Method for manufacturing resistance spot welded joints

Abstract

The present invention provides a method for manufacturing resistance spot welded joints that can achieve excellent cross tensile strength in a shorter welding time than conventional methods, even when high-strength steel plates with a tensile strength of 780 MPa or more are used as the steel plates to be welded. The method comprises a main current process and a post-heat treatment process, in which a nugget is formed in the main current process, and in the post-heat treatment process, (A) primary cooling, (B) heating, (C) secondary cooling, and (D) current holding are performed under conditions that satisfy the relationships in formulas (1) to (6).

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a resistance spot welded joint. [Background technology]

[0002] On automobile assembly lines, resistance spot welding is the primary method used to join steel plates that form automobile components.

[0003] The joint strength of a welded joint obtained by resistance spot welding (hereinafter also referred to as a resistance spot welded joint) is generally evaluated by the tensile shear strength (TSS), which is the tensile strength in the shear direction, and the cross tension strength (CTS), which is the tensile strength in the peel direction.

[0004] In recent years, the use of high-strength steel sheets has been increasing in automobile bodies in order to reduce weight for improved fuel economy and ensure crashworthiness. The TSS of resistance spot-welded joints tends to increase with the tensile strength of the steel sheets being joined, i.e., the steel sheet that serves as the base material for the resistance spot-welded joint (hereinafter also referred to as the base steel sheet). However, as the tensile strength of the base steel sheet increases, especially when it reaches 780 MPa or higher, the CTS of the resistance spot-welded joint may decrease.

[0005] When the CTS of a resistance spot welded joint decreases, the fracture mode of the resistance spot welded joint transitions from plug fracture to interfacial fracture or partial plug fracture, resulting in a decrease in crashworthiness. Here, plug fracture is a fracture mode in which ductile fracture occurs in the base steel sheet or heat-affected zone (hereinafter also referred to as HAZ) around the nugget. Interfacial fracture and partial plug fracture are fracture modes in which brittle fracture occurs inside the nugget. Here, a nugget is a portion of steel sheets that melts and solidifies at the contact point between overlapping steel sheets when current is passed through them by resistance spot welding. The steel sheets are joined at a point by this nugget.

[0006] One of the reasons for the decrease in CTS is that the resistance spot welded joint is rapidly cooled after the resistance spot welding is completed, which hardens the nugget edge.

[0007] In order to avoid such a decrease in CTS, for example, Patent Documents 1 and 2 disclose a technique in which a post-heat treatment step is performed on the nugget for heat treatment after the main current application step for forming the nugget. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6763483 [Patent Document 2] Patent No. 6958765 Summary of the Invention [Problem to be solved by the invention]

[0009] The techniques of Patent Documents 1 and 2 prevent a decrease in CTS and provide excellent joint strength in resistance spot welded joints. However, these techniques require a post-heat treatment step, which increases the welding time. Therefore, from the standpoints of productivity and workability, there is a strong demand for shortening the welding time, especially the time required for the post-heat treatment step.

[0010] The present invention was developed to meet the above demands, and aims to provide a method for manufacturing a resistance spot welded joint that can achieve excellent CTS in a shorter welding time than conventional methods, even when high-strength steel sheets with a tensile strength of 780 MPa or more are used as the steel sheets to be welded. Note that in this disclosure, all numerical ranges expressed using "to" mean ranges that include the numerical values ​​before and after "to" as the lower and upper limits, respectively, except when written as "more than" or "less than." [Means for solving the problem]

[0011] The inventors have conducted extensive research to achieve the above object, and as a result have found that the desired object can be achieved by performing a heat treatment on the nugget formed in the main current application step by carrying out (A) primary cooling, (B) temperature increase, (C) secondary cooling, and (D) current application holding in the post-heat treatment step, and by satisfying the relationships of the formulas (1) to (6) described below.

