First steel plate 10: Metal plating layer 2:Second steel plate 20: Metal plating layer 3: Electrode 4: Electrode 5: Welded part 6: Corona Bond Section 7: Discharge section C: Board set dc: Discharge diameter dn: nugget diameter

Claims

1. A sheet assembly including a first steel sheet having a metal plating layer and a second steel sheet overlapped opposite the metal plating layer is sandwiched between a pair of electrodes, and while applying pressure, a current is passed between the electrodes to form a molten part between the first steel sheet and the second steel sheet, thereby joining the sheet assembly; A method for determining welding conditions for resistance spot welding in which a component of the metal plating layer is extruded to form an extrusion portion between the first steel sheet and the second steel sheet in an outer region in a direction along the sheet surfaces of the sheet assembly as viewed from the fusion zone, the method comprising: When the diameter of the fusion portion in the direction along the plate surface of the plate assembly is the nugget diameter (dn) [mm], the diameter of the outer periphery of the discharge portion is the discharge diameter (dc) [mm], and the target value of the nugget diameter (dn) is the target diameter (D) [mm], Among the welding conditions when spot welding was performed as a preliminary test, the method for determining welding conditions for resistance spot welding determines, as appropriate spot welding conditions, welding conditions under which the nugget diameter (dn) is equal to or greater than one-quarter and equal to or less than one-half of the target diameter (D) and a judgment index (k) calculated by dividing the discharge diameter (dc) by the nugget diameter (dn) is equal to or greater than 1.1 and less than 3.

2. 2. The method for determining welding conditions for resistance spot welding according to claim 1, wherein the target diameter (D) is four times or more the square root of the sheet thickness [mm] of the thinner steel sheet of the first steel sheet or the second steel sheet.

3. 3. The method for determining welding conditions for resistance spot welding according to claim 1, wherein the metal plating layer is mainly composed of aluminum or zinc.

4. 4. The method for determining welding conditions for resistance spot welding according to claim 3, wherein the metal plating layer contains aluminum and zinc in a total amount of 85% to 99.5%.

5. 5. The method for determining welding conditions for resistance spot welding according to claim 4, wherein the metal plating layer contains at least one of zinc, magnesium, and tin in a total amount of 0.5% to 15%.

6. In the preliminary test, a first pressing force is set as a pressure between the electrodes, and a time for passing current between the electrodes is set as a first current passing time, and a plurality of joint samples having different welding current values ​​are prepared, and the nugget diameter (dn) of each joint sample is measured, thereby determining a weld lobe, which is a relationship between the welding current value and the nugget diameter (dn); Based on the weld lobe, a proper current range (ΔW) is determined from a welding current value at which the nugget diameter (dn) is equal to four times the square root of the sheet thickness of the thinner steel sheet of the first steel sheet and the second steel sheet, to a welding current value at which expulsion occurs; If the appropriate current range (ΔW) is less than a predetermined target value, a first judgment spot welding test is performed while setting the pressure between the electrodes to the first pressure force and a time for passing current between the electrodes to a second current passing time which is shorter than the first current passing time, and the judgment index is obtained; a second judgment spot welding test is performed based on the value of the judgment index in the first judgment spot welding test, the second judgment spot welding test is performed by setting the applied pressure between the electrodes to a second pressure different from the first pressure and setting a time for passing current between the electrodes to a second current passing time, and the judgment index is obtained; 2. The method of determining welding conditions for resistance spot welding according to claim 1, wherein if the judgment index in the second judgment spot welding test is equal to or greater than 1.1 and less than 3, the second pressure is determined to be an appropriate spot welding condition.

7. 7. The method for determining welding conditions for resistance spot welding according to claim 6, wherein if the value of the judgment index in the second judgment spot welding test is not equal to or greater than 1.1 and less than 3, the second pressure is changed to another value and a next judgment spot welding test is repeated.

8. 8. The method for determining welding conditions for resistance spot welding according to claim 7, wherein a time during which current is passed between the electrodes in the judgment spot welding test is set as the second current passing time.

9. a new pressure between the electrodes is determined based on the value of the judgment index in the immediately preceding judgment spot welding test, and a next judgment spot welding test is performed to obtain a new judgment index; 9. The method for determining welding conditions for resistance spot welding according to claim 8, wherein if the finally obtained judgment index value is equal to or greater than 1.1 and less than 3, the finally determined pressure between the electrodes is determined to be the appropriate spot welding condition.

10. 10. The method for determining welding conditions for resistance spot welding according to claim 9, wherein, when the finally determined judgment index value is not equal to or greater than 1.1 and less than 3, the finally determined inter-electrode pressure is further changed to another value and the judgment spot welding test is repeated.

11. 7. The method of determining welding conditions for resistance spot welding according to claim 6, wherein when the judgment index value in the first judgment spot welding test is less than 1.1, the second applied pressure is set to be greater than the first applied pressure.

12. 7. The method of determining welding conditions for resistance spot welding according to claim 6, wherein when the judgment index value in the first judgment spot welding test is 3 or more, the second applied pressure is set to be smaller than the first applied pressure.

13. The method for determining welding conditions for resistance spot welding according to claim 7 , wherein when the judgment index value in the second judgment spot welding test is less than 1.1, the second applied pressure is changed to a larger value.

14. 8. The method of determining welding conditions for resistance spot welding according to claim 7, wherein when the judgment index value in the second judgment spot welding test is 3 or more, the second applied pressure is changed to a smaller value.

15. 11. The method of determining welding conditions for resistance spot welding according to claim 10, wherein if the last determined value of the judgment index is less than 1.1, the last determined pressure between the electrodes is changed to a larger value.

16. 11. The method for determining welding conditions for resistance spot welding according to claim 10, wherein if the last determined value of the judgment index is 3 or more, the last determined pressure between the electrodes is changed to a smaller value.

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