Resistance spot welded joint and method for manufacturing a resistance spot welded joint

Abstract

One embodiment of the present application relates to a resistance spot welded joint, which is a resistance spot welded joint provided with a plurality of steel sheets overlapping each other and a welded portion having a nugget joining the steel sheets and a plastic metal zone and a heat-affected zone formed around the nugget, one or more of the plurality of steel sheets being a high-strength steel sheet having a tensile strength of 780 MPa or more, one or more of the plurality of steel sheets being a plated steel sheet having a zinc-based plated layer, the high-strength steel sheet and the plated steel sheet abutting each other at an overlapping surface, the heat-affected zone having a diameter of 1.5 times or more the diameter of the nugget, and 40 / 100 μm 2 The equivalent circle diameter of the carbide described above is 0.1 μm or more, the amount of η phase of the zinc-based plated layer in the plastic metal zone is 20 area% or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a resistance spot welded joint and a method for manufacturing a resistance spot welded joint. BACKGROUND

[0002] In recent years, in the automobile field, in order to reduce oil consumption and CO2 emission, etc., it is required to make the vehicle body lightweight. Also, in the automobile field, in order to improve the collision safety, it is required to make the vehicle body member high-strength. In order to meet these requirements, it is effective to set the material of various parts such as the vehicle body member to a high-strength steel sheet.

[0003] In addition, from the viewpoint of high rust prevention of the vehicle body, it is necessary to constitute the member with a steel sheet excellent in corrosion resistance. It is well known that a zinc-based plated steel sheet is excellent in corrosion resistance. From the above-described viewpoints of lightweight and high-strength, a zinc-based plated steel sheet used for automobile use is generally joined with a high-strength steel sheet, or the plated base sheet thereof is set to a high-strength steel sheet.

[0004] In the process of assembling the vehicle body of the automobile and installing the parts, etc., resistance spot welding is mainly used. Resistance spot welding is resistance welding in which a tip of an electrode having a tip appropriately shaped is sandwiched to a base material overlapped, and a current and a pressing pressure are concentrated to a small portion to locally heat, and at the same time, the electrode is pressed to perform. In order to apply resistance spot welding to joining of a high-strength steel sheet, various methods have been proposed.

[0005] In Patent Literature 1, a spot welding method of a high-strength steel sheet is disclosed, which is a spot welding method of a high-strength steel sheet, and spot welding is performed by a process of having a first step of generating a nugget by gradually increasing an electric current to the high-strength steel sheet, a second step of decreasing the electric current after the first step, and a third step of increasing the electric current after the second step to perform a main welding, and gradually decreasing the electric current.

[0006] In Patent Literature 2, a spot welding method of an aluminum-plated steel sheet is disclosed, in which aluminum-plated steel sheets as materials to be welded are overlapped with each other or with other metal sheets, and they are pressed in a state of being sandwiched by a pair of electrode tips, and an electric current is passed between the electrode tips to melt a welding portion of the materials to be welded by Joule heat, and then, the welding portion is cooled and solidified by stopping the passage of the electric current to form a nugget, and the spot welding method is characterized in that, as a preliminary process of a main welding process in which an electric current is passed to the welding portion at a constant AC cycle, an up-slope process in which the amount of the electric current is gradually increased is added to stabilize the temperature rising rate of the welding portion.

[0007] A method for manufacturing a steel / aluminum joint structure is disclosed in Patent Literature 3, which is characterized in that, when an aluminum material and a hot-dip aluminum plated steel sheet are overlapped and integrated by spot welding, a power supply mode in which a ratio Q1 / Q2 of a cumulative current Q1 during an uphill period from the start of power supply until a welding current reaches a set value W to a cumulative current Q2 during a constant current welding period is set to 0.05 to 3.0, and Q1+Q2 is set to 1 to 5 kA-sec, is supplied with a welding current to the materials to be welded.

[0008] However, when resistance spot welding is performed on a high-strength steel sheet having a zinc-based plated layer or a high-strength steel sheet in contact with a zinc-based plated steel sheet, there is a problem that a crack due to liquid metal embrittlement (LME) occurs. The LME crack is a grain boundary crack that occurs due to the invasion of zinc that has become liquid by melting into the grain boundary of the steel sheet.

[0009] The LME crack is considered to easily occur when the following factors are all present.

[0010] (A) The zinc-based plated layer is arranged on the overlapping surface of the steel sheet;

[0011] (B) The zinc-based plated layer of the overlapping surface is provided to the high-strength steel sheet, or is provided to the steel sheet overlapped with the high-strength steel sheet and is in contact with the high-strength steel sheet (hereinafter, this state is referred to as "high-strength steel sheet and zinc-based plated layer abutment").

[0012] (C) A high tensile stress is added to the periphery of the nugget during the process in which the steel sheet and the nugget are cooled after the nugget is formed.

[0013] Furthermore, as the cause of the addition of the high tensile stress, for example, the four kinds of disturbances described in the paragraphs 0021 and Figure 4 in Patent Literature 4 are known. When resistance spot welding is performed in a state in which these factors are all present, the molten zinc invades the grain boundary of the high-strength steel sheet, and a crack is generated in the grain boundary. The tensile stress promotes the invasion of the liquid zinc into the grain boundary.

[0014] In an automobile body, there is a problem that the crack of the welded portion is remarkable and the strength of the member is reduced. Therefore, it is necessary to suppress the occurrence of the LME crack as much as possible. However, the suppression of the LME crack by controlling the composition of the steel sheet or the zinc-based plated layer reduces the degree of freedom of the material selection of the automobile body, and thus is not preferable. Therefore, a resistance spot welding method capable of suppressing the LME crack is sought.

[0015] Prior Art Documents

[0016] Patent Literature

[0017] Patent Literature 1: Japanese Patent Application Laid-Open No. 2003-236674

[0018] Patent Literature 2: Japanese Patent Application Laid-Open No. 2006-212649

[0019] Patent Literature 3: Japanese Patent Application Laid-Open No. 2006-224127

[0020] Patent Literature 4: Japanese Patent No. 6108017 SUMMARY

[0021] As one of means for preventing LME cracks, there is a technique of extending a pressurization holding time. The long-time pressurization holding means that a pressurization pressure applied to a steel sheet at the time of forming a nugget is also held for a prescribed time after the completion of the electric current application. Thereby, it is possible to prevent LME cracks C (refer to FIG. 1) formed in the vicinity of the outer edge of a plastic metal ring zone in the periphery of the nugget. Figure 1A Further, the cracks in the plastic metal ring zone are likely to occur in a case where the nugget is relatively small, or in a case where the degree of interference such as a corner, a clearance, a gap between sheets is large and the radius of the plastic metal ring zone is small.

[0022] However, it has been known that it is difficult to completely prevent LME cracks even by performing the above-described long-time pressurization holding resistance spot welding. The present inventors repeatedly investigated the form of the LME cracks, and as a result, it was ascertained that the pressurization holding does not have an effect of suppressing the LME cracks C (refer to FIG. 1) in the plastic metal ring zone in the periphery of the nugget. Figure 1B

[0023] The resistance spot welding methods described in the above-described patent literatures are all characterized by optimizing the electric current application pattern. However, the LME cracks are not considered in these literatures. In addition, it was confirmed by the present inventors that in a case where the resistance spot welding methods described in the above-described patent literatures are applied to a sheet set in which a zinc-based plated layer and a high-strength steel sheet are combined, it is not possible to suppress the LME cracks in the plastic metal ring zone in the periphery of the nugget.

[0024] In view of the above, an object of the present application is to provide a resistance spot welded joint in which LME cracks in a plastic metal ring zone do not occur, and a method of manufacturing a resistance spot welded joint, despite that a zinc-based plated layer and a high-strength steel sheet are adjacent at an overlapping surface.

[0025] The gist of the present application is as follows.

[0026] (1) A resistance spot welded joint according to an embodiment of the present application is a resistance spot welded joint provided with a plurality of steel sheets that are overlapped, and a welded portion having a nugget that joins the steel sheets, and a plastic metal ring zone and a heat affected zone formed in the periphery of the nugget,

[0027] one or more of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more, ​

[0028] one or more of the plurality of steel sheets is a plated steel sheet having a zinc-based plated layer,

[0029] the high-strength steel sheet and the zinc-based plated layer abut at an overlapping surface,

[0030] the diameter of the heat-affected zone is 1.5 times or more of the diameter of the nugget,

[0031] 40 / 100 μm of the heat-affected zone is distributed, 2 the equivalent circle diameter of the carbide is 0.1 μm or more,

[0032] the amount of η phase of the zinc-based plated layer in the plastic metal ring zone is 20 area% or less.