[0012] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0013] 1. A method for manufacturing a resistance spot welded joint having two or more overlapping steel plates and a nugget joining the steel plates, comprising: At least one of the steel plates is a high-strength steel plate having a tensile strength of 780 MPa or more, The method includes a main current application step and a post-heat treatment step, In the main current application step, current is applied at a current value I1 (kA) to form the nugget, In the post-heat treatment step, (A) Primary cooling, (B) temperature increase, (C) secondary cooling, and (D) power retention are performed. In the (A) primary cooling, Unpowered state and cooling time t c1 (ms) and the cooling time t c1 satisfies the relationship of the following equation (1), In the (B) temperature increase, A current is applied at a current value I2 (kA) for a current application time t2 (ms), and the current value I2 (kA) and the current application time t2 (ms) satisfy the relationships of the following equations (2) and (3), respectively: In the (C) secondary cooling, Unpowered state and cooling time t c2 (ms) and the cooling time t c2 satisfies the relationship of the following equation (4), In the (D) energization holding, A method for manufacturing a resistance spot welded joint, wherein current is applied at a current value I3 (kA) for a current application time t3 (ms), and the current value I3 (kA) and the current application time t3 (ms) respectively satisfy the relationships of the following equations (5) and (6). 40 ≦t c1 ≦ 250 (1) 1.1×I1≦ I2≦ 1.5×I1 (2) 40 ≦ t2 ≦ 200 (3) 40 ≦t c2 ≦ 250 (4) 0.2×I1≦ I3≦ 0.9×I1 (5) 40 ≦ t3 ≦ 500 (6)

[0014] 2. The method for manufacturing a resistance spot welded joint according to 1 above, wherein the (A) primary cooling and the (B) heating are each performed two or more times after the main current application step and before the (C) secondary cooling.

[0015] 3. The method for manufacturing a resistance spot welded joint according to 1 or 2 above, wherein the total time for the post-heat treatment step is 1200 ms or less.

[0016] 4. The high-strength steel plate is In mass%, C: 0.07 to 0.50%, Si: 0.1 to 2.0%, Mn: 1.0-4.5% P: 0.10% or less, S: 0.005% or less, N: 0.010% or less and O: 0.030% or less and Optionally, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less Cr: 1.00% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 1.000% or less, Ca: 0.005% or less, Sn: 0.100% or less and Sb: 0.200% or less 4. The method for producing a resistance spot welded joint according to any one of 1 to 3 above, having a component composition containing one or more selected from the following, with the balance being Fe and unavoidable impurities. [Effects of the Invention]

[0017] According to the present invention, even when high-strength steel plates with a tensile strength of 780 MPa or more are used as the steel plates to be joined, excellent CTS can be obtained in a shorter welding time than conventional methods, which is extremely advantageous in terms of workability and productivity. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a diagram schematically illustrating a procedure for performing resistance spot welding. FIG. [Figure 2] 1 is a schematic diagram showing an example of a current pattern ((A) primary cooling and (B) temperature increase: once each) in a method for producing a resistance spot-welded joint according to one embodiment of the present invention. FIG. [Figure 3] 1 is a schematic diagram showing an example of a current pattern ((A) primary cooling and (B) temperature increase: two times each) in a method for producing a resistance spot-welded joint according to one embodiment of the present invention. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described based on the following embodiments.

[0020] A method for manufacturing a resistance spot welded joint according to one embodiment of the present invention will be described.

[0021] A resistance spot-welded joint manufactured by a manufacturing method for a resistance spot-welded joint according to one embodiment of the present invention includes two or more overlapping steel plates (base steel plates) and a nugget joining the steel plates. At least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or greater. For example, as shown in FIG. 1 , two overlapping steel plates are prepared as the materials to be welded. A high-strength steel plate is used for either steel plate 1 (located vertically below) or steel plate 2 (located vertically above). In the main current application step, the materials to be welded are sandwiched between a pair of electrodes (vertically lower electrode 4 and vertically upper electrode 5) and joined by applying pressure and current. This forms a nugget 3 at the mating surfaces (lapped surfaces) of steel plate 1 and steel plate 2, joining steel plate 1 and steel plate 2. Next, in the post-heat treatment step, the nugget is heat-treated by performing (A) primary cooling, (B) heating, (C) secondary cooling, and (D) current holding to obtain a resistance spot-welded joint. The materials to be joined, the main current application step, and the post-heat treatment step will be described below.

