Method for manufacturing resistance spot welded joint and method for predicting melting point of plating layer

Abstract

A method for manufacturing a resistance spot welded joint capable of preventing LME cracking in resistance spot welds in sheet assemblies using multiple steel sheets, including zinc-based plated steel sheets, is provided. The method includes a main current application process for forming a nugget that joins the steel sheets during a pressure holding period from the start of pressure application by a pair of electrodes until the pair of electrodes are released and pressure application is terminated. Thereafter, optionally, one or more post-current application processes for post-heat treatment are performed. When TR1 is the temperature at one or more evaluation positions in a region including a coating layer of the zinc-based plated steel sheet when pressure application is terminated, and TM1 is the melting point of the coating layer at the evaluation positions when pressure application is terminated, resistance spot welding is performed under conditions such that TR1 and TM1 satisfy the relationship (TR1 - TM1) ≦ 220°C.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a resistance spot-welded joint formed by resistance spot welding two or more overlapping steel sheets, including at least one zinc-based plated steel sheet, and a method for predicting the melting point of the plating layer, and in particular to a method for manufacturing a resistance spot-welded joint and a method for predicting the melting point of the plating layer that are suitable for use as structural parts for automobiles and the like. [Background technology]

[0002] In order to reduce CO2 emissions from the perspective of protecting the global environment, improving automobile fuel efficiency by reducing the weight of automobile bodies while maintaining their strength has always been an important issue in the automotive industry. Furthermore, from the perspectives of collision safety and improved fuel efficiency, weight reduction of automobile bodies while maintaining their strength is also required. To achieve these goals, it is effective to reduce the thickness of steel sheets by increasing the strength of the steel sheets used in automotive parts, and in recent years, the use of steel sheets with a tensile strength (TS) of 980 MPa or more has been increasing.

[0003] In the automobile assembly process, overlapping steel sheets are typically joined by welding when assembling press-formed automobile parts. From the perspectives of cost and manufacturing efficiency, resistance spot welding, a type of lap resistance welding, is often used to join automobile parts. This welding method involves sandwiching two or more overlapping steel sheets between a pair of electrodes from above and below, applying pressure to the steel sheets from above and below with the pair of electrodes, and passing a high welding current between the upper and lower electrodes for a short period of time to join the steel sheets.

[0004] FIG. 1 is a cross-sectional view in the sheet thickness direction of a resistance spot-welded joint when pressure is applied. In the example shown in FIG. 1, two steel sheets 1 and 2 are overlapped and sandwiched between an upper electrode 7 and a lower electrode 8, and resistance spot welding is performed while pressure is applied, producing a resistance spot-welded joint 5. This method utilizes resistance heating generated by passing a high welding current between the electrodes to obtain a nugget 4a. This nugget 4a is the portion where the steel sheets 1 and 2 melt at the contact point when current is passed through the overlapping steel sheets, and then solidifies. This results in a point-like joining of the steel sheets.

[0005] For automotive steel sheets, zinc-plated steel sheets with rust-preventing properties, such as zinc (Zn)-plated steel sheets, are used for parts at risk of corrosion. However, resistance spot welding of overlapping steel sheets, including zinc-plated steel sheets, can result in cracks at the weld. The cracks at the weld are thought to be caused by liquid metal embrittlement (LME) cracking, which occurs when the low-melting-point metal coating on the surface of the zinc-plated steel sheet melts during welding. When tensile stresses due to the electrode pressure and the thermal expansion and contraction of the steel sheet are applied to the weld, the molten low-melting-point metal penetrates the grain boundaries of the base material of the zinc-plated steel sheet, reducing the grain boundary strength and causing cracks. As shown in Figure 1, LME cracks 9 occur at various locations, such as the surfaces of steel sheets 1 and 2 that contact the upper electrode 7 or the lower electrode 8, and the surfaces of the steel sheet mating surfaces 6 where the steel sheets meet.

[0006] Known techniques for preventing such LME cracking include those described in Patent Documents 1 to 3. Patent Document 1 proposes that the chemical composition of the steel sheets used in the sheet assembly be within a specific range, specifically, a chemical composition consisting of, in weight percent, C: 0.003 to 0.01%, Mn: 0.05 to 0.5%, P: 0.02% or less, sol. Al: 0.1% or less, Ti: 48×(N / 14) to 48×{(N / 14)+(S / 32)}%, Nb: 93×(C / 12) to 0.1%, B: 0.0005 to 0.003%, N: 0.01% or less, Ni: 0.05% or less, with the balance being Fe and unavoidable impurities.

[0007] Patent Document 2 proposes a spot welding method for high-strength plated steel sheets, in which spot welding is performed by setting the welding current application time and the pressure holding time after welding current application so as to satisfy the following conditional expressions (A) and (B): 0.25×(10×t+2) / 50≦WT≦0.50×(10×t+2) / 50 (A) 300-500×t+250×t 2 ≦HT (B) Here, t is the thickness of the steel sheet (mm), WT is the welding current flow time (ms), and HT is the pressure holding time (ms) after welding current flow. Patent Document 2 also proposes performing resistance spot welding by appropriately setting the current flow time and the pressure holding time of the electrode after current flow according to the thickness of the steel sheet, and by using a high-strength galvanized steel sheet in which the amount of alloy elements in the steel sheet is below a certain level.

[0008] Patent Document 3 proposes a technique for preventing LME cracking by removing the plating layer from the portion to be welded prior to resistance spot welding.

[0009] On the other hand, Patent Document 4 proposes a resistance spot welding method in which a post-current application step is performed after the main current application step. According to this welding method, the tensile strength of the resistance spot welded joint is improved by the heat input in the post-current application step. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 10-195597 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-103377 [Patent Document 3] International Publication No. 2016 / 159169 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-115706 [Non-patent literature]

[0011] [Non-Patent Document 1] P. Shewmon: Diffusion in Solids, The Minerals, Metals & Materials Society, Warrendale (1989). Summary of the Invention [Problem to be solved by the invention]

[0012] However, the technology proposed in Patent Document 1 requires limiting the amount of alloying elements in the steel sheet, which has the problem of limiting the use of steel sheets that meet the required performance. In particular, in a situation where high alloying is becoming more prevalent in line with the recent trend toward higher strength steel sheets, the application of the technology proposed in Patent Document 1 is extremely limited.

[0013] The technology proposed in Patent Document 2 only proposes a method for suppressing LME cracking when an excessively large welding current is set that causes expulsion, and does not mention LME cracking in a state where expulsion does not occur. In addition, since the influence of the electrode impact angle is not considered, the countermeasure may be insufficient when considering actual construction during automobile assembly.

[0014] The technology proposed in Patent Document 3 requires a step of removing the plating layer in advance, which increases manufacturing costs. In addition, the removal of the plating layer is thought to reduce the corrosion resistance of the welded portion.

[0015] The technology proposed in Patent Document 4 improves the tensile strength of resistance spot welded joints, but does not consider preventing the occurrence of LME cracking.

[0016] The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide a method for manufacturing a resistance spot welded joint that can suppress LME cracking at resistance spot welds in sheet assemblies using multiple steel sheets, particularly zinc-based plated steel sheets. [Means for solving the problem]

[0017] The present inventors have conducted extensive research to achieve the above object and have come to the following findings.

[0018] Cracking during welding is a phenomenon that is influenced by a variety of complex factors and therefore cannot be explained simply. However, the inventors believe as follows: LME cracking in resistance spot welds is likely to occur when excessive tensile residual stress is generated in the resistance spot weld due to welding disturbances or other factors. In particular, it is known that at the steel sheet mating surfaces where steel sheets come into contact, areas of high localized tensile stress are generated when the electrodes are released after the current and pressure application in resistance spot welding are completed, and these areas are prone to LME cracking. Another factor that may be contributing to the occurrence of tensile stress due to differences in transformation behavior during cooling is when the strength of overlapping steel sheets is high.

[0019] As mentioned above, LME cracking occurs when tensile stress is applied to a steel sheet when a liquid metal such as Zn is in contact with the steel sheet. The inventors considered that the value of (TR1 - TM1), i.e., the degree to which the temperature at the evaluation position TR1 is higher than the melting point TM1 of the coating layer at the evaluation position when pressure application by the electrode is terminated (electrode open), might be a good indicator of the risk of LME cracking. They then discovered that LME cracking can be prevented by performing resistance spot welding under conditions where TR1 and TM1 satisfy the following formula (1), leading to the completion of the present invention. (TR1-TM1)≦220℃ (1)

[0020] The gist and configuration of the present invention are as follows.

