Resistance spot welded joint manufacturing method and method for predicting melting point of plated layer

Abstract

The present invention provides a resistance spot welded joint manufacturing method which can prevent an LME crack in a resistance spot welded part in a sheet set in which a plurality of steel sheets including a zinc-based plated steel sheet are used. The method includes: a main energizing step for forming a nugget that bonds steel sheets together during a pressure holding period from the start of applying pressure by a pair of electrodes to the completion of the pressure application by releasing the pair of the electrodes; and thereafter, a post-energizing step for post-heat treatment, which is performed arbitrarily at least one time. A resistance spot welding is performed under a condition that TR1 and TM1 satisfy (TR1-TM1) ≤ 220°C, where TR1 represents a temperature at one or two or more evaluation positions set in a region including a plated layer provided in a zinc-based plated steel sheet at the completion of the pressure application, and TM1 represents a melting point of the plated layer at the evaluation position at the completion of the pressure application.

Description

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

[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. In particular, the present invention relates 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.

[0002] From the perspective of global environmental conservation ２ In order to reduce emissions, 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, it is also necessary to reduce the weight of automobile bodies while maintaining their strength. To achieve these goals, it is effective to reduce the thickness of steel sheets used as materials for automotive parts by increasing their strength, and in recent years, the use of steel sheets with a tensile strength (TS) of 980 MPa or more has been promoted.

[0003] In the automobile assembly process, overlapping steel sheets are typically joined by welding when assembling press-formed automobile parts. From the viewpoints 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 during pressure application. In the example shown in Fig. 1, two steel sheets 1, 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 a portion where the steel sheets 1, 2 melt at the contact point when current is passed through the overlapping steel sheets and then solidify. This results in a point-like joining of the steel sheets.

[0005] For automotive steel sheets, corrosion-resistant surface-treated steel sheets, such as zinc (Zn)-plated steel sheets, are used for parts at risk of corrosion. However, resistance spot welding of overlapping steel sheets, including surface-treated 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 a low-melting-point metal plating layer on the surface of the surface-treated steel sheet melts during welding. When tensile stress due to the electrode pressure and the thermal expansion and contraction of the steel sheet is applied to the weld, the molten low-melting-point metal penetrates the grain boundaries of the base material of the surface-treated steel sheet, reducing the grain boundary strength and causing cracks. As shown in FIG. 1 , LME cracks 9 occur at various locations, such as the surfaces of steel sheets 1 and 2 in contact with 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] As a countermeasure against such LME cracking, for example, techniques described in Patent Documents 1 to 3 are known. Patent Document 1 proposes that the chemical composition of the steel sheets used in the sheet assembly be set 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) where t is the thickness of the steel sheet (mm), WT is the welding current flow time (ms), and HT is the pressure holding time after welding current flow (ms). 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 using a high-strength galvanized steel sheet in which the amount of alloy elements in the steel sheet is equal to or less than 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.

[0010] Japanese Patent Laid-Open No. 10-195597 Japanese Patent Laid-Open No. 2003-103377 International Publication No. 2016 / 159169 Japanese Patent Laid-Open No. 2010-115706

[0011] P. Shewmon: Diffusion in Solids, The Minerals, Metals & Materials Society, Warrendale (1989).

[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 satisfy required performance. In particular, under the circumstances where high alloying is becoming more prevalent in recent steel sheets due to the increase in strength, 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 such that expulsion occurs, and does not mention LME cracking in a state in which 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.

[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 simply explained. However, the present 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. It is known that, in particular, at the steel sheet mating surfaces where steel sheets come into contact, a region of high localized tensile stress is generated when the electrodes are released after the current and pressure application in resistance spot welding is completed, and LME cracking is likely to occur in this region. Another factor that may be contributing to this is that when the strength of overlapping steel sheets is high, tensile stress is more likely to occur due to differences in transformation behavior during cooling.

[0019] As mentioned above, LME cracking occurs when tensile stress is applied to a steel sheet when liquid metal such as Zn is in contact with the steel sheet. The inventors have determined the temperature of the coating layer at the evaluation position when pressure from the electrode is removed (when the electrode is released) as TR. １ The melting point of the plating layer at the evaluation position when pressure is stopped is defined as TM １ When this is done, (TR １ -TM １ ) value, i.e., the temperature TR at the evaluation position １ is the melting point TM of the plating layer included in the evaluation position １I thought that the extent to which the TR is higher than the LME would be a good indicator of the risk of a breach. １ and TM １ The present inventors have discovered that the occurrence of LME cracking can be suppressed by performing resistance spot welding under conditions that satisfy the following formula (1): １ -TM １ )≦220°C (1)