[0033] (2) The resistance spot welded joint according to the above (1) can also satisfy any one or more of the following three requirements.

[0034] 1: The protrusion of the shoulder of the welded portion protrudes by 0.1 mm or more outward from the surface of the steel sheet in which the protrusion is formed;

[0035] 2: The angle formed by the long diameter direction of the nugget and the surface of the steel sheet surrounding the welded portion is 2° or more;

[0036] 3: The sheet separation at the overlapping surface where the high-strength steel sheet and the zinc-based plated layer abut is 0.3 mm or more.

[0037] (3) Another aspect of the present application relates to a method for manufacturing a resistance spot welded joint, comprising:

[0038] a step of pressing a plurality of steel sheets that are overlapped using a pair of electrodes facing each other;

[0039] a step of forming a nugget and a plastic metal ring zone by passing an electric current between the electrodes while pressing the steel sheets; and

[0040] a step of reducing the current value between the electrodes to zero while maintaining the pressing of the steel sheets,

[0041] one or more of the plurality of steel sheets has a zinc-based plated layer,

[0042] one or more of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more,

[0043] the high-strength steel sheet and the zinc-based plated layer abut at an overlapping surface,

[0044] the average value Iave of the current value between the electrodes in a first period from the time point at which the formation of the nugget is completed to a time point at which the current value becomes 0 satisfies a relationship of 0.30 x I ≤ Iave ≤ 0.90 x I,

[0045] the length of a second period from a time point at which the current value becomes 0.90 x I to a time point at which the current value becomes 0.30 x I is set to 420 milliseconds or more, and

[0046] the pressurizing pressure in the first period is set to 1.1 times or more of the pressurizing pressure P at the time point at which the formation of the nugget is completed.

[0047] (4) The method of manufacturing a resistance spot-welded joint according to the above (3),

[0048] 1 / 2 of the total sheet thickness of the steel sheet in mm is defined as tm,

[0049] When the current value between the electrodes is reduced to 0, the current value between the electrodes can also be held at a constant value in a range from I x 0.9 to I x 0.3 for a period of 265 x tm milliseconds or more and 420 milliseconds or more.

[0050] (5) The method of manufacturing a resistance spot-welded joint according to the above (3) or (4), can further include, after the process of reducing the current value between the electrodes to 0, a process of holding the pressurizing pressure at 0.8 x P or more for 0.04 seconds or more and 0.4 seconds or less in a state in which the current value between the electrodes is 0.

[0051] (6) The method of manufacturing a resistance spot-welded joint according to any one of the above (3) to (5), the Iave and the I can also satisfy a relationship of 0.45 x I ≤ Iave ≤ 0.85 x I.

[0052] (7) The method of manufacturing a resistance spot-welded joint according to any one of the above (3) to (6), 1 / 2 of the total sheet thickness of the steel sheet in mm is defined as tm, and the length of the second period is set to 265 x tm milliseconds or more and 420 milliseconds or more.

[0053] According to the present application, it is possible to provide a resistance spot-welded joint in which LME cracks in a plastic metal ring region do not occur, despite the fact that a zinc-based plated layer and a high-strength steel sheet abut at an overlapping surface, and a method of manufacturing a resistance spot-welded joint. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1Ais a schematic view of an LME crack generated near the outer edge of the plastic metal ring zone.

[0055] Figure 1B is a schematic view of an LME crack generated inside the plastic metal ring zone.

[0056] Figure 2A is a schematic view showing process S1 in the manufacturing method of the resistance spot welded joint to which one embodiment of the present application is applied.

[0057] Figure 2B is a schematic view showing processes S2 to S4 in the manufacturing method of the resistance spot welded joint to which one embodiment of the present application is applied.

[0058] Figure 3 is a graph schematically showing the temporal change in the current and the pressurizing pressure in the manufacturing method of the resistance spot welded joint to which one embodiment of the present application is applied.

[0059] Figure 4 is a graph schematically showing the temporal change in the current and the pressurizing pressure in the manufacturing method of the resistance spot welded joint to which one embodiment of the present application is applied.

[0060] Figure 5A is a graph of simulation results in which the effect of the time required for reducing the current value on the tensile stress in the plastic metal ring zone was examined.

[0061] Figure 5B is a graph of simulation results in which the effect of the time required for reducing the current value on the tensile stress in the plastic metal ring zone was examined.

[0062] Figure 5C is a graph in which the graphs of Figure 5A and Figure 5B are overlapped.

[0063] Figure 6A is a cross-sectional photograph of a resistance spot welded joint manufactured under various conditions.

[0064] Figure 6B is a cross-sectional photograph of a resistance spot welded joint manufactured under various conditions. DETAILED DESCRIPTION

[0065] The present inventors have repeatedly studied a method of suppressing LME cracks, particularly LME cracks in a plastic metal ring zone around a nugget. Further, the nugget means a molten solidified portion generated at a welded portion in lap resistance welding (JIS Z 3001-6:2013). In addition, for convenience, a molten metal before being solidified is also referred to as a nugget. The plastic metal ring zone means a ring-shaped portion welded in a solid phase generated around the nugget in lap resistance welding (JIS Z 3001-6:2013). In addition, in the present embodiment, the term "welded portion" means a region having a nugget, a plastic metal ring zone, and a heat-affected zone.

[0066] Furthermore, the present inventors have found that, in resistance spot welding, after a nugget is formed by pressing a steel sheet and applying electric current, the LME cracks in the plastic metal ring zone around the nugget can be suppressed by gradually reducing the electric current value while the pressing pressure is rising. Specifically, after the nugget is formed, the electric current value is gradually reduced

[0067] (1) the electric current value I at the time when the formation of the nugget is completed and the average value Iave of the electric current value in the first period satisfy the relationship of 0.30 x I ≤ Iave ≤ 0.90 x I,

[0068] (2) the length of the second period is set to 420 milliseconds or more, and

[0069] (3) the pressing pressure in the first period is always set to 1.1 times or more the pressing pressure P at the time point when the formation of the nugget is completed,

[0070] LME cracks can be suppressed. Here, as shown in Figure 3 , the "first period" is a period from the time point when the formation of the nugget is completed to the time point when the electric current value becomes 0. In addition, as shown in Figure 3 , the "second period" is a period from the time point when the electric current value becomes 0.90 x I to the time point when the electric current value becomes 0.30 x I in a process of reducing the electric current value between the electrodes to 0 while the pressing of the steel sheet is maintained. Further, in Figure 3 , the electric current value is continuously gradually reduced after the nugget is formed, but as long as the above requirements are satisfied, there can be a period in which the electric current value is constant after the nugget is formed. For example, the electric current value can be constant in a part or all of the second period. For example, the electric current mode after the nugget is formed can be the Figure 4Such a stepwise pattern (2-stage current reduction pattern). In this case, the current value is constant throughout the 2nd period, and the 1st period and the 2nd period coincide. In addition, the current pattern after the formation of the nugget can also include current value retention at an arbitrary current value Ia lower than the current value I, and current value retention at an arbitrary current value Ib lower than the current value Ia (3-stage current reduction pattern).

[0071] LME cracks are generated due to the invasion of zinc, which becomes a liquid, into the grain boundaries. The 2nd period is a period in which the risk of occurrence of LME cracks is high due to the liquefaction of zinc. It is intuitively thought that the occurrence of LME cracks can be suppressed by shortening this period as much as possible. However, in reality, when the nugget is rapidly cooled after the formation of the nugget, the generation of LME cracks in the plastic metal ring zone is rather promoted. Moreover, the present inventors have reached the conclusion that the nugget should be slowly cooled and the 2nd period should be set to 420 milliseconds or more.

[0072] The reason why such a welding condition suppresses LME cracks in the plastic metal ring zone is not clear at present, but the present inventors presume that it is due to the following mechanism.