[0021] [1] A method for manufacturing a resistance spot welded joint, comprising overlapping two or more steel sheets including at least one zinc-based plated steel sheet, sandwiching the steel sheets between a pair of electrodes, and joining the steel sheets by resistance spot welding in which an electric current is passed through while applying pressure in the sheet thickness direction, The resistance spot welding includes a main current application process for forming a nugget that joins the steel sheets during a pressure holding period from the start of the pressure application by the pair of electrodes to the end of the pressure application by releasing the pair of electrodes, and thereafter, optionally, one or more post-current application processes for post-heat treatment, a method for manufacturing a resistance spot welded joint, characterized in that the resistance spot welding is performed under conditions where TR1 and TM1 satisfy the following formula (1), where TR1 is the temperature at one or more evaluation positions provided in a region including a plating layer of the zinc-based plated steel sheet when the application of pressure is terminated, and TM1 is the melting point of the plating layer at the evaluation positions when the application of pressure is terminated: (TR1-TM1)≦220℃ (1)

[0022] [2] TR1 and TM1 are a first step of creating an analytical model for performing finite element analysis using a computer, the analytical model being composed of a plurality of meshes that divide an area including the two or more steel plates and the pair of electrodes; a second step of providing the one or more evaluation positions in the plurality of meshes including the plating layer of the zinc-based plated steel sheet on the analysis model; a third step of calculating a temperature history at the evaluation position from the start of the main current application process to the end of the pressurization by performing the finite element analysis while providing the conditions of the main current application process and the conditions of resistance spot welding after the end of the main current application process as boundary conditions; a fourth step of calculating a temperature TR1 at the evaluation position when the pressurization is terminated based on the calculated temperature history at the evaluation position; a fifth step of calculating a zinc concentration of the plating layer included in the evaluation position when the pressurization is terminated based on the calculated temperature history at the evaluation position; a sixth step of determining a melting point TM1 of the plating layer included in the evaluation position when the pressing is terminated based on the calculated zinc concentration; The method for manufacturing a resistance spot welded joint according to [1] above, wherein the temperature is determined by the following formula:

[0023] [3] The method for manufacturing a resistance spot welded joint according to [2] above, wherein the conditions of the main current application step given as the boundary conditions are a current value and a current application time.

[0024] [4] The method for manufacturing a resistance spot welded joint according to [2] or [3] above, wherein the conditions for resistance spot welding after the completion of the main current process given as the boundary conditions are whether or not the post-current process is performed and the conditions for that, and the time from the completion of the final current process in the resistance spot welding to the completion of the pressurization.

[0025] [5] The method for manufacturing a resistance spot welded joint according to [4] above, wherein the post-energization process is a single pulse current or two or more pulse currents separated by a non-energization period, and the boundary condition for the post-energization process is one or both of the current value and the number of pulse currents.

[0026] [6] A method for manufacturing a resistance spot welded joint according to any one of [2] to [5] above, wherein, after determining the conditions for the main current flow process, TR1 and TM1 are calculated by executing the first step to the sixth step, and the conditions for resistance spot welding after completion of the main current flow process are adjusted so that the obtained values ​​of TR1 and TM1 satisfy the formula (1).

[0027] [7] When the temperature at the evaluation position when the main current application process is completed is TR0 and the melting point of the plating layer included in the evaluation position when the main current application process is completed is TM0, the current value I of the pulse current application in the post-current application process is p Regarding When (TR0-TM0)<300℃, I p Adjust to 0 (zero) kiloamperes or more, When 300℃≦(TR0-TM0)<400℃, I p Adjust to 4.0 kiloamperes or more, When 400℃≦(TR0-TM0), Ip The method for manufacturing a resistance spot welded joint according to [6] above, wherein the current is adjusted to 6.0 kiloamperes or more.

[0028] [8] Regarding the number of pulse energizations in the post-energization process, (TR0-TM0) When <300℃, adjust to 0 times or more. When 300℃≦(TR0-TM0)<400℃, adjust it to one or more times. The method for manufacturing a resistance spot welded joint according to [6] or [7] above, wherein when 400°C ≦ (TR0 - TM0), the temperature is adjusted three times or more.

[0029] [9] A method for manufacturing a resistance spot welded joint according to any one of [1] to [8] above, wherein at least one of the two or more steel plates is a steel plate having a tensile strength of 590 MPa or more.

[0030]

[10] A method for manufacturing a resistance spot welded joint according to any one of [1] to [9] above, wherein immediately before starting the main current application process, one or more conditions selected from the following (a) to (d) are satisfied: (a) The striking angle of the pair of electrodes is 0.2 degrees or more. (b) The gap between the overlapping steel plates is 0.5 mm or more. (c) A state in which the gap between the fixed electrode of the pair of electrodes and the steel plate is 0.5 mm or more (d) A state in which the amount of misalignment of the pair of electrodes is 0.1 mm or more.

[0031]

[11] A method for predicting the melting point of a coating layer of a zinc-based coated steel sheet in resistance spot welding in which two or more steel sheets including at least one zinc-based coated steel sheet are overlapped, sandwiched between a pair of electrodes, and current is applied while pressure is applied in the sheet thickness direction, comprising: creating an analytical model for performing finite element analysis using a computer, the analytical model being configured with a plurality of meshes that divide an area including the two or more steel plates and the pair of electrodes; providing one or more evaluation positions in the plurality of meshes including the plating layer of the zinc-based plated steel sheet on the analysis model; calculating a temperature history at the evaluation location during the resistance spot welding by performing the finite element analysis with the resistance spot welding conditions as boundary conditions; calculating a zinc concentration of the plating layer included in the evaluation position based on the calculated temperature history at the evaluation position; determining the melting point of the plating layer included in the evaluation position based on the calculated zinc concentration; A method for predicting the melting point of a plating layer, comprising: [Effects of the Invention]

[0032] According to the method for manufacturing a resistance spot welded joint of the present invention, it is possible to suppress the occurrence of LME cracking in the resistance spot weld, regardless of the chemical composition of the steel sheets or construction disturbances during welding, especially in sheet assemblies using multiple steel sheets including zinc-based plated steel sheets. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a cross-sectional view in the plate thickness direction when pressure is applied during resistance spot welding. [Figure 2] 2A and 2B are cross-sectional views in the thickness direction showing the main parts of a resistance spot welded joint, where two steel plates are overlapped and three steel plates are overlapped. [Figure 3] 3A and 3B are time charts showing examples of the timing of applying pressure and energizing by the electrodes, where Fig. 3A shows a case where only main energization is performed, and Fig. 3B shows a case where main energization is performed followed by two post-energizations. [Figure 4] 10 is a flowchart showing the steps for determining TR1 and TM1. [Figure 5] 10 is a graph showing an example of the temperature history at an evaluation position. [Figure 6] 1 is a graph showing an example of a calculated zinc concentration distribution in a plating layer. [Figure 7] FIG. 1 is a diagram for explaining a method for determining the melting point of a plating layer using an Fe—Zn binary equilibrium phase diagram. [Figure 8] 8A is a cross-sectional view in the plate thickness direction for explaining the impact angle in resistance spot welding, and FIG. 8B is a diagram showing a definition of the impact angle, and FIG. 8C is a diagram showing a method for calculating the impact angle from the inclination angle of the nugget. [Figure 9] 9(A) and 9(B) are cross-sectional views in the sheet thickness direction illustrating gaps between steel sheets in resistance spot welding, in which Fig. 9(A) shows the case of continuous welding points and Fig. 9(B) shows the case of an end portion. [Figure 10] FIG. 1 is a cross-sectional view in the plate thickness direction for explaining a gap between an electrode and a steel plate in resistance spot welding. [Figure 11] FIG. 10 is a cross-sectional view in the plate thickness direction for explaining the amount of misalignment of an electrode in resistance spot welding. DETAILED DESCRIPTION OF THE INVENTION

[0034] The method for manufacturing a resistance spot welded joint according to the present invention will be described in detail below, although the present invention is not limited to these embodiments.