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

[0021] [1] A method for manufacturing a resistance spot welded joint, 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 joined together by resistance spot welding in which an electric current is applied while pressure is applied in the sheet thickness direction to obtain a resistance spot welded joint, wherein the resistance spot welding includes a main current application step for forming a nugget that joins the steel sheets together during a pressure holding period from the start of pressure application by the pair of electrodes to the end of pressure application by releasing the pair of electrodes, and thereafter, optionally, one or more post-current application steps for post-heat treatment, and wherein 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 pressure application is completed 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 method for manufacturing a resistance spot welded joint is characterized in that the resistance spot welding is carried out under conditions that satisfy the following formula (1): (TR １ -TM １ )≦220°C (1)

[0022] [2] TR １ and TM １a 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 sheets and the pair of electrodes; a second step of providing the one or more evaluation positions in the plurality of meshes that include the plating layer of the zinc-based plated steel sheet on the analytical 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 pressing process by performing the finite element analysis while giving boundary conditions that are the conditions for the main current application process and the conditions for resistance spot welding after the main current application process; and a third step of calculating a temperature history TR at the evaluation position when the pressing process is ended based on the calculated temperature history at the evaluation position. １ a fourth step of calculating the zinc concentration of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated temperature history at the evaluation position; and a fifth step of calculating the melting point TM of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated zinc concentration. １ and a sixth step of determining the temperature by

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

[0024] [4] The method for manufacturing a resistance spot welded joint according to [2] or [3] above, wherein the boundary conditions for the resistance spot welding after the completion of the main current process are the necessity and conditions for performing the post-current process, 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 the above [4], wherein the post-energization process is one or two or more pulse energizations performed with a non-energization period sandwiched between them, and the condition for the post-energization process given as the boundary condition is one or both of the current value and the number of times of the pulse energization.

[0026] [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

[0023] The method for manufacturing a resistance spot welded joint according to any one of [2] to [5] above, wherein the conditions for the resistance spot welding after completion of the main current application step are adjusted so that the value of (I) satisfies the formula (1).

[0027] [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 the temperature to 0 (zero) kiloamperes or more, and ０ -TM ０ ) < 400 ° C, I ｐ Adjust the temperature to 4.0 kiloamperes or more, and ０ -TM ０ ) when I ｐ 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] The number of pulse energizations in the post-energization step is: (TR ０ -TM ０ )<300℃, adjust to 0 times or more, 300℃≦(TR ０ -TM ０ ) < 400°C, adjust it once or more, and 400°C ≦ (TR ０ -TM ０ 8. The method for manufacturing a resistance spot welded joint according to claim 6 or 7, wherein the number of welding times is adjusted three or more times.

[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] The 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 step, one or more of the following conditions (a) to (d) are satisfied: (a) the impact angle of the pair of electrodes is 0.2 degrees or more, (b) the gap between the overlapping steel sheets is 0.5 mm or more, (c) the gap between the fixed electrode of the pair of electrodes and the steel sheet is 0.5 mm or more, and (d) the misalignment of the pair of electrodes is 0.1 mm or more.

[0031]

[11] A method for predicting the melting point of a plating 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 method comprising: 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 sheets and the 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 a temperature history at the evaluation positions during the resistance spot welding by performing the finite element analysis while providing conditions for the resistance spot welding as boundary conditions; calculating a zinc concentration in the plating layer included at the evaluation position based on the calculated temperature history at the evaluation position; and determining the melting point of the plating layer included at the evaluation position based on the calculated zinc concentration.

[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, particularly in sheet assemblies using multiple steel sheets including zinc-based plated steel sheets.

[0033] 1 is a cross-sectional view in the plate thickness direction during pressure application in resistance spot welding; 2 is a cross-sectional view in the plate thickness direction that schematically shows the main parts of a resistance spot welded joint; 2 (A) shows the case where two steel plates are overlapped, and 2 (B) shows the case where three steel plates are overlapped; 3 is a time chart showing an example of the timing of pressure application by electrodes and current application; 3 (A) shows the case where only main current application is performed, and 3 (B) shows the case where post-current application is performed twice after main current application. TR １ and TM １ 8(A) is a flowchart showing steps for determining the melting point of a coating layer. FIG. 8(B ...