[0073] The present inventors presume that the stress and the temporal change in temperature in the plastic metal ring zone around the nugget affect the LME cracks in this part. However, it is difficult to actually measure the temporal change in the magnitude of the temperature and the tensile stress in the plastic metal ring zone. Therefore, the present inventors have performed various simulations on the temporal change in the temperature and the stress around the plastic metal ring zone at resistance spot welding. As a result, it has been found that in resistance spot welding that satisfies the above conditions (1) to (3), in the 1st period, the tensile stress generated in the plastic metal ring zone in a range within 1 mm from the nugget in a region in the range of 907°C to 420°C is extremely reduced compared to the past with high probability. Hereinafter, the "region in the plastic metal ring zone in a range within 1 mm from the nugget in the range of 907°C to 420°C" will be referred to as a high crack risk region. This is because in the high crack risk region, the high-strength steel sheet and the liquid zinc are in contact, and thus the liquefied zinc invades the grain boundaries of the high-strength steel sheet with extremely high risk.

[0074] The shape of the high crack risk region changes temporally. The temperature of the plastic metal ring zone is not uniform, and the closer to the nugget formation site, the higher the temperature. Therefore, in the process in which the solidification of the molten metal proceeds, the zinc liquefies at a site (the plastic metal ring zone, the very close vicinity outside thereof) adjacent to the nugget, and a high crack risk region is formed. Furthermore, it is thought that the zinc at the nugget formation site melts and evaporates at an early stage of heating before the steel sheet at this site melts, and substantially all of it escapes to the outside of this site. Therefore, it is presumed that the zinc at the nugget formation site does not become a problem.

[0075] Next, as the temperature of the weld decreases, zinc solidifies at a portion adjacent to the nugget (the plastic metal ring zone, the extremely close vicinity thereof on the outside, and also the range within 1 mm from the nugget as a whole, which is particularly targeted for improvement in the resistance spot welding method according to the present embodiment), and the high crack risk region disappears.

[0076] The present inventors simulated the results, and it was estimated that, in the case where the pressurization pressure is raised to a value of 1.1 x P or more after the nugget is formed and maintained, and the length of the second period is set to 420 milliseconds or more, the tensile stress generated in the high crack risk region is substantially suppressed to 200 MPa or less. Here, as described above, the symbol P means the pressurization pressure at the point in time when the formation of the nugget is completed. The tensile stress promotes the invasion of liquid zinc into the grain boundaries. Therefore, it was estimated that the mitigation of the tensile stress in the high crack risk region greatly contributes to the suppression of the occurrence of LME cracks. This is entirely consistent with the fact that the occurrence of LME cracks is suppressed.

[0077] (First Embodiment)

[0078] A method of manufacturing a resistance spot welded joint (resistance spot welding method) according to one aspect of the present invention, which is obtained based on the above knowledge, includes:

[0079] (S1) a step of pressurizing two or more steel sheets 11 that are overlapped using a pair of electrodes A that face each other;

[0080] (S2) a step of forming a nugget 13 and a plastic metal ring zone 14 by passing an electric current between the electrodes A while pressurizing the steel sheets 11; and

[0081] (S3) a step of reducing the current value between the electrodes A to zero while maintaining the pressurization of the steel sheets 11,

[0082] one or more of the steel sheets 11 is a high-strength steel sheet 11' having a tensile strength of 780 MPa or more, a zinc-based plated layer 12 is arranged on the surface of the one or more steel sheets 11, the high-strength steel sheet 11' and the zinc-based plated layer 12 are made to abut at an overlapping surface 15, the average value Iave of the current value between the electrodes A in the first period from the point in time when the formation of the nugget 13 is completed to the point in time when the current value becomes zero satisfies the relationship of 0.30 x I ≤ Iave ≤ 0.90 x I, the length of the second period from the point in time when the current value becomes 0.90 x I to the point in time when the current value becomes 0.30 x I is set to 420 milliseconds or more, and the pressurization pressure in the first period is always set to 1.1 times or more the pressurization pressure P at the point in time when the formation of the nugget 13 is completed. Hereinafter, the method of manufacturing a resistance spot welded joint according to the present embodiment will be described in detail.

[0083] (Step S1)

[0084] In the step S1, the overlapped two or more steel sheets 11 are pressed using the facing pair of electrodes A. Here, one or more of the steel sheets 11 is provided as a steel sheet having a tensile strength of 780 MPa or more. Hereinafter, the steel sheet having a tensile strength of 780 MPa or more is referred to as a high-strength steel sheet 11'. In addition, one or more of the steel sheets 11 is provided as a zinc-based plated steel sheet. Here, the zinc-based plated layer 12 can be provided on the surface of the high-strength steel sheet 11', or on the surface of a steel sheet having a tensile strength of less than 780 MPa. In addition, the zinc-based plated layer 12 can be provided on one side of the steel sheet 11, or on both sides thereof. In Figure 2A and Figure 2B In the illustrated resistance spot welding method, the high-strength steel sheet 11' does not have the zinc-based plated layer 12, and the steel sheet 11 having a tensile strength of less than 780 MPa (low-strength steel sheet) has the zinc-based plated layer 12 on both sides thereof.

[0085] Furthermore, when the steel sheets 11 are overlapped, the high-strength steel sheet 11' and the zinc-based plated layer 12 are brought into contact at the overlapping surface 15 of the steel sheets. Here, the state in which the high-strength steel sheet 11' and the zinc-based plated layer 12 are brought into contact means both a state in which the zinc-based plated layer 12 is provided on the surface of the high-strength steel sheet 11', and a state in which the zinc-based plated layer 12 is provided on the steel sheet overlapped with the high-strength steel sheet 11' and the zinc-based plated layer 12 is in contact with the high-strength steel sheet 11'. Figure 2A and Figure 2B In the illustrated resistance spot welding method, the high-strength steel sheet 11' and the zinc-based plated layer 12 are brought into contact at the overlapping surface 15 of the steel sheets by providing the zinc-based plated layer 12 on the steel sheet overlapped with the high-strength steel sheet 11'.

[0086] In a case where the steel sheets 11 are overlapped and resistance spot welding is performed in a manner such that the high-strength steel sheet 11' and the zinc-based plated layer 12 are brought into contact at the overlapping surface 15 of the steel sheets 11, liquid zinc comes into contact with the high-strength steel sheet 11'. This is one of the factors that cause LME cracking. The resistance spot welding joint to which the present embodiment is directed is directed to suppressing LME cracking, and therefore the high-strength steel sheet 11' and the zinc-based plated layer 12 are brought into contact at the overlapping surface 15 of the steel sheets 11. Thus, it is possible to improve the degree of freedom in design of a mechanical component manufactured by resistance spot welding.

[0087] Furthermore, two or more overlapping steel plates 11 are pressurized using a pair of opposing electrodes A. The shape and structure of the electrodes A are not particularly limited; any electrodes commonly used in resistance spot welding can be used. The pressure applied is also not particularly limited; a value appropriate to the thickness, number, and material of the steel plates 11 to be joined can be set within a normal range. Various pressure conditions preferred for forming the weld nugget 13 can be applied to process S1. Additionally, the composition, microstructure, mechanical properties other than tensile strength, and shape of the steel plates 11 are not particularly limited; they can be appropriately selected according to the application of the resistance spot weld joint. The type of zinc-based coating 12 is not particularly limited; hot-dip galvanized, alloyed hot-dip galvanized, and electroplated zinc coatings can be appropriately selected. The amount of zinc-based coating 12 applied is also not particularly limited.

[0088] (Process S2)

[0089] Next, in process S2, a melt nugget 13 and a plastic metal ring region 14 are formed by pressing the steel plate 11 while energizing the electrodes A.

[0090] There are no particular limitations on the energizing time and current value; they can be appropriately set within the normal range, corresponding to the thickness, number, and material of the steel plates 11 to be joined. Furthermore, in Figure 3 In step S2, the current value immediately reaches its maximum value after energization begins. However, the current value can also be gradually increased in step S2 to reach its maximum value (so-called uphill energization). Alternatively, a small current can be energized (pre-energization) to preheat the steel plate 11 before the large current energization (formal energization) used to form the melt nugget 13. Various energizing conditions preferred for forming the melt nugget 13 can be applied to step S2.

[0091] Furthermore, there are no particular limitations on the pressure applied; it can be set appropriately within a normal range, corresponding to the thickness, number, and material of the steel plates 11 to be joined. Figure 3 In step S2, the pressure applied is set to be constant, but it can also be appropriately varied within a range that allows for the formation of a good weld nugget 13. Various conditions optimized for forming the weld nugget 13 can be applied to step S2. Furthermore, the possibility of unexpected changes in the pressure due to the accuracy of the resistance spot welding device is also considered, but such variations in the pressure are permissible within a range that allows for the formation of a good weld nugget 13.