[0022] [Material to be joined] As described above, the workpieces are composed of two or more overlapping steel plates. At least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or more. That is, as described above, when a steel plate with a tensile strength of 780 MPa or more is used as the workpieces, the CTS of the resistance spot welded joint may decrease, and there is a particularly strong demand for obtaining an excellent CTS in a welding time shorter than conventionally. Therefore, at least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or more, preferably 1180 MPa or more. Note that an excellent CTS can also be obtained when only steel plates with a tensile strength of less than 780 MPa are used as the workpieces.

[0023] The chemical composition of the high-strength steel plate is not particularly limited. For example, the chemical composition of the high-strength steel plate is In mass%, C: 0.07 to 0.50%, Si: 0.1 to 2.0%, Mn: 1.0-4.5% P: 0.10% or less, S: 0.005% or less, N: 0.010% or less and O: 0.030% or less and Optionally, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less Cr: 1.00% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 1.000% or less, Ca: 0.005% or less, Sn: 0.100% or less and Sb: 0.200% or less An example of a composition is one containing one or more selected from the following, with the balance being Fe and inevitable impurities. Hereinafter, "%" relating to the composition means "% by mass" unless otherwise specified.

[0024] C: 0.07 to 0.50% C is an element that contributes to improving the strength of steel. If the C content is less than 0.07%, it becomes difficult to obtain the desired tensile strength. Therefore, the C content is preferably 0.07% or more, more preferably 0.10% or more. On the other hand, if the C content exceeds 0.50%, hard martensite is excessively formed, increasing the number of microvoids. In addition, the nugget formed by resistance spot welding and its surrounding heat-affected zone (hereinafter collectively referred to as the weld zone) become excessively hard. Furthermore, embrittlement also progresses. As a result, it becomes difficult to obtain the desired CTS. Therefore, the C content is preferably 0.50% or less, more preferably 0.40% or less.

[0025] Si: 0.1 to 2.0% Si is an element that contributes to improving the strength of steel. Furthermore, because Si is a ferrite-forming element, it favorably promotes the formation of ferrite at the edge of a nugget formed by resistance spot welding. Therefore, the Si content is preferably 0.1% or more, more preferably 0.2% or more. On the other hand, if the Si content exceeds 2.0%, it may adversely affect toughness. Therefore, the Si content is preferably 2.0% or less, more preferably 1.8% or less.

[0026] Mn: 1.0 to 4.5% Mn is an element that contributes to improving the strength of steel. Therefore, the Mn content is preferably 1.0% or more, more preferably 1.2% or more. On the other hand, if the Mn content exceeds 4.5%, the weld may become embrittled or cracks may occur due to the embrittlement, making it difficult to obtain the desired CTS. Therefore, the Mn content is preferably 4.5% or less, more preferably 3.5% or less. Note that when a steel plate with a Mn content within the above range is used as the joined material, the CTS is likely to decrease, and therefore it is particularly advantageous to apply the manufacturing method for a resistance spot welded joint according to one embodiment of the present invention.

[0027] P:0.10% or less P is an element that is inevitably contained. If the P content exceeds 0.10%, P will segregate strongly at the edge of the nugget, making it difficult to obtain the desired CTS. Therefore, the P content is preferably 0.10% or less, more preferably 0.05% or less, and even more preferably 0.02% or less. The lower limit of the P content is not particularly limited and may be 0%. However, since excessive reduction of P will increase costs, the P content is preferably 0.005% or more.

[0028] S: 0.005% or less S is an element that is inevitably contained. S also segregates at grain boundaries and embrittles steel sheets. Furthermore, S also forms sulfides, reducing the local deformability of steel sheets. Therefore, the S content is preferably 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less. There is no particular lower limit for the S content, and it may be 0%. However, because excessive reduction of S leads to increased costs, the S content is preferably 0.001% or more.

[0029] N: 0.010% or less N is an element that is inevitably contained. Furthermore, N deteriorates the aging resistance of steel. Therefore, the N content is preferably 0.010% or less, and more preferably 0.008% or less. The lower limit of the N content is not particularly limited and may be 0%. However, since excessive reduction of N leads to an increase in costs, the N content is preferably 0.001% or more.

[0030] O: 0.030% or less O (oxygen) is an element that generates nonmetallic inclusions, thereby degrading the cleanliness and toughness of steel. Therefore, the O content is preferably 0.030% or less, and more preferably 0.020% or less. The lower limit of the O content is not particularly limited and may be 0%. The O content is preferably 0.005% or more.