[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] [Method for manufacturing a resistance spot-welded joint] In one embodiment, the present invention is 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 by resistance spot welding in which an electric current is applied while pressure is applied in the sheet thickness direction to obtain a resistance spot-welded joint. The present invention is applicable to a method for manufacturing a resistance spot-welded joint in which at least one of the two or more steel sheets is 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-Zn-Ni alloy-plated steel sheet (Zn-11 mass% Ni), etc. can be used, but the term is 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 controlling the welding pressure and welding current as desired during welding. The welding device's pressure mechanism (e.g., air cylinder or servo motor), type (e.g., stationary, robot gun), and electrode shape are not particularly limited. The electrode tip type may be, for example, a DR (dome radius), R (radius), or D (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. More preferably, the number of steel sheets is two or three.

[0040] Figure 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 Figure 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, including 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") disposed 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] Next, the technical concept of the present invention will be described in detail. Resistance spot welding according to the present invention 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 to the end of pressure application when the pair of electrodes is released, and thereafter, optionally, one or more post-current application processes for post-heat treatment.

[0043] 3A and 3B are time charts showing examples of 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, and the vertical axis of the time chart represents the magnitude of the pressure and the magnitude of the current. Fig. 3A is a time chart for when only main current application is performed, in which overlapped steel sheets are first sandwiched between a pair of electrodes and pressure in the sheet thickness direction is started, then main current application is started, and at time t ０After that, the applied current is stopped and the applied pressure is maintained for a certain period of time. １ At time t, the pressure is stopped and the electrodes are released. FIG. 3B is a time chart showing the case where post-current application is performed twice after the main current application. In this case, the pressure is maintained for a certain period of time after the main current application and post-current application are completed. １ In the step (b), the pressure is stopped and the electrode is released. In the resistance spot welding according to the present invention, the main current application step is an essential step. However, it is optional whether or not to perform post-current application after the main current application.

[0044] In this embodiment, 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 application is finished is defined as TR. １ The melting point of the plating layer at the evaluation position when pressure is stopped is defined as TM １ When this is done, TR １ and TM １ Resistance spot welding is performed under the condition that satisfies the following formula (1): １ -TM １ )≦220°C (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, the temperature TR at the evaluation position when pressure application is completed (when the electrodes are opened) １ is the melting point TM of the plating layer included in the evaluation position １ If the TR is higher than 1 / 3, it indicates that liquid metal exists at the evaluation location and there is a risk of LME cracking. １ -TM １ ) is the temperature TR at the evaluation position １ is the melting point TM of the plating layer included in the evaluation position １ It can be said that this is an indicator of how much higher the risk is, i.e., the magnitude of the risk of a breach of the LME.

[0046] Then, (TR １ -TM １ The smaller the value of (TR １ -TM １It is also considered preferable that the value of ) is a negative value smaller than zero. However, it is also preferable that the diffusion of zinc in the plating layer is promoted to increase the concentration of iron and the melting point TM １ The temperature TR at the evaluation position １ However, it is not easy to raise the temperature to a higher level during the short current application time of resistance spot welding.

[0047] According to the study by the present inventors, (TR １ -TM １ ) is not a negative value less than zero, as shown in equation (1), (TR １ -TM １ It was found that if the value of TR is 220°C or less, the risk of LME cracking can be reduced. １ -TM １ The value of (TR) is preferably 200° C. or less, more preferably 180° C. or less. When evaluation positions are set at multiple locations, １ -TM １ ) is the largest, that is, the evaluation position where the risk of LME cracking is the greatest, resistance spot welding can be performed under conditions that satisfy formula (1).

[0048] (TR １ -TM １ However, the melting point TM of the plating layer included in the evaluation position is not limited to a specific value. １ Compared with the temperature TR at the evaluation position １ If you try to cool it too much, the welding process will take longer. １ -TM １ ) is preferably 0°C or higher. When evaluation positions are set at multiple locations, the value of (TR １ -TM １ ) at the evaluation position where the value of (TR １ -TM １ ) is preferably 0°C or higher.

[0049] In this embodiment, the method for calculating the value of the left side of the formula (1) may be any method, and is not particularly limited. １can be measured directly by, for example, providing a temperature sensor at an evaluation position of a resistance spot welded joint specially prepared for the purpose of measuring the temperature. １ In addition to these methods, TR can also be determined indirectly by using a finite element analysis in which the conditions for resistance spot welding are given as boundary conditions, as will be described later. １ and TM １ The value of can be calculated.