[0092] (Process S3)

[0093] Next, in the process S3, the current value between the electrodes A is reduced to 0. This process S3 is extremely important for suppressing the LME cracks generated inside the plastic metal ring region. In the usual resistance spot welding, the current value between the electrodes A is immediately reduced to 0 after the nugget 13 is formed by the current application. Sometimes, after- current application is performed after the nugget 13 is formed in order to perform heat treatment such as tempering on the nugget 13, but at this time, the current value is temporarily reduced to 0 or the vicinity thereof after the molten metal portion is partially or entirely solidified, and then the current application is performed again. On the other hand, in the resistance spot welding related to the present embodiment, as shown in FIG. 2, when the current value between the electrodes A is reduced to 0, the following three conditions are satisfied. Figure 3

[0094] (1) the current value I at the time when the formation of the nugget 13 is completed and the average value Iave of the current value in the first period satisfy the relation of 0.30 x I < Iave < 0.90 x I,

[0095] (2) the length of the second period is set to 420 milliseconds or more, and

[0096] (3) the pressing pressure in the first period is always set to 1.1 times or more of the pressing pressure P at the time point when the formation of the nugget 13 is completed.

[0097] Here, the "first period" is the period from the time point when the formation of the nugget is completed to the time point when the current value becomes 0. The "second period" is the period from the time point when the current value becomes 0.90 x I to the time point when the current value becomes 0.30 x I in the process of reducing the current value between the electrodes to 0 while the pressing to the steel sheet is maintained. The "time point when the formation of the nugget 13 is completed" refers to the time point when the size of the molten portion of the steel sheet 11 reaches the size of the nugget 13 to be obtained in the process S2. Further, the time point when the process S2, that is, the formal current application is completed can also be regarded as the "time point when the formation of the nugget 13 is completed".

[0098] According to the experimental results of the present inventor, by gradually reducing the current value while maintaining the state in which the pressing pressure is increased, the LME cracks in the plastic metal ring region 14 can be effectively suppressed. The reason is presumed as described below.

[0099] ​It is considered that the LME cracks are generated when the high-strength steel sheet 11' contacts with the liquid zinc and the tensile stress is applied to the high-strength steel sheet 11'. It is known that the stress is introduced to the plastic metal ring zone 14 mainly when the pressure applied by the electrode A is released. In addition, in the plastic metal ring zone 14, the high-strength steel sheet 11' and the liquid zinc can contact when the temperature of the plastic metal ring zone 14 is below 907°C (a temperature at which the liquefaction of the zinc vapor is possible) and above 420°C (a temperature at which the solidification of the liquid zinc is possible). Therefore, by maintaining the pressure applied by the electrode A at a value of 1.1 x P or more until the temperature of the plastic metal ring zone 14 is below about 420°C, the tensile stress generated in the plastic metal ring zone 14 during the period when the high-strength steel sheet 11' contacts with the liquid zinc can be somewhat mitigated.

[0100] Further, in the case where the zinc-based plated layer 12 is an alloyed hot-dip galvanized layer, the iron concentration in the plated layer becomes, for example, about 10% at times. The melting point of such an alloyed hot-dip galvanized layer is about 600°C. Therefore, the lower limit temperature at which the LME cracks occur in the plastic metal ring zone 14 depends on the composition of the zinc-based plated layer formed on the steel sheet 11. The melting point of pure zinc is 420°C, and when a hot-dip galvanized steel sheet in which the plated layer is not alloyed is used, 420°C becomes the lower limit temperature at which the LME occurs. However, even in the case of a hot-dip galvanized steel sheet, some alloying occurs in the heating of the welding, and therefore it is considered that the melting point of the actual plated layer exceeds 420°C. Therefore, depending on the kind of the steel sheet and the zinc-based plated layer, and the welding conditions and the like, the lower limit temperature at which the LME occurs becomes 450°C, or 500°C, or 550°C, or the like at times.

[0101] On the other hand, although the boiling point of pure zinc is 907°C, if iron and zinc are mixed, the boiling point rises. However, the higher the temperature of the steel sheet, the lower the yield strength and the tensile strength of the steel sheet. Therefore, it is considered that the tensile stress generated at the welding portion at a temperature of 800°C or higher is substantially 200 MPa or less. Therefore, within the range of the high-strength steel sheets used industrially, the upper limit temperature at which the LME occurs becomes 850°C or 800°C or the like at times.

[0102] However, the present inventors have simulated the stress distribution and the heat distribution at the time of spot welding in detail, and as a result, it has been known that even if the pressure applied by the electrode A is maintained at 1.1 x P or more, the tensile stress is introduced to the plastic metal ring zone 14 in the process of reducing the current value between the electrodes. It is considered that this is because the cooling medium always flows through the inside of the electrode A, and if the pressure applied by the electrode A is maintained at 1.1 x P or more after the current is turned off, the welding portion is rapidly cooled and shrinks. Since the welding portion is constrained by the surrounding steel sheet 11, if the welding portion shrinks, the welding portion is subjected to the tensile stress by the surrounding steel sheet 11.

[0103] On the other hand, it is known that if the length of the second period during which the current value between the electrodes A is reduced from I x 0.9 to I x 0.3 is set to 420 milliseconds or more and the relationship of 0.30 x I ≤ Iave ≤ 0.90 x I is satisfied, the tensile stress introduced to the plastic metal ring region 14 is reduced during the process of reducing the current value between the electrodes.

[0104] In the case where the length of the second period and the average value Iave of the current value in the first period are set to the above-described ranges, the current value is gradually reduced after the nugget 13 is formed. Here, Figures 5A-5C The simulation results showing the progress of the tensile stress of the plastic metal ring region 14 in the case where the current value is rapidly reduced and in the case where the current value is gradually reduced are shown. As the analysis conditions, a 980 MPa grade zinc-based plated steel sheet having a sheet thickness of 1.6 mm was used, and as the interference conditions, a gap of 2 mm between the sheets and a clearance of 0.5 mm were given. The pressurizing pressure P at the time point when the formation of the nugget is completed was set to 3.9 kN, the pressurizing pressure in the first period was set to 4.5 kN, the current was set to 6 kA, and the current application time was set to 320 milliseconds.

[0105] Figure 5A The graph of FIG. 1 is a simulation result showing the temporal change in the temperature and the stress of the plastic metal ring region under the condition where the pressurizing pressure is maintained at 1.1 x P or more for 10 cycles (= 0.2 seconds) after the current application is completed. Also in the graph, the vertical axis is set to the stress of the plastic metal ring region, and the horizontal axis is set to the temperature of the plastic metal ring region. Regarding the stress, the positive side is the tensile stress, and the negative side indicates the compressive stress. In the graph, the horizontal axis is set to the temperature of the plastic metal ring region, and the vertical axis is set to the stress of the plastic metal ring region. Figure 5A In FIG. 1, the simulation result that first, the tensile stress in the plastic metal ring region increases as the steel sheet is pressurized by the electrodes in the state at room temperature, then the tensile stress decreases as the temperature rises, and further, the tensile stress increases again as the temperature decreases is shown. It is assumed that at the time point when the temperature decreases to about 700°C, the electrodes are released and the tensile stress greatly increases.

[0106] Figure 5B The graph of FIG. 2 is a stress simulation result in the case where the current value between the electrodes A is set to decrease at a slope of 30 cycles, that is, the time required for the current value to decrease from I to 0 is set to 30 cycles (= 600 milliseconds), and the time during which the pressurizing pressure is maintained at 1.1 x P or more is set to 10 cycles (= 200 milliseconds). At this time, the length of the second period is set to 360 milliseconds. It is assumed that: Figure 5B The change in the stress and the temperature of the plastic metal ring region in FIG. 2 is the same as that in FIG. 1 until the middle. However, it is also assumed that in FIG. 2, at the time point when the temperature decreases to about 700°C, the increase in the tensile stress is suppressed compared to FIG. 1. Figure 5A Figure 5B Figure 5A ​​​

[0107] Figure 5C are shown for reference. Figure 5A and Figure 5B are shown for reference. Figure 5C , by adding a slope of the current value, the increase of the tensile stress at the time of cooling is more suppressed in the simulation. Thus, it is shown in the simulation result that, in the case where the current value is gradually reduced, the tensile stress is greatly reduced compared to the case where the current value is rapidly reduced.