[0031] In addition to the basic elements, the above-mentioned composition may optionally contain the following elements (hereinafter also referred to as optional additional elements).

[0032] Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less Cu, Ni, and Mo are elements that contribute to improving the strength of steel. Therefore, the Cu content is preferably 0.005% or more, more preferably 0.006% or more, the Ni content is preferably 0.01% or more, and the Mo content is preferably 0.005% or more, more preferably 0.006% or more. On the other hand, excessive inclusion of Cu, Ni, and Mo leads to a deterioration in toughness. Therefore, when these elements are contained, the Cu content is preferably 0.80% or less, more preferably 0.60% or less, the Ni content is preferably 1.00% or less, more preferably 0.80% or less, and the Mo content is preferably 1.00% or less, more preferably 0.80% or less.

[0033] Cr:1.00% or less Cr is an element that improves the hardenability and thereby the strength of steel. Therefore, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 1.00%, the toughness of the HAZ may deteriorate. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less, more preferably 0.80% or less.

[0034] Nb: 0.080% or less Nb is an element that improves CTS and delayed fracture resistance by forming fine carbonitrides. Therefore, the Nb content is preferably 0.005% or more, more preferably 0.006% or more. On the other hand, excessive Nb content may reduce elongation and may also deteriorate toughness. Therefore, when Nb is added, the Nb content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less.

[0035] V: 0.50% or less V is an element that improves the strength of steel by controlling the structure through precipitation hardening. Therefore, the V content is preferably 0.005% or more, more preferably 0.02% or more. On the other hand, excessive V content may deteriorate the toughness of the HAZ. Therefore, when V is added, the V content is preferably 0.50% or less, more preferably 0.30% or less.

[0036] Ti: 0.20% or less Ti is an element that improves the hardenability and thereby the strength of steel. Therefore, the Ti content is preferably 0.003% or more, more preferably 0.004% or more. On the other hand, excessive Ti content forms carbides, which undergo precipitation hardening, resulting in a deterioration in toughness. Therefore, when Ti is contained, the Ti content is preferably 0.20% or less, more preferably 0.15% or less.

[0037] B: 0.005% or less B is an element that improves the hardenability and thereby the strength of steel. Therefore, the B content is preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, even if B is added in excess, the above effect saturates. Therefore, when B is added, the B content is preferably 0.005% or less, more preferably 0.0010% or less.

[0038] Al: 1.000% or less Al is an element that enables microstructural control by refining austenite grains. Therefore, the Al content is preferably 0.015% or more. On the other hand, excessive Al content leads to a deterioration in toughness. Therefore, when Al is contained, the Al content is preferably 1.000% or less, more preferably 0.500% or less, and even more preferably 0.100% or less.

[0039] Ca: 0.005% or less Ca is an element that contributes to improving the workability of steel. Therefore, the Ca content is preferably 0.001% or more. On the other hand, excessive Ca content leads to a deterioration in toughness. Therefore, when Ca is added, the Ca content is preferably 0.005% or less, more preferably 0.004% or less.

[0040] Sn: 0.100% or less Sn is an element that suppresses nitriding and oxidation of the steel sheet surface. By suppressing nitriding and oxidation of the steel sheet surface, the reduction of martensite on the steel sheet surface can be suppressed. Therefore, the Sn content is preferably 0.001% or more. On the other hand, excessive Sn content leads to a decrease in toughness. Therefore, when Sn is contained, the Sn content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less.

[0041] Sb: 0.200% or less Sb is an element that suppresses nitriding and oxidation of the steel sheet surface. On the other hand, excessive Sb content leads to a decrease in toughness. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. There is no particular lower limit for the Sb content. From the viewpoint of fully obtaining the above effects, the Sb content is preferably 0.002% or more.

[0042] The balance other than the above elements is Fe and unavoidable impurities. In addition, any of the above optional added elements may be 0%.

[0043] As described above, at least one of the steel plates to be joined must be a high-strength steel plate having a tensile strength of 780 MPa or more, and the remaining steel plates may or may not be high-strength steel plates having a tensile strength of 780 MPa or more.