[0035] [Manufacturing method for resistance spot welded joints] In one embodiment, the present invention relates to a method for manufacturing a resistance spot welded joint, in which two or more steel sheets are overlapped, sandwiched between a pair of electrodes, and joined together by resistance spot welding in which an electric current is passed through while pressure is applied in the sheet thickness direction to obtain a resistance spot welded joint. The present invention is applied to a method for manufacturing a resistance spot welded joint in which at least one of the two or more steel sheets comprises a zinc-based plated steel sheet. The melting point of the zinc-based plated layer in the zinc-based plated steel sheet is usually lower than the melting point of the steel sheet that is the base material.

[0036] In this specification, the term "zinc-based plated steel sheet" refers to a steel sheet coated with a zinc-based plating by various methods such as hot-dip plating, electroplating, vapor deposition plating, thermal spraying, etc. Specifically, hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), hot-dip Zn-5 mass% Al alloy-plated steel sheet (GF), hot-dip Zn-55 mass% Al alloy-plated steel sheet (GL), hot-dip Zn-Al-Mg alloy-plated steel sheet, electrogalvanized steel sheet (EG), electrogalvanized zinc-Ni alloy-plated steel sheet (Zn-11 mass% Ni), etc. can be used, but are not limited to these, and all known zinc-based plated steel sheets containing zinc can be applied.

[0037] Resistance spot welding in the present invention includes a main current application process for forming a nugget that joins steel sheets during the pressure holding period from the start of pressure application by a pair of electrodes to the end of pressure application when the electrodes are released, and then, optionally, one or more post-current application processes for post-heat treatment.

[0038] A welding device that can be used in the method for manufacturing a resistance spot-welded joint according to the present invention includes a pair of upper and lower electrodes, each capable of freely controlling the welding pressure and welding current during welding. The welding device's pressure mechanism (e.g., air cylinder or servo motor), type (e.g., stationary type, robot gun), and electrode shape are not particularly limited. The electrode tip type may be, for example, a DR type (dome radius type), an R type (radius type), or a D type (dome type), as defined in Japanese Industrial Standard JIS C 9304:1999. The electrode tip diameter is preferably, for example, 4 mm or more and 16 mm or less.

[0039] According to the present invention, it is possible to manufacture a resistance spot welded joint having a resistance spot weld formed by resistance spot welding two or more overlapping steel sheets. As described above, at least one of the two or more overlapping steel sheets is a zinc-based plated steel sheet. There is no particular upper limit on the number of steel sheets constituting the resistance spot welded joint, but it is preferably five or less. It is more preferable that the number of steel sheets is two or three.

[0040] FIG. 2(A) shows an example of a cross-sectional view in the sheet thickness direction of a resistance spot welded joint 5 after two overlapping steel sheets 1, 2 have been welded by an upper electrode 7 and a lower electrode 8. In this example, either or both of the lower steel sheet 2 (hereinafter sometimes referred to as the "lower sheet") and the upper steel sheet 1 (hereinafter sometimes referred to as the "upper sheet") are zinc-based plated steel sheets. In the case of the resistance spot welded joint 5 of FIG. 2(A), a resistance spot weld 4 is formed on the steel sheet mating surface (steel sheet overlapping surface) 6 where the upper sheet 1 and the lower sheet 2 come into contact. The resistance spot weld 4 is composed of a central nugget 4a and a surrounding HAZ (heat affected zone) 4b.

[0041] As an example of a joint using three or more steel plates, FIG. 2(B) shows an example of a cross-sectional view in the plate thickness direction of a resistance spot welded joint 5 formed by welding three steel plates: a lowermost steel plate (lower plate) 2, an uppermost steel plate (upper plate) 1, and a steel plate 3 (hereinafter sometimes referred to as the "middle plate") positioned between them. In the resistance spot welded joint 5 of FIG. 2(B), at least one of the three steel plates is a zinc-based plated steel plate. In the resistance spot welded joint 5 of FIG. 2(B), the resistance spot weld 4 is formed so as to include a steel plate mating surface 6a where the upper plate 1 and the middle plate 3 meet, and a steel plate mating surface 6b where the middle plate 3 and the lower plate 2 meet.

[0042] [Conditions for resistance spot welding] The technical concept of the present invention will now be described in detail. Resistance spot welding according to the present invention includes a main current application process for forming a nugget that joins steel sheets during a pressure holding period from the start of pressure application by a pair of electrodes to the end of pressure application when the pair of electrodes are released, and then, optionally, one or more post-current application processes for post-heat treatment.

[0043] FIG. 3 is a time chart showing an example of the timing of electrode pressure and current application in resistance spot welding according to the present invention. The horizontal axis of the time chart represents time t. The vertical axis of the time chart represents the magnitude of the pressure and the magnitude of the current. FIG. 3(A) is a time chart showing the case where only main current application is performed. First, overlapped steel sheets are sandwiched between a pair of electrodes and pressure in the sheet thickness direction is initiated. After main current application, main current application is terminated at time t0. The pressure is then maintained for a certain period of time, and the pressure is terminated and the electrodes are released at time t1. FIG. 3(B) is a time chart showing the case where post-current application is performed twice after main current application. In this case, the pressure is also maintained for a certain period of time after the main current application and the post-current application, and the pressure is terminated and the electrodes are released at time t1. In resistance spot welding according to the present invention, the main current application process is an essential process. However, whether or not post-current application after main current application is performed can be determined arbitrarily.

[0044] In this embodiment, when the temperature at one or more evaluation positions provided in a region including the plating layer of the zinc-based plated steel sheet when pressure application is terminated is defined as TR1, and the melting point of the plating layer at the evaluation positions when pressure application is terminated is defined as TM1, resistance spot welding is performed under conditions where TR1 and TM1 satisfy the following formula (1): (TR1-TM1)≦220℃ (1)

[0045] As mentioned above, LME cracking occurs when tensile stress is applied to a steel sheet with a liquid metal such as Zn in contact with it. In other words, if the temperature TR1 at the evaluation position when pressure is removed (when the electrode is released) is higher than the melting point TM1 of the coating layer included in the evaluation position, this suggests that liquid metal is present at the evaluation position and there is a risk of LME cracking. The value of (TR1 - TM1) in this case can be considered an index of how much higher the temperature TR1 at the evaluation position is than the melting point TM1 of the coating layer included in the evaluation position, i.e., the magnitude of the risk of LME cracking.

[0046] Therefore, the smaller the value of (TR1 - TM1), the lower the risk of LME cracking, and it is considered preferable that the value of (TR1 - TM1) is a negative value less than 0. However, it is not easy to promote the diffusion of zinc in the coating layer, increase the iron concentration, and raise the melting point TM1 at the evaluation position to a temperature higher than the temperature TR1 at the evaluation position during the short current flow time of resistance spot welding.

[0047] According to the inventors' investigations, it was found that even if the value of (TR1-TM1) is not a negative value smaller than zero, the risk of LME cracking can be reduced as long as the value of (TR1-TM1) is 220°C or less, as shown in formula (1). The value of (TR1-TM1) is preferably 200°C or less, and more preferably 180°C or less. When multiple evaluation positions are set, resistance spot welding should be performed under conditions that satisfy formula (1) at the evaluation position with the largest value of (TR1-TM1), i.e., the evaluation position with the highest risk of LME cracking.

[0048] There is no particular lower limit for the value of (TR1 - TM1). However, if the temperature TR1 of the evaluation position is excessively cooled compared to the melting point TM1 of the coating layer included in the evaluation position, the time required for the welding process will be longer. Therefore, it is preferable that the value of (TR1 - TM1) is 0°C or higher. Note that when evaluation positions are set at multiple locations, it is preferable that the value of (TR1 - TM1) at the evaluation position with the smallest value of (TR1 - TM1) be 0°C or higher.

[0049] In this embodiment, any method for determining the value of the left side of Equation (1) may be used, and is not particularly limited. The temperature TR1 at the evaluation position when pressure application is terminated can be measured directly, for example, by installing a temperature sensor at the evaluation position of the resistance spot-welded joint specially prepared for this purpose. The melting point TM1 of the coating layer at the evaluation position when pressure application is terminated can be determined indirectly, for example, by continuously measuring the temperature of the resistance spot-welded joint using a temperature sensor while continuing to pass current through the joint, and measuring the temperature at the time when the temperature temporarily drops due to the latent heat of fusion when the coating layer melts. In addition to these methods, the values ​​of TR1 and TM1 can also be determined by finite element analysis, in which the resistance spot welding conditions are applied as boundary conditions, as described below.