[0108] If the current value I at the time point when the formation of the nugget is completed and the average value Iave of the current value in the first period satisfy the relation of 0.30 x I < Iave < 0.90 x I, and the length of the second period is set to 420 milliseconds or more, the cooling speed of the welded portion is reduced. In addition, in this case, the heat dissipation due to the electrode A is small, and the heat transfer from the welded portion to the surrounding steel plate 11 is promoted. As a result, when the temperature of the welded portion is reduced, the shrinkage of the welded portion becomes slow, and on the other hand, the restraint force of the surrounding steel plate 11 to the welded portion becomes small. The present inventors presume that, by such a mechanism, the tensile stress in the welded portion is more reduced. Iave and I can also satisfy the relation of 0.45 x I < Iave < 0.85 x I.

[0109] Further, the present inventors set the period in which the current value between the electrodes A is controlled to the period from when the current value between the electrodes A becomes I x 0.9 until it is reduced to I x 0.3. This is because it is presumed that the temperature of the plastic metal ring zone 14 when the current value between the electrodes A is I x 0.9 substantially coincides with the boiling point of zinc, and when the current value between the electrodes A is I x 0.3, the temperature of the plastic metal ring zone 14 substantially coincides with the melting point of zinc. Based on this presumption, the second period is a period in which molten zinc that causes LME cracks exists around the nugget 13. In reality, by controlling the length of the second period thus set, the effect of suppressing LME cracks in the plastic metal ring zone is confirmed.

[0110] In addition, according to the experimental results of the present inventors, the longer the second period is, the lower the frequency of occurrence of LME cracks in the plastic metal ring zone 14 is. Therefore, the length of the second period can also be specified to 450 milliseconds, 480 milliseconds or more, 500 milliseconds or more, 600 milliseconds or more, or 800 milliseconds or more.

[0111] Furthermore, it is preferable that the greater the thickness of the steel plate, the longer the second period. Therefore, the length of the second period can also be set according to the thickness of the steel plate. For example, if half of the total thickness of the steel plate in mm is defined as "tm", the length of the second period can be set to at least 265 × tm and at least 420 milliseconds in milliseconds. In other words, the lower limit of the length of the second period can also be set to the longer of the lower limit corresponding to the plate thickness, 265 × tm (milliseconds), and the aforementioned lower limit of 420 milliseconds.

[0112] The pressure applied during the first phase is always set to 1.1 × P or higher. This is because it is anticipated that if the first phase includes periods with pressures below 0.1 × P, tensile stress will be introduced into the weld, promoting LME cracking. Figure 3 and Figure 4 As illustrated, the pressure applied during the first period can be increased to a value of 1.1×P or higher simultaneously with the end of the nucleus-forming process S2, and then maintained at a constant value. On the other hand, after the pressure is increased to a value of 1.1×P or higher simultaneously with the end of the nucleus-forming process S2, fluctuations in the pressure within the range of 1.1×P or higher during the first period are also permitted.

[0113] In process S3, where the pressure on the steel plate is maintained at 1.1 × P or higher while the current value between the electrodes is reduced to 0, the rate of decrease in current value can be as follows: Figure 3 As shown, the current is constant, but it can also vary. This is because, in order to reduce tensile stress during the period when liquid zinc can exist, it is sufficient to reduce the current value over a specified period of time or longer.

[0114] For example, such as Figure 4In the stepwise curve shown in FIG. 6, a period in which the current value is constant can also be provided when the current value is reduced. Specifically, when the current value between the electrodes is reduced to 0, the current value between the electrodes can be held at a constant value in the range from I x 0.9 to I x 0.3 for a period of 265 x tm or more in milliseconds and 420 milliseconds or more. In other words, the lower limit value of the length of the second period can be set to the longer one of the lower limit value 265 x tm (milliseconds) corresponding to the plate thickness and the above-mentioned lower limit value 420 milliseconds. The current in the first period can also be divided into two or more parts, for example, in a manner such that the first half is I x 0.8 and the second half is I x 0.6. However, for example, in a welding condition including the main welding in which the nugget 13 is formed and a post-welding in which the nugget 13 is temporarily cooled and then reheated, it is not allowed to rapidly cool the welded portion after the main welding and slowly cool the welded portion after the post-welding as described above. This is because an LME crack is generated at the time of rapid cooling after the main welding. That is, when the current value is made I x 0.9 or less after the nugget 13 is formed, slow cooling based on the above-mentioned condition is required.

[0115] (Step S4)

[0116] The resistance spot welding method according to the present embodiment can further include, after the step S3 in which the pressure applied to the steel sheet is maintained at 1.1 x P or more while the current value between the electrodes is reduced to 0, a step S4 in which the pressure applied by the electrode A is maintained at 0.8 x P or more in a state in which the current value between the electrodes A is 0 (so-called holding time). Thereby, the plastic metal ring zone 14 and the LME crack outside thereof can be suppressed, and thus the LME crack can be more reliably prevented.

[0117] In actual production, in a case where the disturbance is larger than assumed, even when the length of the first period is set to 420 milliseconds or more after the nugget is formed, there is a possibility that the temperature of the plastic metal ring zone is not lower than 420°C. In that case, if the electrode is released immediately after the current value becomes 0, there is a possibility that the LME crack occurs in the plastic metal ring zone 14 and outside thereof. It is considered that by not completely releasing the electrode after the first period and maintaining the pressure at 0.8 x P or more as described above, the case where the pressure is released in a state where liquid zinc remains and thus the tensile stress is introduced can be more reliably avoided.

[0118] In the process S4, the length of the period during which the pressurizing pressure is maintained at 0.8 x P or more after the current value between the electrodes A is made to be 0 is preferably 0.04 seconds (40 milliseconds) or more. From the viewpoint of suppressing LME cracks, it is considered that the longer the maintenance time is, the more preferable it is. Therefore, in the process S4, the time during which the pressurizing pressure is maintained at 0.8 x P or more can also be 0.04 seconds (40 milliseconds) or more, 0.06 seconds (60 milliseconds) or more, or 0.08 seconds (80 milliseconds) or more. However, when the maintenance time is excessively long, the effect of suppressing LME cracks saturates, and on the other hand, the welding efficiency decreases. Also, if the time during which the pressurizing pressure is maintained at 0.8 x P or more exceeds 400 milliseconds, after the electrodes are released, the self-tempering (self-tempering) of the nugget that is quenched during the process of decreasing the current does not proceed, and the joint strength, the hydrogen embrittlement resistance can decrease. Therefore, in the process S4, the time during which the pressurizing pressure is maintained at 0.8 x P or more can also be 0.4 seconds (400 milliseconds) or less, 0.3 seconds (300 milliseconds) or less, or 0.2 seconds (200 milliseconds) or less.

[0119] In the process S4, the pressurizing pressure can also be maintained at 0.9 x P or more, 1.0 x P or more, 1.1 x P or more, or 1.2 x P or more. In addition, in the process S4, the pressurizing pressure can also be set to a constant value as exemplified above. On the other hand, it is also permissible for the pressurizing pressure to vary within the range of 0.8 x P or more during the maintenance period. Figure 3

[0120] Next, a resistance spot welded joint relating to another aspect of the present application will be described. A resistance spot welded joint 1 relating to another aspect of the present application is a resistance spot welded joint provided with a plurality of steel sheets 11 that overlap, and a welded portion having a nugget 13 that joins the steel sheets 11, and a plastic metal ring zone 14 and a heat affected zone 16 formed around the nugget 13, one or more of the plurality of steel sheets 11 being a high-strength steel sheet 11' having a tensile strength of 780 MPa or more, one or more of the plurality of steel sheets 11 being a plated steel sheet 11 having a zinc-based plated layer 12, the high-strength steel sheet 11' and the zinc-based plated layer 12 abutting at an overlapping surface 15, the diameter of the heat affected zone 16 being 1.5 times or more the diameter of the nugget 13, the number of carbides having an equivalent circle diameter of 0.1 μm or more being 40 / 100 μm 2 The equivalent circle diameter of the carbides described above is 0.1 μm or more in the plastic metal ring zone 14, and the amount of η phase of the zinc-based plated layer 12 is 20 area% or less.

[0121] ​The resistance spot-welded joint 1 according to the present embodiment has a plurality of steel sheets 11 overlapped, one or more of which is a high-strength steel sheet 11' having a tensile strength of 780 MPa or more. The composition, metal structure, mechanical properties other than the tensile strength, and shape of the steel sheet 11 are not particularly limited and can be appropriately selected according to the use of the resistance spot-welded joint.