[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] [TR １ and TM １ In a preferred embodiment, the TR １ and TM １ The present invention relates to an analytical model for performing finite element analysis using a computer, the analytical model comprising a first step of creating 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 providing one or more evaluation positions at nodes of the plurality of meshes that include a plating layer of a zinc-based plated steel sheet on the analytical model; a third step of calculating the temperature history at the evaluation position from the start of the current application process to the end of pressurization by performing finite element analysis while giving boundary conditions that are the conditions for the current application process and the conditions for resistance spot welding after the current application process; and a third step of calculating the temperature history TR at the evaluation position when pressurization is ended based on the calculated temperature history at the evaluation position. １a fourth step of calculating the zinc concentration of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated temperature history at the evaluation position; a fifth step of calculating the melting point TM of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated zinc concentration; １ and the sixth step is to calculate the temperature.

[0052] FIG. 4 shows the TR in this preferred embodiment. １ and TM １ 1 is a flowchart showing the steps for determining the following. 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 components are superimposed. In this case, the entire plating layer having the 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 a plurality of meshes present in an area including a coating layer of the zinc-based coated steel sheet. Since LME cracking does not occur in areas where a coating layer is not present nearby, there is no need 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 contacts 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 contact 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 in which the conditions for the main current application process and the conditions for resistance spot welding after the main current application process are given as boundary conditions, and a finite element analysis is performed to calculate the temperature history at the evaluation position from the start of the main current application process to the end of pressurization. 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 pressurization.

[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 viewpoint 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 the analysis accuracy.

[0059] FIG. 5 shows the calculation results, as an example of the temperature history calculation in the third step, performed using the finite element analysis software SORPAS 2D ("SORPAS" is a registered trademark) for the case where three post-current processes are performed after the main current process. Prior to the calculation, the calculation accuracy 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 located 600 μm inside the HAZ (heat affected zone) from the position of the nugget end on the surface of the steel sheet mating surface. As shown in FIG. 5, at this evaluation position, the temperature history was calculated from the time when the main current process was started to the time when the main current was stopped, t ０ The temperature increases monotonically during the period from the end of the post-current application step to the end of the pressurization step. １ The calculation results showed that the temperature decreased monotonically during the period up to

[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. In order to obtain accurate calculation results, it is preferable to reflect the conditions when actual resistance spot welding is performed 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 provided 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 completion of the main current flow are whether or not a post-current flow is performed and the conditions for this, as well as the time from the end of the final current flow to the end of pressurization in the resistance spot welding, and by providing these conditions as boundary conditions in the third step, the heating and cooling conditions after the completion of the main current flow are reflected in the calculation.

[0063] The conditions for resistance spot welding after the main current application step include the number of current applications and the welding current value in the post-current application step, as well as the case where the post-current application step itself is not performed. That is, if formula (1) is satisfied without the post-current application step, the post-current application step can be omitted. In this case, the main current application step is the final current application step.

[0064] The time from the end of the final current application step to the end of pressure application in resistance spot welding is related to the cooling conditions. After the end of the final current application step in resistance spot welding, rather than immediately ending the pressure and releasing the electrode, cooling may be promoted by maintaining the pressure for a certain period of time and then ending the pressure. For example, if the electrode used in resistance spot welding is equipped with a water-cooling mechanism, maintaining the pressure promotes cooling of the weld. This promotes solidification of the coating layer and suppresses 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. By applying pulse current, the temperature of the welded part is not excessively increased, and the diffusion of zinc in the plating layer is promoted, and the final melting point TM of the plating layer is reached. １ The pulse current may be a direct current. If the current value of the pulse current is too low, the heat generation is insufficient, and the melting point TM of the plating layer is lowered. １ It becomes difficult to increase the temperature. 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 excessively high, the temperature of the welded portion rises 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 current duration of the pulse current is not particularly limited, but can be, for example, 30 ms or more and 100 ms or less.

[0067] Referring again to Fig. 3(B), when a post-current application step using pulse current is performed after the main current application step, it is preferable to provide a cooling time between the main current application step 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 step 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 In the fourth step (S4), the temperature TR at the evaluation position when the pressure application is completed is calculated based on the temperature history at the evaluation position calculated in the third step. １ Referring again to FIG. 5, in this calculation result, the time t １ Temperature TR at the evaluation position １ 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 described in known literature, 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 be 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 calculation accuracy of the zinc concentration described above is not particularly limited as long as it can quantitatively measure the zinc concentration. The zinc concentration can be measured, for example, by EPMA or SEM-EDS. Measurement conditions for measuring the zinc concentration using SEM-EDS include, for example, using a Ka radiation 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 the zinc concentration using these analytical methods, the measurement should be performed at the same position as the evaluation position set in the second step described above. However, if the measured zinc concentration at that evaluation position is extremely unstable compared to the measured values ​​at adjacent locations, the measurement may be performed within a range of ±20 μm from the evaluation position, avoiding the unstable region 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 starting the 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 pressing 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 pressure and heat applied by the electrodes. In general, when the coating layer thickness is correctly set, the maximum zinc concentration at the center of the coating layer is significantly lower than when calculated using the original thickness. Performing the calculation based on the actual coating layer thickness improves calculation accuracy.