[0050] In this embodiment, the magnitude of the pressing force in the resistance spot welding is not particularly limited. From the viewpoint of automotive applications, the pressing force is preferably adjusted to a range of 2.0 kN or more and 8.0 kN or less.

[0051] [Steps to calculate TR1 and TM1] In a preferred embodiment, TR1 and TM1 are temperatures calculated by the following steps: a first step of creating an analytical model for performing finite element analysis using a computer, the first step being to create an analytical model composed of a plurality of meshes that divide an area including two or more steel sheets and a pair of electrodes; a second step of establishing one or more evaluation positions at nodes of the plurality of meshes that include plating layers of the zinc-based plated steel sheets on the analytical model; a third step of calculating the temperature history at the evaluation positions from the start of the main current process to the end of pressurization by performing finite element analysis while providing boundary conditions that are the conditions for the main current process and the conditions for resistance spot welding after the end of the main current process; a fourth step of calculating the temperature TR1 at the evaluation position when pressurization is completed based on the calculated temperature history at the evaluation position; a fifth step of calculating the zinc concentration of the plating layer included in the evaluation position when pressurization is completed based on the calculated temperature history at the evaluation position; and a sixth step of calculating the melting point TM1 of the plating layer included in the evaluation position when pressurization is completed based on the calculated zinc concentration.

[0052] 4 is a flowchart showing the steps for determining TR1 and TM1 in this preferred embodiment. Each of the six steps included in this flowchart will now be described in detail.

[0053] (1) First step The first step (S1) is a step of creating an analytical model for performing finite element analysis using a computer, the analytical model being composed of a plurality of meshes that divide an area including two or more steel plates and a pair of electrodes. The analytical model can be created using finite element analysis software. This analytical model includes two or more steel plates and a pair of electrodes.

[0054] (2) Second step The second step (S2) is a step of providing one or more evaluation positions in a plurality of meshes including the plating layer of the zinc-based plated steel sheet on the analytical model. In this specification, the "plating layer of the zinc-based plated steel sheet" may be a plating layer of any composition as long as it contains zinc. That is, the plating layer may be a pure zinc plating layer or a zinc alloy plating layer. Furthermore, the plating layer may have a multilayer structure in which plating layers having various compositions are superimposed. In this case, the entire plating layer having a multilayer structure is referred to as the "plating layer."

[0055] In the second step, one or more evaluation positions are set at the positions of multiple meshes present in an area containing a coating layer of the zinc-based coated steel sheet. Since LME cracking does not occur in areas where no coating layer is present nearby, it is not necessary to set evaluation positions in such areas. The number and positions of the evaluation positions are not particularly limited. However, as mentioned above, LME cracking is often observed on the surface of the steel sheet 1 or 2 that comes into contact with the upper electrode 7 or the lower electrode 8 shown in FIG. 1 , or on the surface of the steel sheet mating surface 6 where the steel sheets come into contact with each other. Therefore, it is preferable to set evaluation positions at meshes present in these positions. It is more preferable to set evaluation positions at the HAZ (heat-affected zone) 4b on the surface of the steel sheet mating surface 6.

[0056] The evaluation position set in the region including the coating layer does not have to be a pinpoint, but may be a region with a certain degree of spread. For example, when the entire region of a mesh having a certain size in the relevant element analysis described below is set as the evaluation position, the mesh may include only the coating layer, or may include both the coating layer and the steel sheet. Furthermore, when a region including both the coating layer and the steel sheet is set as the evaluation position, the steel sheet included in the evaluation position may be a zinc-based coated steel sheet that originally has a coating layer, or a steel sheet that does not have a coating layer itself but is in contact with the coating layer of the other steel sheet that is mated with it, or may include both of these steel sheets.

[0057] (3) Third step The third step (S3) is a step of calculating the temperature history at the evaluation position from the start of the main current application process to the end of pressure application by performing finite element analysis using the conditions of the main current application process and the conditions of resistance spot welding after the end of the main current application process as boundary conditions. The period for calculating the temperature history at the evaluation position is the period from the start of the main current application process to the end of pressure application.

[0058] The analysis software used to calculate the temperature history is not particularly limited as long as it is finite element analysis software capable of analyzing the temperature, stress, and strain of resistance spot welding. However, from the perspective of ensuring calculation accuracy, it is preferable to use finite element analysis software that can calculate the difference between the calculated and measured nugget diameters within ±0.7 mm when analysis is performed under the same conditions as actual resistance spot welding. It is even more preferable if the difference between the calculated and measured values ​​can be calculated within ±0.5 mm. Note that if it is difficult to calculate the difference between the calculated and measured values ​​within ±0.7 mm, the calculation preconditions may be adjusted to improve analysis accuracy.

[0059] Figure 5 shows the results of calculations using the finite element analysis software SORPAS 2D ("SORPAS" is a registered trademark) for an example of the temperature history calculation in the third step, in which three post-current processes are performed after the main current process. Prior to the calculations, the accuracy of the calculation of the nugget diameter was verified, and it was confirmed that the calculation was possible with an accuracy of within ±0.7 mm of the experimental nugget diameter. Then, the temperature history was calculated at an evaluation position on the surface of the steel sheet mating surface, which was located 600 μm from the edge of the nugget into the HAZ (heat-affected zone). As shown in Figure 5, the calculation results showed that at this evaluation position, the temperature monotonically increased from the start of the main current process to the time t0 when the main current was stopped, remained approximately constant during the post-current processes, and then monotonically decreased from the end of the post-current process to the time t1 when the pressing was completed.

[0060] To calculate the temperature history in the third step, the heating and cooling conditions from the start of the main current application process to the end of the pressure application must be given as boundary conditions. To obtain accurate calculation results, it is preferable to reflect the conditions in the actual resistance spot welding as accurately as possible in the calculation.

[0061] In a preferred embodiment, the boundary conditions for the main current application process are the current value and the current application time. In the third step, the current value and the current application time for the main current application process are set as boundary conditions, so that the heating conditions for the main current application process are reflected in the calculation.

[0062] In a preferred embodiment, the boundary conditions for the resistance spot welding after the main current application step are whether or not a post-current application step is performed and the conditions for that, as well as the time from the end of the final current application step to the end of pressurization in the resistance spot welding, are set as boundary conditions in the third step, so that the heating and cooling conditions after the main current application step are reflected in the calculation.

[0063] The conditions for resistance spot welding after the main current process include the number of currents and the welding current value in the post-current process, as well as the case where the post-current process is not performed at all. In other words, if formula (1) is satisfied without the post-current process, the post-current process can be omitted. In this case, the main current process is the final current process.

[0064] The time between the end of the final current application process and the end of pressure application in resistance spot welding is related to the cooling conditions. After the end of the final current application process in resistance spot welding, rather than immediately ending the pressure and releasing the electrode, cooling can sometimes be accelerated by maintaining pressure for a certain period of time and then terminating the pressure. For example, if the electrode used in resistance spot welding is equipped with a water-cooling mechanism, maintaining pressure accelerates cooling of the weld. This accelerates solidification of the coating layer and prevents LME cracking.

[0065] In a preferred embodiment, the post-current step is a single pulse current step or two or more pulse current steps with a non-current period between them, and the boundary conditions for the post-current step are one or both of the current value and the number of pulse current steps. By providing these conditions as boundary conditions in the third step, the heating conditions for the post-current step are reflected in the calculation.

[0066] Here, "pulse current" refers to the application of current between electrodes for a short period of time. Pulse current promotes zinc diffusion in the coating layer, increases the melting point TM1 of the final coating layer, and reduces the risk of LME cracking without excessively increasing the temperature of the weld. The pulse current may be direct current. If the current value of the pulse current is too low, insufficient heat is generated, making it difficult to increase the melting point TM1 of the coating layer. Therefore, the current value of the pulse current is preferably 4.0 kA or more, and more preferably 5.0 kA or more. If the current value of the pulse current is too high, the temperature of the weld increases rapidly, increasing the risk of LME cracking. Therefore, the current value of the pulse current is preferably 12 kA or less, and more preferably 11 kA or less. The duration of the pulse current is not particularly limited, but may be, for example, 30 ms or more and 100 ms or less.