[0122] In the resistance spot-welded joint 1 according to the present embodiment, one or more of the steel sheets 11 has a zinc-based plated layer 12 on the surface thereof. The type of the zinc-based plated layer 12 is also not particularly limited and can be appropriately selected from among a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, and the like. The attached amount of the zinc-based plated layer 12 is also not particularly limited.

[0123] In the resistance spot-welded joint 1 according to the present embodiment, the high-strength steel sheet 11' and the zinc-based plated layer 12 are in contact at one or more of the overlapping surfaces 15. Here, the state in which the high-strength steel sheet 11' and the zinc-based plated layer 12 are in contact means either of the following:

[0124] (a) a state in which the high-strength steel sheet 11' has the zinc-based plated layer 12 on the surface thereof; and

[0125] (b) a state in which a steel sheet having a tensile strength of less than 780 MPa that overlaps the high-strength steel sheet 11' has the zinc-based plated layer 12, and the zinc-based plated layer 12 is in contact with the high-strength steel sheet 11'.

[0126] The resistance spot-welded joint 1 according to the present embodiment has a weld 17 having a nugget 13 that joins the steel sheets 11, and a plastic metal zone 14 and a heat-affected zone (HAZ) 16 formed around the nugget 13.

[0127] In the weld 17, the diameter of the heat-affected zone 16 is set to be 1.5 times or more the diameter of the nugget 13. Here, the diameter of the heat-affected zone 16 and the nugget 13 is a value observed in a cross section that is perpendicular to the plate surface of the steel sheet 11 and passes through the center of the nugget 13. Also, in the heat-affected zone 16, 40 / 100 μm 2 The equivalent circle diameter of the carbide described above is 0.1 μm or more. Also, in the plastic metal zone 14 of the resistance spot-welded joint 1 according to the present embodiment, the amount of the η phase of the zinc-based plated layer is 20 area% or less. The η phase of the zinc-based plated layer 12 means a phase that has Zn as the main component and contains other elements such as Fe in a solid solution state.

[0128] As described above, in the manufacturing method of the resistance spot welded joint 1 according to the present embodiment, the period (i.e., the second period) during which the current value is decreased from I x 0.9 to I x 0.3 after the current application for forming the nugget is longer than that of the conventional resistance spot welding. When the resistance spot welding is performed under such conditions, the heat input is increased more than ever, and the diameter of the heat-affected zone 16 (a region in which the temperature reaches above the Ac1 point and below the melting point in welding) is enlarged. Further, since the cooling speed after the electrode release becomes slow, self-tempering (auto-tempering) is performed in the martensite formed in the heat-affected zone 16 during cooling. As a result, carbides having an equivalent circle diameter of 0.1 μm or more are distributed in the heat-affected zone 16, and the number density thereof becomes 40 pieces per 100 μm 2 The above. Further, when the resistance spot welding is performed under such conditions, the heat input is increased more than ever, and the alloying of the zinc-based plated layer with the steel sheet progresses. As a result, the amount of the η phase mainly composed of zinc in the plastic metal ring zone 14 becomes 20 area% or less. In other words, it is presumed that the resistance spot welded joint in which the size of the heat-affected zone 16, the number density of the carbides having an equivalent circle diameter of 0.1 μm or more possessed by the heat-affected zone 16, and the amount of the η phase of the zinc-based plated layer in the plastic metal ring zone 14 are within the above-described ranges is obtained according to the manufacturing method of the resistance spot welded joint according to the present embodiment. The distribution density of the carbides is preferably 45 pieces per 100 μm 2 The above is further preferably 50 pieces per 100 μm 2 The above.

[0129] The method of measuring the diameter of the heat-affected zone 16 and the diameter of the nugget 13 is as follows. First, the resistance spot welded joint is cut with a plane passing through the center of the nugget 13 and perpendicular to the plate surface. Next, the cross section is polished, and the polished surface is etched using a bitter water solution. Thereby, the outer edge of the nugget 13 and the heat-affected zone 16 can be visually recognized. The etched surface is observed using an optical microscope with an appropriate magnification in the range of 10 to 50 times, whereby the diameter of the heat-affected zone 16 and the diameter of the nugget 13 can be measured.

[0130] The number density of carbides having an equivalent circle diameter of 0.1 μm or more in the heat-affected zone 16 is measured as follows. As in the case of the diameter of the nugget 13, the resistance spot welded joint is cut, and the cross section is polished and etched. Ten 5 μm x 5 μm measurement regions in the heat-affected zone 16 in the etched surface are selected, and a photograph is taken at a magnification of 20,000 times using a scanning electron microscope (SEM). The carbides are easily visible in the SEM photograph. Then, using an image processing device, the area of each carbide contained in the above measurement region is measured. Then, the shape of the carbide is assumed to be a circle, and the diameter of each carbide is calculated from the area of the carbide. Then, the number of carbides having a diameter of 0.1 μm or more is counted, and the number density of the carbides is obtained by dividing the number by the total area of the measurement regions. The area ratio of the η phase of the zinc-based plated layer 12 in the plastic metal ring zone 14 is measured as follows. The Zn and Fe element distribution images of the plastic metal ring zone in the cross section of the welded portion are taken using an SEM-EDS. The η phase in the image is defined as a region in which the Zn concentration is 95% or more and the Fe concentration is 5% or less. The portions satisfying the definition and the other portions are binarized using image analysis software, and the area ratio of the η phase in the plated layer in the plastic metal ring zone is calculated. The area ratio of the η phase can be measured throughout the entire region of the plastic metal ring zone, or representative portions such as three or more portions like the two ends and the center of the plastic metal ring zone can be selected for measurement.

[0131] The resistance spot welded joint 1 according to the present embodiment can satisfy any one or more of the following listed requirements.

[0132] 1: The raised portion of the shoulder portion 18 of the welded portion 17 protrudes by 0.1 mm or more outward from the surface of the steel sheet 11 in which the raised portion is formed.

[0133] If interference occurs during resistance spot welding, the shoulder portion 18 of the steel sheet 11 is somewhat raised at the outer edge. The shoulder portion 18 of the welded portion 17 refers to the outer edge of the indentation portion (indentation) in the surface obtained by cutting the welded portion 17 in the thickness direction of the steel sheet (see Figure 2B ). In the case where there is no interference during resistance spot welding, the shoulder portion 18 is not raised as described in Figure 2B . However, if interference occurs during resistance spot welding, a significant raised portion (see, for example, the cross-sectional photograph of Figure 6A ) is formed in the shoulder portion 18 due to the pressure of the electrode A.

[0134] In the cross section, there are four shoulder portions 18. The size of the protrusion in these shoulder portions 18 is evaluated with the surface of the steel sheet 11 in which the protrusion is formed as a reference. The specific method of measuring the size of the protrusion is as follows. A cross section is produced which is perpendicular to the surface of the steel sheet and passes through the center of the nugget and the largest portion of the protrusion of the shoulder portion of the weld portion. The measurement is performed in this cross section. An imaginary line along the surface of the steel sheet in which the protrusion is formed on the outside of the weld portion 17 is entered into the photograph of the cross section. Then, the distance of the apex of the protrusion from the imaginary line is measured. In the case where this distance is 0.1 mm or more, it is determined that the protrusion of the shoulder portion 18 of the weld portion 17 protrudes 0.1 mm or more to the outside of the surface of the first steel material 11 in which the protrusion is formed. Of course, the surface of the steel sheet other than the surface of the steel sheet in which the protrusion to be evaluated is formed cannot be used as the reference line when evaluating the size of the protrusion.

[0135] 2: The angle formed by the long diameter direction of the nugget 13 and the surface of the steel sheet 11 at the periphery of the weld portion 17 is 2° or more.

[0136] The long diameter direction of the nugget 13 means the direction parallel to the major axis of the ellipse in the case where the outer edge of the nugget 13 is regarded as an ellipse. The angle of the major axis is the angle of the nugget 13 due to interference. The angle of the major axis is measured with the surface of the steel sheet at the periphery of the weld portion (specifically, the region within 20 mm from the outer edge 20 of the heat affected zone 16 of the weld portion 17) as a reference. That is, the angle formed by the surface of the steel sheet at the periphery of the weld portion and the long diameter direction of the nugget 13 is used as the value for evaluating the angle of the nugget 13. In the case where the shape of the nugget 13 is not a substantially elliptical shape, a straight line is drawn at the portion where the nugget diameter becomes the largest in the cross section of the weld portion, and the angle formed by this straight line and the surface of the steel sheet at the periphery of the weld portion is regarded as the angle of the nugget 13. In the case where the surface of the steel sheet at the portion away from the weld portion 17 is used as a reference, the deformation of the steel sheet can possibly affect the evaluation result of the angle of the nugget 13. The angle formed by the long diameter direction of the nugget 13 and the surface of the steel sheet 11 at the periphery of the weld portion 17 is a value measured in the cross section which is perpendicular to the surface of the steel sheet and passes through the center of the nugget.