[0044] In addition, the steel sheet to be joined may be a plated steel sheet having a plating layer on one or both sides of a base steel sheet. An example of the plating layer is a zinc plating layer. A zinc plating layer is a plating layer containing zinc as a main component (containing 50% by mass or more). The zinc plating layer can be formed by subjecting the base steel sheet to a conventional zinc plating process, for example, a hot-dip galvanizing process and an electroplating process. Examples of the zinc plating layer include a hot-dip galvanizing layer, a galvannealed zinc plating layer, an electrogalvanized layer, a Zn-Al plating layer, and a Zn-Ni plating layer. Note that the galvannealed zinc plating layer can be formed by performing an alloying process after a zinc plating process.

[0045] The thickness of the steel plate to be joined is preferably, for example, 0.4 to 2.2 mm, and in this case, general steel plates for automobiles can be used.

[0046] The number of steel sheets to be joined (overlapped) is not particularly limited, and may be two, three, or four. These steel sheets may be the same type or different types. These steel sheets may have the same or different thicknesses. Furthermore, plated steel sheets and unplated steel sheets may be used together. When a plated steel sheet and an unplated steel sheet are overlapped to form a sheet assembly, for example, these steel sheets may be overlapped so that the surface of the plated steel sheet having the plating layer faces the unplated steel sheet.

[0047] [Main energization process] In the main current application step, a current of I1 (kA) is applied to form a nugget and join the steel sheets together. In automotive components, the diameter of the nugget (hereinafter also referred to as nugget diameter) is, for example, 3.0√t to 6.0√t. Here, t (mm) is the thickness of the thinnest steel sheet among the steel sheets that make up the materials to be joined. There are no particular restrictions on the current application conditions in the main current application step, and they may be set in accordance with the usual method depending on the target nugget diameter (hereinafter also referred to as target nugget diameter).

[0048] For example, the current value I1 (kA) in the main current application step is preferably 3.0 to 9.0 kA. If I1 is too low, it becomes difficult to stably obtain the target nugget diameter. On the other hand, if I1 is too high, the nugget diameter may become too large. Furthermore, the steel sheet may be excessively melted, increasing the possibility of expulsion (spattering of molten metal). Therefore, I1 is preferably 3.0 to 9.0 kA. I1 is more preferably 3.5 kA or more. I1 is more preferably 8.0 kA or less. I1 may or may not be constant during the main current application step. Note that if I1 is not constant, the average value of I1 (= the time integral value of I1 from the start to the end of current application in the main current application step divided by t1) is used to determine whether the relationships in Equations (2) and (5) described below are satisfied. The current application time t1 (ms) in the main current application step is preferably 120 to 400 ms from the viewpoint of stably obtaining the target nugget diameter. From the viewpoint of stably obtaining the target nugget diameter, the pressure (kN) in the main current application process is preferably 2.0 to 7.0 kN. The pressure (kN) in the main current application process is more preferably 3.0 kN or more. The pressure (kN) in the main current application process is more preferably 6.5 kN or less.

[0049] [Post-heat treatment process] In the post-heat treatment process, the nugget formed in the main current application process is heat-treated by performing (A) primary cooling, (B) heating, (C) secondary cooling, and (D) current application holding in this order. That is, (A) primary cooling is performed to satisfy the relationship in equation (1) described below, and then (B) heating is performed by applying a short, high current to satisfy the relationships in equations (2) and (3) described below. This achieves the effect of mitigating segregation at the edge of the nugget, particularly the segregation of segregating elements such as P and S (hereinafter also referred to as the segregation mitigation effect). Then, (C) secondary cooling is performed to satisfy the relationship in equation (4) described below, and (D) current application holding is performed to satisfy the relationships in equations (5) and (6) described below. This achieves the effect of softening the edge of the nugget and improving its toughness (hereinafter also referred to as the softening effect). By combining these effects, even when using a high-strength steel plate with a tensile strength of 780 MPa or more as the base steel plate, it is possible to obtain excellent CTS while shortening the total time in the post-heat treatment process, particularly the cooling time and current holding time throughout the entire post-heat treatment process.

[0050] (A) Primary cooling In the primary cooling, the power is turned off and the cooling time is t c1 (ms), and cooling time t c1 The relationship of the following formula (1) is satisfied for the above. Note that the non-energized state means that the current value is 0 (kA). The same applies hereinafter.