[0067] Referring again to Fig. 3(B), when a post-current application process using pulse current is performed after the main current application process, it is preferable to provide a cooling time between the main current application process and the first pulse current application, as shown in Fig. 3(B). By providing a cooling time, the temperature increased by the main current application process can be temporarily lowered, and the temperature can be prevented from becoming excessively high. Similarly, when pulse current application is performed multiple times, it is preferable to provide a cooling time between pulse current applications.

[0068] (4) Fourth step The fourth step (S4) is a step of calculating the temperature TR1 at the evaluation position when pressurization is completed based on the temperature history at the evaluation position calculated in step 3. Referring again to Figure 5, in this calculation result, the temperature TR1 at the evaluation position at time t1 when pressurization is completed is 962°C.

[0069] (5) Fifth step The fifth step (S5) calculates the zinc concentration of the coating layer included at the evaluation position when pressing is terminated, based on the temperature history at the evaluation position calculated in the third step. The zinc concentration is calculated by assuming that the initial concentration of the coating layer of the zinc-based coated steel sheet is constant, and using a diffusion equation, the distribution of zinc concentration in the sheet thickness direction when zinc atoms thermally diffuse into the base material of the steel sheet in contact with the coating layer is calculated. The diffusion equation used in the calculation is not particularly limited, as long as it is one described in a known document, such as Non-Patent Document 1. Furthermore, known values ​​can be used for the physical constants, such as the zinc diffusion coefficient, frequency term, and activation energy, required to solve the diffusion equation. However, from the perspective of ensuring calculation accuracy, it is preferable to use a diffusion equation and physical constants that allow the difference between the calculated and measured zinc concentrations to be within ±7% when calculations are performed under conditions identical to those of actual resistance spot welding. It is even more preferable if the difference between the calculated and measured values ​​can be calculated to an accuracy of within ±5%. If it is difficult to achieve a difference between the calculated value and the measured value within ±7%, the preconditions for the calculation may be adjusted to improve the accuracy of the analysis.

[0070] The method for measuring the concentration of an actual sample in the process of verifying the accuracy of the zinc concentration calculation is not particularly limited as long as it can quantitatively measure the zinc concentration. The zinc concentration can be measured, for example, using an EPMA or SEM-EDS. Measurement conditions for measuring zinc concentration using SEM-EDS include, for example, using a Ka ray source as the X-ray source, a magnification of 1500x or more, an acceleration voltage of 15 kV, and a scan time of 1 minute or more. The measurement mode can be, for example, line analysis or point analysis. When measuring zinc concentration using these analytical methods, the measurement should be performed at the same position as the evaluation position set in the second step above. However, if the measured zinc concentration at that evaluation position is extremely unstable compared to the measured values ​​at neighboring areas, the measurement may be performed within a range of ±20 μm from the evaluation position, avoiding the unstable area from the measured value.

[0071] Figure 6 shows the results of an analysis of the zinc concentration distribution using the diffusion equation described in Non-Patent Document 1 based on the temperature history shown in Figure 5 as an example of a calculation of the zinc concentration distribution. Prior to the calculation, the accuracy of the zinc concentration calculation was verified, and it was confirmed that the calculation was possible with an accuracy of within ±7% of the measured zinc concentration. As shown by the dashed line in Figure 6, at this evaluation position, the zinc concentration distribution at the time of the start of current application in this current application process was uniform at 90 mass%. The thickness of the coating layer was 2 μm. Furthermore, as shown by the solid line in Figure 6, at this evaluation position, the zinc-rich area distribution at the end of pressurization showed that zinc atoms had diffused toward the base steel sheet compared to the initial state, and the thickness of the diffusion layer was approximately 4 μm. Furthermore, the maximum zinc concentration at the center of the coating layer was 85 mass%.

[0072] In calculating the zinc concentration distribution shown in FIG. 6 , correctly setting the initial coating layer thickness is crucial for obtaining accurate calculation results in the fifth step. The inventors' research has revealed that during resistance spot welding, the coating layer thickness becomes significantly thinner than the coating layer thickness of the zinc-based coated steel sheet before welding. For example, if the coating layer thickness of a galvannealed hot-dip galvannealed steel sheet (GA) is 20 μm, the coating layer thickness may be as thin as 2 μm immediately after the start of the current application process. This is thought to be the result of most of the coating layer being expelled toward the outside of the corona bond due to the pressure and heat applied by the electrode. In general, when calculations are performed using a correctly set coating layer thickness, the maximum zinc concentration at the center of the coating layer is significantly lower than when calculations are performed using the original thickness. Performing calculations based on the actual coating layer thickness improves calculation accuracy.

[0073] (6) Sixth Step The sixth step is to determine the melting point TM1 of the coating layer at the evaluation position when pressure application is terminated, based on the zinc concentration calculated in the fifth step. The melting point TM1 can be easily determined using an Fe-Zn binary equilibrium phase diagram. Figure 7 shows an example of how to determine the melting point TM1 using the sixth step. The maximum zinc concentration at the center of the coating layer determined in Figure 6 was 85 mass%. Therefore, in the Fe-Zn binary equilibrium phase diagram shown in Figure 7, the solidus temperature indicated by the bold line when the zinc concentration is 85 mass% is read as 672°C. Therefore, in this example, the melting point TM1 is determined to be 672°C.

[0074] As explained above, by performing steps 1 to 4, the temperature TR1 at one evaluation position when pressure application at that position is completed can be determined. Furthermore, by subsequently performing steps 5 and 6, the melting point TM1 of the plating layer included in the same evaluation position can be determined. If multiple evaluation positions are set, the above steps are performed for each evaluation position.

[0075] [Adjustment of resistance spot welding conditions after the main current application process] In a preferred embodiment, after determining the conditions for this current application process, TR1 and TM1 are calculated by executing steps 1 to 6, and the conditions for resistance spot welding are adjusted so that the obtained values ​​of TR1 and TM1 satisfy equation (1).

[0076] To satisfy the formula (1), the values ​​of TR1 and TM1 can be increased by, for example, promoting interdiffusion between Fe and the zinc-based plating metal to increase the melting point TM1 of the plating layer. To achieve this, adjusting the resistance spot welding conditions after the main current application step to hold the welding at a high temperature for a long period of time so that alloying between Fe and the zinc-based plating metal progresses during current application is effective. However, the required degree of alloying varies depending on the sheet assembly and welding conditions, and therefore the required holding time at a high temperature varies. Therefore, in a preferred embodiment, the conditions of subsequent processes are determined based on the difference (TR0 - TM0) between the temperature at the evaluation position at the end of the main current application step (TR0) and the melting point TM0 of the plating layer at the evaluation position, and the welding conditions are appropriately adjusted. An example of a specific method for adjusting the resistance spot welding conditions is described below.

[0077] (1) Adjusting the current value of pulse current In a preferred embodiment, when the temperature at the evaluation position when the main current application process is completed is defined as TR0 and the melting point of the plating layer included in the evaluation position when the main current application process is completed is defined as TM0, the current value I of the pulse current in the post-current application process is p Regarding (TR0-TM0)<300℃, I p Adjust to 0 (zero) kiloamperes or more, and when 300℃≦(TR0-TM0)<400℃, I p Adjust to 4.0 kiloamperes or more, and when 400℃≦(TR0-TM0), I p Adjust to 6.0 kiloamperes or more.

[0078] The temperature TR0 of the evaluation position at time t0 when the main current application process is completed and the melting point TM0 of the plating layer included in the evaluation position when the main current application process is completed can be calculated using the same method as steps 1 to 4 used to calculate the temperature TR1 of the evaluation position at time t1 when the pressurization process is completed and the melting point TM1 of the plating layer included in the evaluation position. However, when performing the calculation in step 3 in this case, it is sufficient to provide the conditions for the main current application process as boundary conditions, and the conditions for resistance spot welding after the main current application process are not required.

[0079] When (TR0-TM0)<300°C, the melting point TM0 of the plating layer included in the evaluation position at the time t0 when the main current application process is completed is already sufficiently high, so the post-current application process is omitted or the current value I p Even if the value of (TR0-TM0) is small, equation (1) can be satisfied. When 300°C≦(TR0-TM0)<400°C, the magnitude of (TR0-TM0) is moderate, so I p By controlling the current to 4.0 kA or more to perform the post-current application process, the melting point TM1 of the coating layer at time t1 when the pressing is completed can be appropriately increased. When 400°C ≦ (TR0-TM0), the melting point TM0 of the coating layer at time t0 when the main current application process is completed is low, so that an I of 6.0 kA or more can be used. p By applying current, the melting point TM1 of the plating layer at time t1 when pressure is stopped can be sufficiently increased.