[0137] 3: The warpage at the overlapping surface 15 where the high-strength steel sheet 11' and the zinc-based plated layer 12 abut is 0.3 mm or more.

[0138] Here, the warpage is the size of the gap between the steel sheets 11 at the overlapping surface 15, and is defined as a value measured at a portion 2 mm away from the end of the plastic metal ring zone 14 in the cross section which is perpendicular to the surface of the steel sheet 11 and passes through the center of the nugget 13.

[0139] As described above, the interference at the time of resistance spot welding increases the tensile stress in the welded portion, promoting the LME crack. In addition, the interference causes the bulging of the surface of the steel sheet 11, the inclination of the nugget 13, and / or the lifting described above. Therefore, the resistance spot welded joint 1 satisfying one or more of the three requirements described above can be said to be formed under the condition where the interference exists and the LME crack is likely to occur. In the manufacture of a general resistance spot welded joint, if such interference exists, the LME crack occurs, and the joint thus obtained is presumed to be unable to constitute the body as a mechanical member. However, the resistance spot welded joint 1 according to the present embodiment is formed under the condition where the number density of the carbides having the prescribed equivalent circle diameter of the heat-affected zone 16 is within the range described above, and thus the LME crack does not occur. Therefore, the resistance spot welded joint 1 according to the present embodiment satisfying one or more of the requirements described above can be said to have further superiorities over the general resistance spot welded joint.

[0140] Example

[0141] The effects of one embodiment of the present application are further specifically described by way of examples. However, the conditions in the examples are merely one example of conditions adopted for confirming the workability and effects of the present application. The present application is not limited to this one example of conditions. The present application can adopt various conditions as long as the gist of the present application is not deviated from and the object of the present application is achieved.

[0142] (Example 1)

[0143] Various resistance spot welded joints were manufactured by resistance spot welding including the following processes: a process of pressing two steel sheets overlapped using a pair of electrodes facing each other; a process of forming a nugget and a plastic metal zone by passing electric current between the electrodes while pressing the steel sheets; and a process of reducing the current value between the electrodes to 0 while maintaining the pressing of the steel sheets.

[0144] The steel sheets were arranged substantially horizontally, and a pair of electrodes were arranged so as to sandwich the steel sheets. The electrode arranged above the steel sheets was used as a movable electrode, and the electrode arranged below the steel sheets was used as a fixed electrode. The steel sheets were pressed by moving the upper electrode toward the lower electrode. At the start of the passage of electric current, the current value was instantaneously increased to a prescribed value, and then the current value was maintained constant until the nugget was completed.

[0145] Other welding conditions are shown below. The so-called angle of attack is the angle formed by the axis of the movable electrode and the direction perpendicular to the surface of the steel sheet. The so-called clearance is the interference described in paragraph (b) of the 0021 paragraph of Patent Literature 4 and in FIG. 5. The angle of attack and the clearance are interference factors of resistance spot welding and are factors that cause LME cracks. By setting the angle of attack and the clearance as described below, it is made easy to cause LME cracks. As for the pressurizing pressure, it is kept at P during the period described below after the nugget formation.

[0146] • Welding machine: Servo pressurizing stationary welding machine single-phase alternating current (frequency 50 kHz)

[0147] • Electrode: Round top radius (DR) Cr-Cu

[0148] • Shape of the electrode tip: R 40 mm

[0149] • Pressurizing pressure P at the time point when the formation of the nugget is completed: 3.9 kN

[0150] • Current value I at the time point when the formation of the nugget is completed: 6 kA (conditions for forming a nugget of 4√t or less, where t is the thinnest sheet thickness (mm) among the overlapped steel sheets).

[0151] • Current application time when the nugget is formed: 15 cycles (0.3 seconds)

[0152] • Retention time of the pressurizing pressure from the time point when the current becomes zero: 4 cycles or 99 cycles (0.08 seconds or 1.98 seconds)

[0153] • Angle of attack: 3°

[0154] • Clearance: 0.3 mm

[0155] • Types of steel sheets: Both were alloyed hot-dip galvanized (GA) 980 MPa grade steel (at the overlapping surface, a high-strength steel sheet of 980 MPa was in contact with an alloyed galvanized layer)

[0156] • Thickness of the steel sheets: Both were 1.6 mm (tm = 1.6)

[0157] • The length of the second period after the nugget formation was changed in the range of 0 to 500 milliseconds.

[0158] • The pressurizing pressure P' after the nugget formation was changed in the range of 1.0 times P to 1.4 times P.

[0159] Resistance spot welding was performed 10 times under each condition. The resistance spot welded joint thus obtained was cut with a plane passing through the center of the nugget and perpendicular to the surface of the steel sheet, and a cross section was prepared as appropriate, and observed with an optical microscope. The observation results are shown inFigure 6A and Figure 6B .

[0160] The length of the period (i.e., the 2nd period) during which the current value between the electrodes is reduced from I x 0.9 to I x 0.3 when the current value between the electrodes is reduced to 0 was set to less than 420 milliseconds in the comparative example of resistance spot welding, and the occurrence of LME cracks was scattered. It was observed that the shorter the length of the period, the higher the frequency of occurrence of LME cracks. On the other hand, according to the inventive example of resistance spot welding in which the length of the 2nd period is set to 420 milliseconds or more when the current value between the electrodes is reduced to 0, LME cracks can be suppressed.

[0161] The resistance spot welded joints obtained in the inventive example using the condition in which the length of the 2nd period is 420 milliseconds were observed in detail. Specifically, the ratio of the diameter of the heat-affected zone to the diameter of the nugget, the number density of carbides having an equivalent circle diameter of 0.1 μm or more in the heat-affected zone, the size of the raised portion of the weld portion shoulder, the angle of the long diameter direction of the nugget and the overlapping surface constituting the side of the steel sheet adjacent to the weld portion, and the warpage were evaluated.

[0162] The warpage, the angle of the long diameter direction of the nugget and the surface of the side of the steel sheet adjacent to the weld portion, and the height of the raised portion of the weld portion shoulder were measured using the following method: the resistance spot welded joint was cut with a plane passing through the center of the nugget of the spot welded portion and the largest portion of the raised portion of the weld portion shoulder and perpendicular to the plate surface, the cross section was polished, the polished surface was etched with a picric acid aqueous solution, and the etched surface was observed using an optical microscope with appropriate magnification in the range of 10 to 50 times. Further, in the case where the raised portion of the weld portion shoulder was not observed by visual observation of the appearance, the measurement was performed by cutting the nugget with any plane passing through the center of the nugget of the spot welded portion and perpendicular to the plate surface.

[0163] The size of the heat-affected zone and the nugget diameter (diameter) were measured by observing the etched surface produced using the above steps with an optical microscope.

[0164] The density of carbides having an equivalent circle diameter of 0.1 μm or more was calculated from an image obtained by photographing a 5 μm x 5 μm region in the heat-affected zone in 10 places of the etched surface produced using the above steps using a scanning electron microscope (SEM) at a magnification of 20,000 times. Here, the equivalent circle diameter of each carbide contained in the field of view was measured, the area of each carbide was calculated using an image processing device, and the equivalent circle diameter was calculated from the value to calculate. Then, the carbides having an equivalent circle diameter of 0.1 μm or more were determined, and the distribution density of the carbides was calculated by dividing the total number of the carbides by the total area of the photographed region.

[0165] As a result, the lift-off was 0.14 mm. The angle of the long diameter direction of the nugget with respect to the normal line of the steel sheet base material adjacent to the welding portion was 3°. The height of the bulging portion of the welding portion shoulder was 0.16 mm. The heat-affected zone diameter / nugget diameter was 2.3, and the density of carbides having a particle diameter of 0.1 μm or more was 55 pieces / 100 μm 2 The resistance spot welding joint of the present application, since the bulging of the welding portion shoulder, the inclination angle of the nugget, and the lift-off are generated, can be said to be formed under the disturbed conditions in which LME cracks are easily generated, but is a joint in which the plastic metal ring zone does not have LME cracks.