[0051] 40 ≦ t c1 ≦ 250 (1) t c1 If t becomes too short, cooling will be insufficient, and the temperature will become excessively high during (B) heating, which will be described later, increasing the risk of the nugget remelting. c1 is 40 ms or more, preferably 50 ms or more, and more preferably 60 ms or more. c1 If t becomes too long, the total time for the post-heat treatment process and therefore the welding time will increase. c1 is 250 ms or less, preferably 230 ms or less, more preferably 200 ms or less, and even more preferably 180 ms or less.

[0052] (B) Temperature increase In the temperature rise, current is applied at a current value I2 (kA) for a current application time t2 (ms), and the current value I2 (kA) and current application time t2 (ms) satisfy the relationships of the following equations (2) and (3), respectively.

[0053] 1.1×I1≦ I2≦ 1.5×I1 (2) In order to fully obtain the above-mentioned segregation mitigation effect and achieve the intended purpose, I2 is 1.1 × I1 or more, preferably 1.2 × I1 or more. On the other hand, if I2 exceeds 1.5 × I1, the nugget may remelt, which may result in a decrease in joint strength. Therefore, I2 is 1.5 × I1 or less, preferably 1.4 × I1 or less. Note that, as long as I2 is within the above range, it may or may not be constant during (B) the temperature rise.

[0054] 40 ≦ t2 ≦ 200 (3) To fully obtain the above-mentioned segregation mitigation effect and achieve the intended purpose, t2 is 40 ms or more, preferably 50 ms or more, and more preferably 60 ms or more. On the other hand, if t2 exceeds 200 ms, the nugget may remelt, resulting in a decrease in joint strength. This is also disadvantageous in terms of shortening the welding time. Therefore, t2 is 200 ms or less, preferably 180 ms or less, and more preferably 160 ms or less.

[0055] Furthermore, from the viewpoint of further enhancing the above-described segregation mitigation effect, (A) primary cooling and (B) temperature increase may each be performed two or more times after the main current application step and before the (C) secondary cooling described below. For reference, FIG. 2 shows an example of a current application pattern for a method for producing a resistance spot welded joint according to an embodiment of the present invention, in which (A) primary cooling and (B) temperature increase are each performed once. FIG. 3 shows an example of a current application pattern for a method for producing a resistance spot welded joint according to an embodiment of the present invention, in which (A) primary cooling and (B) temperature increase are each performed (repeatedly) twice. The conditions for each of (A) primary cooling and (B) temperature increase may be the same or different, as long as they satisfy the above formulas (1) to (3).

[0056] Furthermore, when (A) primary cooling and (B) temperature increase are each performed two or more times, the total cooling time for (A) primary cooling (total cooling time for each cooling) is preferably 1100 ms or less, more preferably 1000 ms or less, and even more preferably 900 ms or less. Similarly, the total current application time for (B) temperature increase (total current application time for each cooling) is preferably 400 ms or less, more preferably 360 ms or less, and even more preferably 320 ms or less. Hereinafter, when (A) primary cooling and (B) temperature increase are each performed two or more times, the cooling time for the first (A) primary cooling is referred to as t c1 (ms), the cooling time t in the kth (A) primary cooling c1-k Similarly, the current value and current flow time in the first (B) temperature rise are expressed as I2 (kA) and t2 (ms), respectively, and the current value and current flow time in the kth (B) temperature rise are expressed as I 2-k (kA) and t 2-k The time is expressed as (ms). Here, k is an integer from 2 to n, and n is the number of times (A) primary cooling and (B) heating are performed. Preferably, n is, for example, 2, 3, or 4. Note that (A) primary cooling and (B) heating are performed the same number of times. That is, when (A) primary cooling and (B) heating are performed n times, the post-heat treatment process is performed in the following order: (A) primary cooling (first time) → (B) heating (first time) → (A) primary cooling (second time) → (B) heating (second time) → (A) primary cooling (nth time) → (B) heating (nth time) → (C) secondary cooling → (D) energization hold.

[0057] (C) Secondary cooling In the secondary cooling, the cooling time is t c2 (ms), and cooling time t c2 The relationship of the following equation (4) is satisfied.