[0073] (6) Sixth Step In the sixth step, the melting point TM of the plating layer included in the evaluation position when the pressure is stopped is calculated based on the zinc concentration calculated in the fifth step. １ This is the step to calculate the melting point TM １ can be easily obtained using the Fe-Zn binary equilibrium phase diagram.１ An example of how to calculate the melting point TM is shown below. The maximum zinc concentration at the center of the coating layer calculated 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, １ It can be seen that the temperature is 672°C.

[0074] As described above, by performing the first to fourth steps, the temperature TR of the evaluation position when the pressure application at one evaluation position is completed is calculated. １ Furthermore, by subsequently performing the fifth and sixth steps, the melting point TM of the plating layer included in the same evaluation position can be grasped. １ When multiple evaluation positions are set, the above steps are performed for each evaluation position.

[0075] [Adjustment of the resistance spot welding conditions after the main current application process] In a preferred embodiment, after the conditions of the main current application process are determined, the first to sixth steps are performed to adjust the TR. １ and TM １ Calculate the TR obtained １ and TM １ The resistance spot welding conditions are adjusted so that the value satisfies the formula (1).

[0076] TR １ and TM １ In order for the value of to satisfy the formula (1), for example, the melting point TM of the plating layer due to the promotion of interdiffusion between Fe and zinc-based plating metal is １ To achieve this, it is effective to adjust the conditions of the resistance spot welding after the completion of the main current application process so that the wire is held in a high temperature range for a long time so that alloying of the Fe and the zinc-based plated metal progresses during current application. However, the required degree of alloying differs depending on the plate assembly and welding conditions, and therefore the required holding time in the high temperature range also differs. Therefore, in a preferred embodiment, the temperature TR at the evaluation position when the main current application process is completed is used. ０ and the melting point TM of the plating layer included in the evaluation position ０ Difference with (TR０ -TM ０ The conditions for the subsequent processes are determined based on the results of the above, and the welding conditions are adjusted appropriately. An example of a specific method for adjusting the resistance spot welding conditions will be described below.

[0077] (1) Adjustment of the current value of pulse current. In a preferred embodiment, the temperature at the evaluation position when the current application process is completed is referred to as TR. ０ The melting point of the plating layer included in the evaluation position after the current application process is completed is defined as TM ０ When the current value I of the pulse current in the post-current process is ｐ Regarding (TR ０ -TM ０ ) < 300 ° C, I ｐ is adjusted to 0 (zero) kiloamperes or more, and 300°C ≦ (TR ０ -TM ０ ) < 400 ° C, I ｐ Adjust the temperature to 4.0 kiloamperes or more, and set the temperature to 400°C or less (TR ０ -TM ０ ) when I ｐ Adjust to 6.0 kiloamperes or more.

[0078] The time t when this current application process is completed ０ Temperature TR at the evaluation position ０ and the melting point TM of the plating layer included in the evaluation position when this current application process is completed. ０ is the time t when the pressure is stopped. １ Temperature TR at the evaluation position １ and the melting point TM of the plating layer included in the evaluation position １ However, when performing the calculation of the third step in this case, it is sufficient to provide the conditions for the main current application process as boundary conditions, and the conditions for the resistance spot welding after the main current application process are not required.

[0079] (TR ０ -TM ０ ) < 300 ° C., the time t ０ The melting point TM of the plating layer included in the evaluation position ０Since the current value I is already sufficiently high, the post-energization step is omitted or the current value I ｐ Even if the temperature is small, the formula (1) can be satisfied. ０ -TM ０ ) < 400 ° C., (TR ０ -TM ０ ) is of medium size, so I ｐ The current is controlled to 4.0 kiloamperes or more to carry out the post-energization step, and at the time t １ Melting point TM of the plating layer １ can be increased moderately. ０ -TM ０ ) when the current application process is completed, ０ Melting point TM of the plating layer ０ is low, so I of 6.0 kiloamperes or more ｐ The current is turned on and pressure is stopped at time t １ Melting point TM of the plating layer １ can be sufficiently increased.