[0166] (Example 2)

[0167] With respect to the alloyed hot-dip galvanized steel sheet described in Table 1, various resistance spot welding joints were manufactured using the conditions described in Table 2. Further, in each of the welding joints, the types of the two steel sheets were set to be the same. In addition, resistance spot welding was performed 10 times under each condition. The current profile was set to be stepped as exemplified in Table 1. That is, the current value in the second stage was set to be constant. Then, the number of nuggets in which LME cracks were generated in the manufactured resistance spot welding joints was confirmed, and is described in Table 2. Also, the η area ratio of the zinc-based plated layer in the plastic metal ring zone was measured, and is described in Table 1. Figure 4

[0168] Further, "the current I" of Table 2 corresponds to "the current value between the electrodes at the time point at which the formation of the nugget is completed". "The pressure of the second stage" of Table 2 corresponds to "the pressure during the first period". "The current of the second stage" of Table 2 corresponds to "the average value of the current value between the electrodes during the first period Iave". "The current time of the second stage" of Table 2 corresponds to "the length of the second period". The welding conditions not described in Table 2 were in accordance with Example 1 described above. The length of the S4 step, that is, the holding time was set to be 10 cycles (200 msec). In addition, "the number density of carbides in the heat-affected zone" of Table 2 refers to the number density of carbides having an equivalent circle diameter of 0.1 μm or more.

[0169]

[0170]

[0171] Conditions 4 and 8 are welding conditions that satisfy the following requirements:

[0172] (1) the current value between the electrodes at the time point at which the formation of the nugget is completed I and the average value of the current value between the electrodes during the first period Iave satisfy the relationship 0.30 x I ≤ Iave ≤ 0.90 x I,

[0173] ​(2) the length of the second period is set to 420 milliseconds or more, and

[0174] (3) the pressurizing pressure in the first period is set to 1.1 times or more of the pressurizing pressure P at the time point at which the formation of the nugget is completed.

[0175] In the resistance spot welded joint obtained by adopting the condition 4 and the condition 8, the number of cracks is 0. That is, the manufacturing method of the resistance spot welded joint satisfying the above requirements (1) to (3) can manufacture the resistance spot welded joint in which the occurrence of the LME crack in the plastic metal ring zone does not occur although the zinc-based plated layer and the high-strength steel sheet abut at the overlapping face. Further, in the resistance spot welded joint obtained by adopting the condition 4 or the condition 8,

[0176] (A) the diameter of the heat-affected zone is 1.5 times or more of the diameter of the nugget,

[0177] (B) 40 / 100 μm 2 the equivalent circle diameter of the above carbide is 0.1 μm or more,

[0178] (C) in the plastic metal ring zone, the amount of the η phase of the zinc-based plated layer is 20 area% or less.

[0179] On the other hand, the other welding conditions do not satisfy one or more of the above requirements (1) to (3).

[0180] Specifically, in the condition 1, both of the pressurizing pressure of the second section (i.e., the pressurizing pressure in the first period) and the current-on time of the second section (i.e., the length of the second period) are insufficient, and the above requirements (2) and (3) are not satisfied.

[0181] In the condition 2, the current-on time of the second section (i.e., the length of the second period) is insufficient, and the above requirement (2) is not satisfied.

[0182] In the condition 3, the current-on time of the second section (i.e., the length of the second period) is insufficient, and the above requirement (2) is not satisfied.

[0183] In the condition 5, the current of the second section (i.e., the average value Iave of the current value between the electrodes in the first period) is insufficient, and the above requirement (1) is not satisfied.

[0184] In the condition 6, both of the pressurizing pressure of the second section (i.e., the pressurizing pressure in the first period) and the current-on time of the second section (i.e., the length of the second period) are insufficient, and the above requirements (2) and (3) are not satisfied.

[0185] In the condition 7, the current-on time of the second section (i.e., the length of the second period) is insufficient, and the above requirement (2) is not satisfied.

[0186] In the resistance spot welded joint obtained using these conditions, an LME crack occurred. Furthermore, in the resistance spot welded joint obtained using these conditions, one or more of (A) the diameter of the heat-affected zone, (B) the number density of carbides in the heat-affected zone, and (C) the amount of η phase within the plastic metal ring zone are outside the scope of the present application.

[0187] Industrial applicability

[0188] According to the present application, it is possible to provide a resistance spot welded joint in which an LME crack in a plastic metal ring zone does not occur, despite the fact that a zinc-based plated layer and a high-strength steel sheet abut at an overlapping surface, and a method of manufacturing a resistance spot welded joint. Therefore, the present application has high industrial applicability.

[0189] Explanation of reference numerals

[0190] 1 Resistance spot welded joint

[0191] 11 Steel sheet

[0192] 11' High-strength steel sheet

[0193] 12 Zinc-based plated layer

[0194] 13 Nugget

[0195] 14 Plastic metal ring zone

[0196] 15 Overlapping surface

[0197] 16 Heat-affected zone (HAZ)

[0198] 17 Welded portion

[0199] 18 Shoulder portion

[0200] A Electrode

[0201] C LME crack

Claims

1. A resistance spot welded joint that is a resistance spot welded joint provided with a plurality of steel sheets that overlap, and a welded portion having a nugget that joins the steel sheets, and a plastic metal zone and a heat affected zone that are formed around the nugget, one or more of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more, one or more of the plurality of steel sheets is a plated steel sheet having a zinc-based plating layer, the high-strength steel sheet and the zinc-based plating layer abut at an overlapping surface, a diameter of the heat affected zone is 1.5 times or more of a diameter of the nugget, 40 per 100 μm in the heat affected zone 2 carbides having an equivalent circle diameter of 0.1 μm or more, an amount of η phase of the zinc-based plating layer in the plastic metal zone is 20 area% or less.

2. The resistance spot weld joint of claim 1, wherein, any one or more of the following three requirements is satisfied, 1: a raised portion of a welded portion shoulder portion protrudes by 0.1 mm or more to an outside of a surface of the steel sheet in which the raised portion is formed; 2: an angle formed by a long diameter direction of the nugget and a surface of the steel sheet that surrounds the welded portion is 2° or more; 3: a separation at the overlapping surface where the high-strength steel sheet and the zinc-based plating layer abut is 0.3 mm or more.

3. A method of manufacturing a resistance spot welded joint, comprising: a process of pressing a plurality of steel sheets that overlap using a pair of electrodes that face each other; a process of forming a nugget and a plastic metal zone by passing an electric current between the electrodes while pressing the steel sheets; and a process of reducing a current value between the electrodes to 0 while maintaining the pressing of the steel sheets, one or more of the plurality of steel sheets has a zinc-based plating layer, one or more of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of 780 MPa or more, the high-strength steel sheet and the zinc-based plating layer abut at an overlapping surface, an average value Iave of a current value between the electrodes at a time point at which formation of the nugget is completed and a current value between the electrodes in a first period from the time point at which the formation of the nugget is completed to a time point at which the current value becomes 0 satisfies a relationship of 0.30 x I ≤ Iave ≤ 0.90 x I, a length of a second period from a time point at which the current value becomes 0.90 x I to a time point at which the current value becomes 0.30 x I is set to 420 milliseconds or more, and a pressing pressure in the first period is set to 1.1 times or more of a pressing pressure P at the time point at which the formation of the nugget is completed.

4. The method of manufacturing a resistance spot welded joint according to claim 3, wherein 1 / 2 of a total sheet thickness of the steel sheets in mm is defined as tm, when the current value between the electrodes is reduced to 0, the current value between the electrodes is maintained at a constant value in a range from I x 0.9 to I x 0.3 for a period of 265 x tm milliseconds or more and 420 milliseconds or more.

5. The method of manufacturing a resistance spot welded joint according to claim 3 or 4, wherein After the process of reducing the current value between the electrodes to 0, the process of maintaining the pressurizing pressure at 0.8 x P or more for 0.04 seconds or more and 0.4 seconds or less while the current value between the electrodes is 0 is further provided.

6. The method of producing a resistance spot welded joint according to any one of claims 3 to 5, characterized by The Iave and the I satisfy the relationship of 0.45 x I ≤ Iave ≤ 0.85 x I.

7. A method of producing a resistance spot welded joint according to any one of claims 3 to 6, characterized in that, 1 / 2 of the total sheet thickness of the steel sheet in mm is defined as tm, The length of the second period is set to 265 x tm milliseconds or more and 420 milliseconds or more.

Images