[0058] 40 ≦ t c2 ≦ 250 (4) In order to fully obtain the softening effect and achieve the intended purpose in (D) energization and holding, which will be described later, t c2 is 40 ms or more, preferably 50 ms or more, and more preferably 60 ms or more.c2 If t becomes too long, the total time for the post-heat treatment process and therefore the welding time will increase. c2 is 250 ms or less, preferably 220 ms or less, more preferably 200 ms or less, and even more preferably 180 ms or less.

[0059] (D) Holding current In the energization maintenance, the current is applied at a current value I3 (kA) for a current application time t3 (ms), and the current value I3 (kA) and the current application time t3 (ms) satisfy the relationships in the following equations (5) and (6), respectively.

[0060] 0.2×I1≦ I3≦ 0.9×I1 (5) During the energization hold, the edge of the nugget is softened. This achieves the softening effect described above. To fully obtain the softening effect and achieve the intended purpose, I3 is set to 0.2×I1 or more and 0.9×I1 or less. I3 is preferably 0.3×I1 or more. I3 is preferably 0.8×I1 or less. Note that I3 may or may not be constant during (D) energization hold, as long as it is within the above range.

[0061] 40 ≦ t3 ≦ 500 (6) To fully obtain the softening effect and achieve the intended purpose, t3 is set to 40 ms or more and 500 ms or less. t3 is preferably 50 ms or more, and more preferably 60 ms or more. t3 is preferably 480 ms or less, more preferably 450 ms or less, even more preferably 420 ms or less, and even more preferably 400 ms or less.

[0062] In order to shorten the welding time, the total time for the post-heat treatment step is preferably 1200 ms or less, more preferably 1000 ms or less, and even more preferably 800 ms or less. The total time for the post-heat treatment step is the time from the start of the (first) (A) primary cooling to the end of (D) energization holding.

[0063] Conditions other than those mentioned above are not particularly limited and may be those of the ordinary method. For example, the pressure in the post-heat treatment step is preferably 2.0 to 7.0 kN. The pressure in the post-heat treatment step is more preferably 3.0 kN or more. The pressure in the post-heat treatment step is more preferably 6.5 kN or less. The pressure in the main current application step and the pressure in the post-heat treatment step may be the same or different. Furthermore, after the end of the above-mentioned (D) current application holding, a pressure holding step may be performed in which pressure is applied with electrodes in a non-current-applied state. [Example]

[0064] 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.

[0065] Steel plates (balance: Fe and unavoidable impurities, long side: 150 mm, short side: 50 mm) with the chemical composition shown in Table 1 were stacked in the combination shown in Table 2 to prepare the welded materials, and resistance spot welding was performed on the welded materials in the manner shown in Figure 1 under the conditions shown in Table 3 to produce resistance spot welded joints. Note that the "first," "second," and "third" in the "position of steel plate" in Table 2 refer to the "first," "second," and "third" positions from the bottom in each plate combination, respectively. Also, "GA" in Table 2 means that the plated layer has a galvannealed coating layer. In addition, in Table 2, "780 MPa class" under "tensile strength" means that the tensile strength is within the range of 601 to 900 MPa, "tensile strength: 980 MPa class" means that the tensile strength is within the range of 901 MPa to 1110 MPa, "tensile strength: 1180 MPa class" means that the tensile strength is within the range of 1111 MPa to 1400 MPa, and "tensile strength: 1470 MPa class" means that the tensile strength is within the range of 1401 MPa to 1600 MPa. In Table 3, "-" means that the process in question was not carried out.

[0066] The resistance welding machine used here was a servo motor-driven C-gun (servo spot welding gun) with a DC power supply. The electrodes (vertically lower electrode 4 and vertically upper electrode 5) were both chromium copper DR-type electrodes with a tip diameter of 6 mm and a tip curvature radius of 40 mm. The nugget diameters were all in the range of 3.0√t to 5.5√t. The pressure during the main current application process and the post-current application process were both constant (3.5 kN).

[0067] Then, whether or not a short post-heat treatment time and excellent CTS could be achieved at the same time (hereinafter also referred to as the compatibility of short welding time and excellent CTS) was evaluated in the following manner. The evaluation results are also shown in Table 3.