[0080] (2) Adjustment of the Number of Pulse Currents In a preferred embodiment, the number of pulse currents in the post-current process is adjusted to (TR ０ -TM ０ ) < 300°C, adjust to 0 times or more, and 300°C ≦ (TR ０ -TM ０ ) < 400°C, adjust it once or more, and 400°C ≦ (TR ０ -TM ０ ) adjust it to three or more times. (TR ０ -TM ０ ) < 300 ° C., the time t ０ The melting point TM of the plating layer included in the evaluation position ０ Since the temperature is already sufficiently high, the formula (1) can be satisfied even if the post-current application step is omitted or the number of pulse current applications is small. ０ -TM ０ ) < 400 ° C., (TR ０ -TM ０ Since the magnitude of the pulse current is medium, the number of pulse current applications is adjusted to one or more times to perform the post-current application process.１ Melting point TM of the plating layer １ can be increased moderately. ０ -TM ０ ) when the current application process is completed, ０ Melting point TM of the plating layer ０ Since the time t １ Melting point TM of the plating layer １ can be sufficiently increased.

[0081] Referring again to FIGS. 5 and 7, in these cases, the time t ０ Temperature TR at the evaluation position ０ is 1155°C, and time t ０ The melting point TM of the plating layer included in the evaluation position ０ is estimated to be 672°C, (TR ０ -TM ０ ) is 483°C. If 483°C is the maximum value among all the evaluation positions where the calculation was performed, the above 400°C≦(TR ０ -TM ０ Therefore, in this case, the conditions for the post-energization step are preferably to adjust the current value Ip to 6.0 kiloamperes or more and / or to adjust the number of pulse energizations to 3 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 by 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 steps from step 1 to step 6 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 process, steps 1 to 6 can be performed first, and the manufacturing method for a resistance spot-welded joint according to the present invention can be repeatedly performed while keeping the post-main current-flow resistance spot welding conditions that have been adjusted accordingly. 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 that have been adjusted by steps 1 to 6 that have been previously performed.

[0083] (3) Changing the Current Value and Wet Time of the Main Wet Process While the above description has focused on adjusting the conditions for resistance spot welding after the completion of the main wet process, this does not preclude changing the conditions for the main wet process. That is, it is permissible in the present invention to satisfy formula (1) by changing at least one of the current value and wet time of the main wet process, in addition to or without adjusting the conditions for the post-wet process.

[0084] [Tensile strength of steel plate] In a preferred embodiment, at least one of the two or more steel plates has a tensile strength of 590 MPa or more. As described above, in the current situation where high alloying is being promoted in line with the recent trend toward higher strength steel plates, the application of the technology proposed in Patent Document 1, which specifies a specific range for the chemical composition of the steel plate, 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 plate. More preferably, at least one of the two or more steel plates has a tensile strength of 980 MPa or more.

[0085] [Welding Disturbance] Next, we will explain welding disturbance, which is considered to be one of the main factors that induce LME cracking. Here, "welding disturbance" refers to a state in which one or more of the welding conditions for resistance spot welding deviates from the preferred conditions.

[0086] In a preferred embodiment, immediately before the start of the main current application process, one or more of the following conditions (a) to (d) may be satisfied: (a) the impact angle of the pair of electrodes is 0.2 degrees or more, (b) the gap between the overlapping steel sheets is 0.5 mm or more, (c) the gap between the fixed electrode of the pair of electrodes and the steel sheet is 0.5 mm or more, and (d) the misalignment of the 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 pair of electrodes attached to the welding device have completed positioning at the welding position and pressure application begins.

[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) State in which the impact angle of a pair of electrodes is 0.2 degrees or more. The "impact angle" of the electrode refers to the angle at which the electrode is inclined relative to the steel sheet, i.e., the "angle between the electrode pressure direction and the steel sheet thickness direction." Figure 8(A) shows a schematic diagram illustrating the definition of the impact angle. If the impact 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 FIG. 8(B), when resistance spot welding is performed with a stroke angle, the nugget formed is tilted at an angle equal to the stroke angle. In other words, this "stroke angle" is one of the welding disturbances. However, for the reasons described above, the "nugget tilt" in the resistance spot welded joint obtained after welding may be substituted for this "stroke angle." More specifically, as shown in FIG. 8(B), a line connecting the outermost portions (i.e., the boundary portion 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 steel sheet side of the outermost layer 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 passing through the intersections of these lines 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, i.e., the inclination of the nugget, is regarded as the strike 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 makes nugget formation unstable and can 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 more preferably 8.0 degrees or less.