[0080] (2) Adjusting the number of pulse energizations In a preferred embodiment, the number of pulse currents in the post-current step is adjusted to 0 or more when (TR0 - TM0) < 300°C, adjusted to 1 or more when 300°C ≦ (TR0 - TM0) < 400°C, and adjusted to 3 or more when 400°C ≦ (TR0 - TM0). When (TR0 - TM0) < 300°C, the melting point TM0 of the coating layer included in the evaluation position at time t0 when the main current step is completed is already sufficiently high, so that the post-current step can be omitted or even with a small number of pulse currents, formula (1) can be satisfied. When 300°C ≦ (TR0 - TM0) < 400°C, the magnitude of (TR0 - TM0) is moderate, so the number of pulse currents is adjusted to 1 or more to perform the post-current step, and the melting point TM1 of the coating layer at time t1 when pressing is completed can be appropriately increased. When 400°C≦(TR0-TM0), the melting point TM0 of the plating layer at time t0 when the main current application process is completed is low, so by adjusting the number of pulse current applications to three or more times and performing a post-current application process, the melting point TM1 of the plating layer at time t1 when the pressure application is completed can be sufficiently increased.

[0081] 5 and 7 again, in these cases, the temperature TR0 at the evaluation position at time t0 when the main current application process was completed is estimated to be 1155°C, and the melting point TM0 of the coating layer included in the evaluation position at time t0 is estimated to be 672°C, resulting in a value of (TR0 - TM0) of 483°C. If 483°C is the maximum value among all the evaluation positions for which calculations were performed, this corresponds to the above-mentioned case of 400°C ≦ (TR0 - TM0). Therefore, as conditions for the post-current application process in this case, it is preferable to implement either or both of the following: adjusting the current value Ip to 6.0 kiloamperes or more and adjusting the number of pulse current applications to three or more.

[0082] In a preferred embodiment, resistance spot welding is performed according to the post-main current-flow conditions that have been adjusted in advance using the method described above. However, when implementing the method for manufacturing a resistance spot-welded joint according to the present invention, it is not necessary to adjust the first through sixth steps and the post-main current-flow conditions every time. For example, if the manufacturing of a resistance spot-welded joint according to the present invention is repeatedly performed while maintaining the same conditions, such as the plate assembly, pressure, and current and current duration of the post-main current-flow, steps one through six can be performed first, and the post-main current-flow resistance spot welding conditions adjusted by these steps can be fixed and the manufacturing method for a resistance spot-welded joint according to the present invention can be repeatedly performed. Furthermore, even if the post-main current-flow conditions are changed midway through the manufacturing process, as long as the changed conditions are the same as those used to manufacture a resistance spot-welded joint in the past, the manufacturing of a resistance spot-welded joint can be performed by applying the post-main current-flow resistance spot welding conditions adjusted by the previous steps one through six.

[0083] (3) Changes in the current value and duration of this current application process While the above description has focused on adjusting the conditions for resistance spot welding after the completion of the main current process, the present invention does not preclude changing the conditions for the main current process. That is, the present invention allows for satisfying formula (1) by changing at least one of the current value and current duration of the main current process, in addition to or without adjusting the conditions for the post-current process.

[0084] [Tensile strength of steel plate] In a preferred embodiment, at least one of the two or more steel sheets has a tensile strength of 590 MPa or more. As mentioned above, in the current situation where high alloying is being promoted in line with the recent trend toward higher strength steel sheets, the application of the technology proposed in Patent Document 1, which specifies a specific range for the chemical composition of the steel sheet, is extremely limited. According to the present invention, it is possible to suppress the occurrence of LME cracking in resistance spot welds regardless of the chemical composition of the steel sheet. More preferably, at least one of the two or more steel sheets has a tensile strength of 980 MPa or more.

[0085] [Disturbance during welding] Next, we will explain welding process disturbances, which are thought to be one of the main factors that induce LME cracking. Here, "process disturbances" refers to a state in which one or more of the resistance spot welding process conditions deviates from the desired conditions.

[0086] In a preferred embodiment, immediately before the start of the current application step, one or more conditions selected from the following (a) to (d) may be satisfied. (a) The striking angle of the pair of electrodes is 0.2 degrees or more. (b) The gap between overlapping steel sheets is 0.5 mm or more. (c) A state in which the gap between the fixed electrode of the pair of electrodes and the steel plate is 0.5 mm or more (d) The misalignment of a pair of electrodes is 0.1 mm or more. Here, "immediately before the start of the main current application process" refers to the state immediately before the start of pressure application after the pair of electrodes attached to the welding device have completed positioning of the welding position.

[0087] The above conditions (a) to (d) all qualify as welding process disturbances. These process disturbances locally increase the temperature and / or tensile stress of the weld when the electrodes are released. Therefore, resistance spot-welded joints subjected to resistance spot welding involving process disturbances are prone to LME cracking. However, by applying the method for manufacturing a resistance spot-welded joint according to the present invention, LME cracking can be suppressed even in the presence of these process disturbances, thereby expanding the tolerance for process disturbances when manufacturing a resistance spot-welded joint. In other words, the effect of preventing LME cracking according to the present invention is more pronounced in situations where process disturbances are present. Each process disturbance is described in detail below.

[0088] (a) The striking angle of the pair of electrodes is 0.2 degrees or more. The "strike angle" of the electrode refers to the angle at which the electrode is tilted relative to the steel sheet, i.e., the angle between the direction of the electrode pressure and the direction of the steel sheet thickness. Figure 8(A) shows a schematic diagram illustrating the definition of the strike angle. If the strike angle 10 is large, bending stress is applied to the weld, causing large localized compressive plastic deformation, which increases the tensile stress after cooling.

[0089] As shown in Figure 8(B), when resistance spot welding is performed with a stroke angle, the resulting nugget is tilted at an angle equal to the stroke angle. This "stroke angle" is a welding process disturbance. However, for the reasons described above, the "nugget tilt" of the resistance spot-welded joint obtained after welding can be used as a substitute for this "stroke angle." More specifically, as shown in Figure 8(B), a line connecting the outermost portions (i.e., the boundary where deformation due to electrode pressure is no longer observed) of the left and right shoulders on the upper sheet 1 side (i.e., the outermost steel sheet side of the sheet assembly) is defined as the reference line 1a. Next, two perpendicular lines are drawn from positions 500 μm to the left and right of the center of the reference line 1a, and lines are drawn that intersect with the outer periphery of the nugget 4a. Here, the "two perpendicular lines" refer to two perpendicular lines parallel to a midline drawn from the center of the reference line perpendicular to the reference line. Finally, the angle between this straight line and the reference line 1a, that is, the inclination of the nugget, is regarded as the hitting angle 10.

[0090] The effects of the present invention are most pronounced when the impact angle is 0.2 degrees or greater. An excessively large impact angle can lead to unstable nugget formation and cause expulsion, so the impact angle is preferably 10.0 degrees or less. The impact angle is more preferably 1.0 degrees or greater, and even more preferably 8.0 degrees or less.

[0091] (b) The gap between overlapping steel sheets is 0.5 mm or more. The gap between the steel sheets constituting a resistance spot-welded joint may change slightly due to deformation of the steel sheets during welding. However, if a gap exists between the steel sheets in the cross section after welding, it is highly likely that a gap also existed between the steel sheets before welding. Therefore, in the present invention, the cross section after welding is observed to determine the gap between the steel sheets, and the determined value is considered to be the value of the gap between the steel sheets before welding. In other words, the "gap between the steel sheets" described in (b) above is one of the construction disturbances during welding, but for the reasons described above, the "gap between the steel sheets after welding" in the resistance spot-welded joint obtained after welding may be substituted for this "gap between the steel sheets."

[0092] The "gap between steel sheets after welding" can be determined by the method shown in Figure 9(A). In the case of continuous welding points as shown in Figure 9(A), the difference between the maximum thickness of steel sheets 1 and 2 between two adjacent nuggets 4a and the total thickness of the overlapped steel sheets 1 and 2 is calculated, and this is taken as the "gap between steel sheets after welding." In other cases, such as an end portion as shown in Figure 9(B), the difference between the maximum thickness of steel sheets 1 and 2 from the nugget 4a to the end of the sheet combination and the total thickness of the overlapped steel sheets 1 and 2 is calculated, and this is taken as the "gap between steel sheets after welding." The obtained gap between the steel sheets after welding is then regarded as the gap 11 between the steel sheets.