[0068] [Compatibility between short welding times and excellent CTS] Using the resistance spot welded joints produced as described above, cross tensile tests were performed to measure the CTS according to the procedure specified in JIS Z 3137: 1999. The compatibility of short welding times with excellent CTS was evaluated based on the total time for post-heat treatment steps in producing the resistance spot welded joints and the measured CTS, according to the following criteria. Pass A (Excellent): The total time of the post-heat treatment process is 800 ms or less, and the CTS is 3.4 kN (JIS Class A) or more. Pass B (Very good): The total time of the post-heat treatment process is more than 800ms and less than 1000ms, and the CTS is 3.4kN (JIS Class A) or more. Pass C (Excellent): The total time of the post-heat treatment process is over 1000ms and 1200ms or less, and the CTS is 3.4kN (JIS Class A) or more. Fail (bad): CTS is less than 3.4kN (JIS Class A) and / or the total time of the post-heat treatment process is more than 1200ms

[0069] [Table 1]

[0070] [Table 2]

[0071] [Table 3] TIFF0007823801000004.tif233131

[0072] In all of the inventive examples, even when high-strength steel sheets with a tensile strength of 780 MPa or more were used as the steel sheets to be joined, it was possible to achieve both short welding times and excellent CTS. On the other hand, in all of the comparative examples, it was not possible to achieve both short welding times and excellent CTS. [Explanation of symbols]

[0073] 1, 2 steel plate 3. Nuggets 4, 5 electrodes

Claims

1. A method for manufacturing a resistance spot welded joint having two or more overlapping steel plates and a nugget joining the steel plates, comprising: At least one of the steel plates is a high-strength steel plate having a tensile strength of 780 MPa or more, The method includes a main current application step and a post-heat treatment step, In the main current application step, the current value I 1 (kA) to form the nugget, In the post-heat treatment step, (A) Primary cooling, (B) temperature increase, (C) secondary cooling, and (D) current retention are performed. In the (A) primary cooling, No-energization state and cooling time t c１ (ms), and the cooling time t c１ satisfies the relationship of the following equation (1), In the (B) temperature increase, Current value I ２ (kA) and current application time t 2 (ms), and the current value I ２ (kA) and the current application time t 2 (ms) satisfy the relationships of the following equations (2) and (3), respectively: In the (C) secondary cooling, No-energization state and cooling time t c２ (ms), and the cooling time t c２ satisfies the relationship of the following equation (4), In the (D) energization holding, Current value I ３ (kA) and current application time t ３ (ms), and the current value I ３ (kA) and the current application time t ３ (ms) respectively satisfy the relationships of the following equations (5) and (6). 40 ≦t c１ ≦ 250 ・・・(1) 1.1×I １ ≦ I ２ ≦ 1.5×I １ ・・・(2) 40 ≦ t ２ ≦ 200 ・・・(3) 40 ≦t ｃ２ ≦ 250 ・・・(4) 0.2×I １ ≦ I ３ ≦ 0.9×I １ ・・・(5) 40 ≦ t 3 ≦ 500 ・・・(6)

2. 2. The method for producing a resistance spot welded joint according to claim 1, wherein the (A) primary cooling and the (B) heating are each performed two or more times after the main current application step and before the (C) secondary cooling.

3. The method for manufacturing a resistance spot welded joint according to claim 1 or 2, wherein a total time for the post-heat treatment step is 1200 ms or less.

4. The high-strength steel plate In mass%, C: 0.07-0.50%, Si: 0.1-2.0%, Mn: 1.0 to 4.5%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less and O: 0.030% or less and Optionally, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 1.000% or less, Ca: 0.005% or less, Sn: 0.100% or less and Sb: 0.200% or less 3. The method for producing a resistance spot welded joint according to claim 1, having a component composition comprising one or more selected from the group consisting of:

5. The high-strength steel plate, In mass%, C: 0.07-0.50%, Si: 0.1-2.0%, Mn: 1.0 to 4.5%, P: 0.10% or less, S: 0.005% or less, N: 0.010% or less and O: 0.030% or less and Optionally, Cu: 0.80% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 1.000% or less, Ca: 0.005% or less, Sn: 0.100% or less and Sb: 0.200% or less The method for producing a resistance spot welded joint according to claim 3, having a component composition containing one or more selected from the group consisting of:

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