[0091] (b) A state in which the gap between overlapping steel sheets is 0.5 mm or more. The gap between the steel sheets constituting the 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, 2 between two adjacent nuggets 4a and the total thickness of the overlapped steel sheets 1, 2 is calculated, and this is defined 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, 2 from the nugget 4a to the end of the sheet combination and the total thickness of the overlapped steel sheets 1, 2 is calculated, and this is defined as the "gap between steel sheets after welding." The resulting 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 effectively achieved when the gap between the overlapping steel sheets is 0.5 mm or more. If this gap is too large, nugget formation becomes unstable and causes 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 of a pair of electrodes and the steel sheet is 0.5 mm or more. As in the case of (b) above, when 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 contacts the upper sheet 1 and the time when the lower sheet 2 contacts the lower electrode 8, and bending stress is applied to the weld, making LME cracking more likely to occur.

[0095] The "gap between the fixed electrode and the steel plate" is determined by the method shown in Fig. 10. As shown in Fig. 10, the distance between the surface of the lower electrode 8, which is the fixed electrode, and the lower plate 2 facing it is determined, 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 determined 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) A state in which the misalignment amount between a pair of electrodes is 0.1 mm or more. "Misalignment" of the electrodes refers to a state in which the central axes of the pair of electrodes are not aligned. Also, "amount of misalignment" refers to the magnitude of the misalignment. As in the case of 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. This misalignment amount is determined using the method shown in Figure 11. As shown in Figure 11, 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 is determined and taken as misalignment amount 13.

[0098] The effects of the present invention are effectively achieved when the misalignment amount is 0.1 mm or more. If the misalignment amount is too large, nugget formation becomes unstable and causes expulsion, so the misalignment amount is preferably 5.0 mm or less. The misalignment amount is more preferably 0.2 mm or more, and even more preferably 3.0 mm or less.

[0099] [Method for Predicting Melting Point of Galvanized 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 thereafter, 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 the same as the TR mentioned above. １ and TM １The steps except for the fourth step are almost the same as those of the six steps from the first step to the sixth step for determining the value of the zinc concentration in the fifth step. Therefore, the above explanation will be cited here, and a detailed explanation of these five steps will be omitted. However, the zinc concentration in the fifth step and the melting point of the plating phase in the sixth step are determined by the time t １ In contrast to the above, in the present embodiment, the time is not limited as long as it is 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.

[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 for performing finite element analysis on a computer was created, which consisted of two galvannealed steel sheets (GA) with a tensile strength of 1470 MPa and multiple meshes dividing an 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. ２The 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 during 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 sheet of 2.0 mm, and a misalignment of the electrode of 1.0 mm. Table 1 shows the pressure applied to the electrode and the current value I for the current application process, which were set as conditions for the current application process. ０ , and current application time T ０ Shows.

[0106] Next, the temperature TR of the evaluation position at the end of the current application process is measured by the same method as in the first to sixth steps of the present invention. ０ and the melting point TM of the plating layer included in the evaluation position ０ The evaluation position set in the second step was a position on the steel sheet mating surface where the coating layer was present, 600 μm inward from the edge of the nugget toward the HAZ (heat affected zone). In the third step, the temperature history at the evaluation position was calculated using the finite element analysis software SORPAS 2D ("SORPAS" is a registered trademark). In the fifth step, the zinc concentration distribution was calculated using the diffusion equation described in Non-Patent Document 1. In step S6, the melting point TM of the coating layer included in the evaluation position was calculated using an Fe-Zn binary equilibrium phase diagram. ０ The calculated values ​​obtained by the above steps and (TR ０ -TM ０ ) 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 TR of the evaluation position when the pressure application at the same evaluation position is completed by the first to sixth steps of the present invention. １ , the melting point TM of the plating layer included in the evaluation position １, and (TR １ -TM １ ) was 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, actual plate assemblies were prepared, set in a resistance spot welding machine, and sandwiched between a pair of electrodes. Resistance spot welding was performed under the conditions shown in Table 1 to obtain resistance spot-welded joints No. 1 to 7. The resistance spot welding device used had an electrode attached to a welding gun and driven by a servomotor pressure system, and was powered by a single-phase AC (50 Hz). Next, using the obtained resistance spot-welded joints, LME cracking was evaluated in the resistance spot welds using the method described below.

[0110] <Evaluation of LME cracking> The center of the weld of the resulting resistance spot welded joint was cut with a microcutter, and then the cross section of the weld was observed to evaluate the presence or absence of LME cracking. The position where the presence or absence of LME cracking was confirmed in the cross section observation was between the plates (i.e., on the mating surface side of the steel plates). Five samples for each manufacturing condition were evaluated according to the following evaluation criteria. The results of the crack evaluation are shown in Table 1. Here, evaluation results of A and B were considered to be "pass."