[0093] The effects of the present invention are most pronounced when the gap between the overlapping steel sheets is 0.5 mm or more. If this gap is too large, nugget formation becomes unstable, which can cause expulsion, so this gap is preferably 4.0 mm or less. The gap is more preferably 1.0 mm or more, and more preferably 3.0 mm or less.

[0094] (c) A state in which the gap between the fixed electrode and the steel plate of a pair of electrodes is 0.5 mm or more As in the case of (b) above, if there is a gap between either electrode and the steel sheet immediately before the start of pressure application, bending deformation occurs in the steel sheet, and bending stress is applied to the weld, making LME cracking more likely to occur. Specifically, in the example shown in Figure 10, when the upper electrode 7 is a movable electrode and the lower electrode 8 is a fixed electrode, if there is a large gap between the lower electrode 8 and the lower sheet 2, bending deformation occurs in the upper sheet 1 and the lower sheet 2 between the time when the upper electrode 8 starts to move and comes into contact with the upper sheet 1 and the time when the lower sheet 2 comes into contact with the lower electrode 8, and bending stress is applied to the weld. This makes LME cracking more likely to occur.

[0095] This "gap between the fixed electrode and the steel plate" is found by the method shown in Figure 10. As shown in Figure 10, the distance between the surface of the lower electrode 8, which is the fixed electrode, and the lower plate 2 facing it is found, and the obtained distance is taken as the gap 12 between the fixed electrode and the steel plate. When three or more steel plates are stacked, the distance between the fixed electrode and the steel plate can be found by the same method.

[0096] The effects of the present invention are effectively achieved when the gap between the fixed electrode and the steel sheet thus determined is 0.5 mm or more. If this gap is too large, nugget formation becomes unstable and causes expulsion, so this gap is preferably 5.0 mm or less. The gap is more preferably 1.0 mm or more, and more preferably 3.0 mm or less.

[0097] (d) The misalignment of a pair of electrodes is 0.1 mm or more. "Misalignment" of electrodes refers to a state in which the central axes of a pair of electrodes are not aligned. "Misalignment amount" refers to the magnitude of the misalignment. As with the impact angle described above, if the misalignment amount is large, bending stress is applied to the weld, making LME cracking more likely to occur. The misalignment amount is determined using the method shown in Figure 11. As shown in Figure 11, the misalignment amount 13 is determined by determining the amount of misalignment between the axis of the upper electrode 7 and the axis of the lower electrode 8 in the sheet width direction.

[0098] The effects of the present invention are most pronounced when the misalignment is 0.1 mm or greater. Excessive misalignment can lead to unstable nugget formation and expulsion, so the misalignment is preferably 5.0 mm or less. The misalignment is more preferably 0.2 mm or greater, and even more preferably 3.0 mm or less.

[0099] [Method for predicting the melting point of the plating layer] In another embodiment, the present invention relates to a method for predicting the melting point of a coating layer of a zinc-based plated steel sheet during resistance spot welding, in which two or more steel sheets, including at least one zinc-based plated steel sheet, are overlapped, sandwiched between a pair of electrodes, and current is applied while pressure is applied in the sheet thickness direction. The resistance spot welding is not particularly limited, but may include a main current application step for forming a nugget that joins the steel sheets during a pressure holding period from the start of pressure application by the pair of electrodes to the end of pressure application when the pair of electrodes is released, and then, optionally, one or more post-current application steps for post-heat treatment.

[0100] A method for predicting the melting point of a plating layer according to the present invention includes the steps of: creating an analytical model for performing finite element analysis using a computer, the analytical model being composed of a plurality of meshes that divide an area including two or more steel sheets and a pair of electrodes; providing one or more evaluation positions in the plurality of meshes that include the plating layers of the zinc-based plated steel sheets on the analytical model; calculating the temperature history at the evaluation positions during resistance spot welding by performing finite element analysis while providing resistance spot welding conditions as boundary conditions; calculating the zinc concentration of the plating layer included at the evaluation positions based on the calculated temperature history at the evaluation positions; and determining the melting point of the plating layer included at the evaluation positions based on the calculated zinc concentration.

[0101] These five steps are almost identical to the six steps (steps 1 to 6) for determining TR1 and TM1 described above, excluding step 4. Therefore, the above description will be cited here, and a detailed description of these five steps will be omitted. However, while the zinc concentration in step 5 and the melting point of the coating phase in step 6 are determined as values ​​at time t1 when pressing is completed, in this embodiment, the time is not limited as long as they are during the resistance spot welding.

[0102] According to the method for predicting the melting point of a coating layer according to the present invention, it is possible to predict with high accuracy the melting point of a coating layer provided on a zinc-based coated steel sheet during resistance spot welding.

[0103] In a preferred embodiment, in the method for predicting the melting point of a coating layer according to the present invention, in the step of calculating the zinc concentration of the coating layer included in the evaluation position, the initial coating layer thickness used in the calculation is set to a value smaller than the coating layer thickness on the zinc-based coated steel sheet before welding. As described above, during the process of resistance spot welding, the coating layer thickness becomes significantly thinner than the coating layer thickness on the zinc-based coated steel sheet before welding. By setting the initial coating layer thickness used in the calculation to a value smaller than the coating layer thickness on the zinc-based coated steel sheet before welding based on the actual thickness of the coating layer, the accuracy of the calculation can be further improved. [Example]

[0104] The functions and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0105] First, an analytical model was created for performing finite element analysis on a computer, consisting of two galvannealed steel sheets (GA) with a tensile strength of 1470 MPa and multiple meshes dividing the area including a pair of electrodes. The galvannealed steel sheets had a thickness of 1.8 mm and a zinc coating weight of 45 g / m. 2 The electrode was a chromium copper DR-type electrode with a tip curvature radius R of 40 mm and a tip diameter of 6 mm. The disturbances in the process immediately before starting the current application process were an impact angle of 5.0 degrees, a gap between the fixed electrode and the steel plate of 2.0 mm, and a misalignment of the electrode of 1.0 mm. Table 1 shows the pressure applied to the electrode, the current value I0 for the current application process, and the current application time T0, which were set as conditions for the current application process.

[0106] Next, the temperature TR0 at the evaluation position at the end of this current application process and the melting point TM0 of the coating layer included in the evaluation position were calculated using the same method as steps 1 to 6 of the present invention. The evaluation position set in step 2 was a position on the steel sheet mating surface where the coating layer was present, 600 μm from the edge of the nugget toward the HAZ (heat-affected zone). In step 3, the temperature history at the evaluation position was calculated using the finite element analysis software SORPAS 2D ("SORPAS" is a registered trademark). In step 5, the zinc concentration distribution was calculated using the diffusion equation described in Non-Patent Document 1. In step S6, the melting point TM0 of the coating layer included in the evaluation position was determined using an Fe-Zn binary equilibrium phase diagram. The calculated values ​​obtained in the above steps and the value of (TR0 - TM0) are shown in Table 1.

[0107] Next, the current value Ip of the pulse current in the post-current process following the main current process, the number of pulse currents, and the pressurization holding time from the end of the final current process to the end of pressurization were set as shown in Table 1. The current time when pulse current was performed was 60 ms. When pulse current was performed only once, the cooling time between the main current process and the pulse current was 80 ms. When pulse current was performed multiple times, the cooling time (non-current time) between pulse currents was also 80 ms.

[0108] Next, the temperature TR1 of the evaluation position when pressure application at the same evaluation position was completed using steps 1 to 6 of the present invention, the melting point TM1 of the plating layer included in the evaluation position, and (TR1-TM1) were calculated. The calculation results are shown in Table 1. As shown in Table 1, No. 1 and 3 to 6 are invention examples whose calculation results satisfy formula (1). On the other hand, No. 2 and 7 are comparative examples whose calculation results do not satisfy formula (1).

[0109] Next, prepare the actual plate assembly, set the plate assembly on a resistance spot welding machine, sandwich it with a pair of electrodes, and perform resistance spot welding under the conditions shown in Table 1 to obtain resistance spot weld joints numbered from 1 to 7. The resistance spot welding apparatus had electrodes attached to a welding gun and driven by a servo motor pressure type, and a resistance spot welding machine using single-phase alternating current (50 Hz) as the power supply was used. Next, using the obtained resistance spot weld joints, the evaluation of LME cracking in the resistance spot welded portion was performed by the method shown below.

[0110] <Evaluation of LME Cracking> After cutting the center of the welded portion of the obtained resistance spot weld joint with a micro cutter, the cross-section of the welded portion was observed to evaluate the presence or absence of LME cracking. The position for confirming the presence or absence of LME cracking in the cross-section observation was set as the plate-plate interface (i.e., the side of the steel plate mating surface). For 5 samples under each manufacturing condition, evaluation was performed according to the following evaluation criteria. The results of the crack evaluation are shown in Table 1. Here, when the evaluation result was judged as A or B, it was regarded as "qualified".

[0111] <Evaluation Criteria> Judgment A: 0 / 5 (i.e., the number of cracked specimens among 5 specimens is 0) Judgment B: 1 / 5 (i.e., the number of cracked specimens among 5 specimens is 1) Judgment C: 2 / 5 - 4 / 5 (i.e., the number of cracked specimens among 5 specimens is 2 - 4)

[0112]

Table 1

[0113] As shown in Table 1, in the resistance spot weld joints of the inventive examples numbered 1 and 3 to 6 where the conditions of the resistance spot welding after the end of this energization satisfied Equation (1), all the evaluation results of LEM cracking were qualified. On the other hand, in the resistance spot weld joints of the comparative examples numbered 2 and 7 that did not satisfy the conditions of Equation (1), all the results of the LEM cracking evaluation were judged as C.

Explanation of Reference Signs

[0114] : 1 Steel plate (upper plate) 1a Reference Line 2 Steel plate (lower plate) 3 Steel plate (medium plate) 4 Resistance spot welds 4a Nugget 4b HAZ (heat affected zone) 5 Resistance spot welded joints 6 Steel plate mating surface 6a Steel plate mating surface (between the upper and middle plates) 6b Steel plate mating surface (between middle plate and bottom plate) 7 Upper electrode 8 Lower electrode 9 LME crack 10 batting angle 11 Gap between steel plates 12 Gap between the fixed electrode and the steel plate 13 Electrode misalignment x rolling direction y Sheet width direction z Thickness direction t0 The time when the current application process was completed t1 Time when pressure application ended Steps S1 to S6 of the manufacturing method for resistance spot welded joints

Claims

1. A method for manufacturing a resistance spot welded joint, comprising: overlapping two or more steel sheets including at least one zinc-based plated steel sheet; sandwiching the steel sheets between a pair of electrodes; and joining the steel sheets by resistance spot welding in which an electric current is passed through the steel sheets while applying pressure in a thickness direction of the steel sheets, the method comprising: The resistance spot welding includes a main current application process for forming a nugget that joins the steel sheets during a pressure holding period from the start of the pressure application by the pair of electrodes to the end of the pressure application by releasing the pair of electrodes, and thereafter, optionally, one or more post-current application processes for post-heat treatment, The temperature at one or more evaluation positions provided in the region including the plating layer of the zinc-based plated steel sheet when the pressure is stopped is defined as TR. １ The melting point of the plating layer at the evaluation position when the pressure is stopped is defined as TM １ When this is done, TR １ and TM １ The resistance spot welding is performed under the condition that satisfies the following formula (1), TR 1 and TM 1 are a first step of creating an analytical model for performing finite element analysis using a computer, the analytical model being configured with a plurality of meshes that divide an area including the two or more steel plates and the pair of electrodes; a second step of providing the one or more evaluation positions in the plurality of meshes including the plating layer of the zinc-based plated steel sheet on the analysis model; a third step of calculating a temperature history at the evaluation position from the start of the main current application process to the end of the pressurization by performing the finite element analysis while giving the conditions of the main current application process and the conditions of resistance spot welding after the end of the main current application process as boundary conditions; a fourth step of calculating a temperature TR 1 at the evaluation position when the pressurization is completed based on the calculated temperature history at the evaluation position; a fifth step of calculating a zinc concentration of the plating layer included in the evaluation position when the pressurization is terminated based on the calculated temperature history at the evaluation position; a sixth step of determining a melting point TM 1 of the plating layer included in the evaluation position when the pressurization is terminated based on the calculated zinc concentration; A method for manufacturing a resistance spot welded joint, characterized in that the temperature is determined by the following formula: (TR １ -TM １ )≦220℃ (1)

2. The method for manufacturing a resistance spot welded joint according to claim 1 , wherein the boundary conditions for the main current application step are a current value and a current application time.

3. 2. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the boundary conditions for the resistance spot welding after completion of the main current flow process include whether or not the post-current flow process is required and the conditions for that process, and the time from the end of the final current flow process in the resistance spot welding to the end of the pressurization.

4. 4. The method for manufacturing a resistance spot welded joint according to claim 3, wherein the post-energization step is one or two or more pulse energizations that are performed with a non-energization period between them, and the condition for the post-energization step that is given as the boundary condition is one or both of a current value and a number of times of the pulse energization.

5. After determining the conditions of the main current application process, the first step to the sixth step are performed to obtain the TR. １ and TM １ Calculate the TR obtained １ and TM １ 4. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the conditions for the resistance spot welding after completion of the main current application step are adjusted so that the value of

6. After determining the conditions of the main current application process, the first step to the sixth step are performed to obtain the TR. １ and TM １ Calculate the TR obtained １ and TM １ 5. The method for manufacturing a resistance spot welded joint according to claim 4, wherein the conditions for the resistance spot welding after completion of the main current application step are adjusted so that the value of

7. The temperature at the evaluation position when the main current application process is completed is TR ０ The melting point of the plating layer included in the evaluation position when the main current application process is completed is defined as TM ０ When the current value I of the pulse current in the post-current application step is ｐ Regarding (TR ０ -TM ０ ) < 300 ° C, I ｐ Adjust to 0 (zero) kiloamperes or more, 300°C≦(TR ０ -TM ０ ) < 400 ° C, I ｐ Adjust to 4.0 kiloamperes or more, 400°C≦(TR ０ -TM ０ ) when I ｐ The method for manufacturing a resistance spot welded joint according to claim 6, wherein the current is adjusted to 6.0 kiloamperes or more.

8. Regarding the number of pulse energizations in the post-energization step, (TR ０ -TM ０ ) When <300 ° C, adjust to 0 times or more, 300°C≦(TR ０ -TM ０ ) When < 400 ° C, adjust it once or more, 400°C≦(TR ０ -TM ０ 8. The method for manufacturing a resistance spot welded joint according to claim 6, wherein the adjustment is made three or more times when the welding time is 100 s.i.

9. The method for manufacturing a resistance spot welded joint according to claim 1, wherein at least one of the two or more steel plates is a steel plate having a tensile strength of 590 MPa or more.

10. 2. The method for manufacturing a resistance spot welded joint according to claim 1, wherein immediately before starting the main current application step, one or more conditions selected from the following (a) to (d) are satisfied: (a) A state in which the strike angle of the pair of electrodes is 0.2 degrees or more (b) A state in which the gap between the overlapping steel plates is 0.5 mm or more (c) A state in which the gap between the fixed electrode of the pair of electrodes and the steel plate is 0.5 mm or more. (d) A state in which the amount of misalignment between the pair of electrodes is 0.1 mm or more.

11. A method for predicting the melting point of a coating layer of a zinc-based plated steel sheet in resistance spot welding in which two or more steel sheets including at least one zinc-based plated steel sheet are overlapped, sandwiched between a pair of electrodes, and current is applied while pressure is applied in the sheet thickness direction, comprising: creating an analytical model for performing finite element analysis using a computer, the analytical model being configured with a plurality of meshes that divide an area including the two or more steel plates and the pair of electrodes; providing one or more evaluation positions in the plurality of meshes including the plating layer of the zinc-based plated steel sheet on the analysis model; calculating a temperature history at the evaluation location during the resistance spot welding by performing the finite element analysis with the resistance spot welding conditions as boundary conditions; calculating a zinc concentration of the plating layer included in the evaluation position based on the calculated temperature history at the evaluation position; determining the melting point of the plating layer included in the evaluation position based on the calculated zinc concentration; A method for predicting the melting point of a plating layer, comprising:

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