[0111] <Evaluation criteria> A rating: 0 / 5 (i.e., 0 out of 5 bodies were found to have cracks) B rating: 1 / 5 (i.e., 1 out of 5 bodies was found to have cracks) C rating: 2 / 5 to 4 / 5 (i.e., 2 to 4 out of 5 bodies were found to have cracks)

[0112]

[0113] As shown in Table 1, the resistance spot-welded joints of Inventive Examples No. 1 and 3 to 6, in which the resistance spot welding conditions after the end of main current flow satisfied formula (1), all received a pass rating for LEM cracking. On the other hand, the resistance spot-welded joints of Comparative Examples No. 2 and 7, in which the conditions of formula (1) were not satisfied, all received a C rating for LEM cracking.

[0114] REFERENCE SIGNS LIST 1 steel plate (upper plate) 1a reference line 2 steel plate (lower plate) 3 steel plate (middle plate) 4 resistance spot welded portion 4a nugget 4b HAZ (heat affected zone) 5 resistance spot welded joint 6 steel plate mating surface 6a steel plate mating surface (between upper plate and middle plate) 6b steel plate mating surface (between middle plate and lower plate) 7 upper electrode 8 lower electrode 9 LME crack 10 impact angle 11 gap between steel plates 12 gap between fixed electrode and steel plate 13 misalignment of electrode x rolling direction y plate width direction z plate thickness direction t ０ The time t when this current application process is completed １ Steps S1 to S6 in the method for manufacturing a resistance spot welded joint

Claims

1. A method for manufacturing a resistance spot welded joint, 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 joined together by resistance spot welding in which an electric current is applied while pressure is applied in the sheet thickness direction to obtain a resistance spot welded joint, wherein the resistance spot welding includes a main current application process for forming a nugget that joins the steel sheets together 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 thereafter, optionally, one or more post-current application processes for post-heat treatment, and wherein 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 pressure application is completed 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 method for manufacturing a resistance spot welded joint is characterized in that the resistance spot welding is carried out under conditions that satisfy the following formula (1): (TR １ -TM １ )≦220°C (1) 2. TR １ and TM １ 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 sheets and the pair of electrodes; a second step of providing the one or more evaluation positions in the plurality of meshes that include the plating layer of the zinc-based plated steel sheet on the analytical 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 pressing process by performing the finite element analysis while giving boundary conditions that are the conditions for the main current application process and the conditions for resistance spot welding after the main current application process; and a third step of calculating a temperature history TR at the evaluation position when the pressing process is ended based on the calculated temperature history at the evaluation position. １ a fourth step of calculating the zinc concentration of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated temperature history at the evaluation position; and a fifth step of calculating the melting point TM of the plating layer included in the evaluation position when the pressure application is terminated based on the calculated zinc concentration. １ and a sixth step of determining the temperature by:

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

4. A method for manufacturing a resistance spot welded joint as described in claim 2 or 3, wherein the conditions for resistance spot welding after completion of the main current flow process given as the boundary conditions are whether or not to perform the post-current flow process and the conditions under which it is performed, and the time from the end of the final current flow process in the resistance spot welding to the end of the pressure application.

5. A method for manufacturing a resistance spot welded joint as described in claim 4, 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.

6. After determining the conditions for the main current application process, the first to sixth steps are carried out to obtain the TR. １ and TM １ Calculate the TR obtained １ and TM １ 6. The method for manufacturing a resistance spot welded joint according to claim 2, wherein conditions for 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 called 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 the temperature to 0 (zero) kiloamperes or more, and ０ -TM ０ ) < 400 ° C, I ｐ Adjust the temperature to 4.0 kiloamperes or more, and 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. The number of pulse energizations in the post-energization step is: (TR ０ -TM ０ )<300℃, adjust to 0 times or more, 300℃≦(TR ０ -TM ０ ) < 400°C, adjust it once or more, and 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. A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 8, 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. A method for manufacturing a resistance spot welded joint as described in any one of claims 1 to 9, wherein immediately before starting the main current application process, 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 of the pair of electrodes is 0.1 mm or more.

11. A method for predicting the melting point of a plating 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, comprising 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 the two or more steel sheets and the pair of electrodes; providing one or more evaluation positions on 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 the resistance spot welding by performing the finite element analysis while providing the resistance spot welding conditions as boundary conditions; calculating the zinc concentration of the plating layer included at the evaluation position based on the calculated temperature history at the evaluation position; and determining the melting point of the plating layer included at the evaluation position based on the calculated zinc concentration.

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