Method for manufacturing resistance spot welded joint

Abstract

Provided is a method for manufacturing a resistance spot welded joint with which it is possible to obtain excellent cross tensile strength with a shorter welding time than in the past, even when a high-strength steel sheet having a tensile strength of 780 MPa or more is used as a steel plate serving as the material to be joined. The method includes a main energization step and a post-heat treatment step, wherein: a nugget is formed in the main energization step; and in the post-heat treatment step, (A) primary cooling, (B) temperature increase, (C) secondary cooling, and (D) maintained energization are performed under conditions satisfying the relationships of expressions (1) to (6).

Description

Method for manufacturing a resistance spot welding joint

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

[0002] In an automobile assembly line, resistance spot welding is mainly used as a method for joining steel plates that are members of an automobile.

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

[0004] In recent years, in automobile bodies, the application of high-strength steel plates has been promoted from the viewpoints of weight reduction for fuel efficiency improvement and ensuring collision safety. The TSS of a resistance spot welding joint tends to increase along with the tensile strength of the steel plate used as the joined material, that is, the steel plate that becomes the base material of the resistance spot welding joint (hereinafter also referred to as the base material steel plate). However, when the tensile strength of the base material steel plate becomes high, particularly when it is 780 MPa or more, the CTS of the resistance spot welding joint may decrease.

[0005] When the CTS of a resistance spot welding joint decreases, the fracture mode of the resistance spot welding joint transitions from plug fracture to interfacial fracture or partial plug fracture, leading to a decrease in collision safety. Here, plug fracture is a fracture mode in which ductile fracture occurs in the base material steel plate or the 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, the nugget is a portion where the steel plates melt and solidify at the contact location between the steel plates when an electric current is passed through the steel plates overlapped by resistance spot welding. The steel plates are joined pointwise by this nugget.

[0006] Examples of the cause of the decrease in CTS include, for example, the hardening of the nugget end due to the rapid cooling of the resistance spot welding joint after the completion of resistance spot welding.

[0007] To avoid such a decrease in CTS, for example, Patent Documents 1 and 2 disclose a technique in which a post-heat treatment process is performed on the nugget for heat treatment after the main energizing process that forms the nugget.

[0008] Patent No. 6763483, Patent No. 6958765

[0009] The technologies described in Patent Documents 1 and 2 suppress the decrease in CTS and achieve excellent joint strength in resistance spot welded joints. However, these technologies involve a post-heat treatment process, which increases the welding time. Therefore, from the viewpoint of productivity and workability, there is a strong demand for shortening the welding time, especially the time required for the post-heat treatment process.

[0010] The present invention was developed to meet the above requirements and aims to provide a method for manufacturing resistance spot welded joints that can obtain excellent CTS in a shorter welding time than conventional methods, even when using high-strength steel plates with a tensile strength of 780 MPa or more as the steel plates to be joined. In this disclosure, numerical ranges expressed using "~" all include the numerical values ​​before and after "~" as the lower and upper limits, respectively, except when "greater than" or "less than" is indicated.

[0011] The inventors then diligently conducted research to achieve the above objectives. As a result, they discovered that the desired objectives could be achieved by performing heat treatment on the nuggets formed in the main energizing process in a post-heat treatment step, which involves (A) primary cooling, (B) heating, (C) secondary cooling, and (D) holding the energized state, while satisfying the relationship shown in equations (1) to (6) described later.

[0012] This invention was completed based on the above findings and further investigations. Specifically, the gist of this invention is as follows:

[0013] 1. A method for manufacturing a resistance spot welding joint having two or more overlapping steel plates and a nugget for joining the steel plates together, wherein at least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or more, and the method includes a main energization step and a post-heat treatment step. In the main energization step, energization is performed 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) energization holding are carried out. In the (A) primary cooling, it is held in a non-energized state and a cooling time t １ , ,

[0014] , 3 (ms), and the cooling time t c１ satisfies the relationship of the following formula (1). In the (B) temperature increase, energization is performed at a current value I ２ (kA) and an energization time t2 (ms), and the current value I ２ (kA) and the energization time t2 (ms) respectively satisfy the relationships of the following formulas (2) and (3). In the (C) secondary cooling, it is held in a non-energized state and a cooling time t c２ (ms), and the cooling time t c２ satisfies the relationship of the following formula (4). In the (D) energization holding, energization is performed at a current value I ３ (kA) and an energization time t ３ (ms), and the current value I ３ (kA) and the energization time t ３ (ms) respectively satisfy the relationships of the following formulas (5) and (6). A method for manufacturing a resistance spot welding joint. 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)

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

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

[0016] 4. The high-strength steel sheet has the following composition in mass%, C: 0.07-0.50%, Si: 0.1-2.0%, 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 A method for manufacturing a resistance spot welding joint according to any one of the above 1 to 3, having a component composition containing one or more Sb selected from 0.200% or less, with the remainder being Fe and unavoidable impurities.

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

[0018] This is a schematic diagram illustrating the procedure for performing resistance spot welding. This is a schematic diagram showing an example of the energizing pattern ((A) primary cooling and (B) heating: once each) of the method for manufacturing a resistance spot welded joint according to one embodiment of the present invention. This is a schematic diagram showing an example of the energizing pattern ((A) primary cooling and (B) heating: twice each) of the method for manufacturing a resistance spot welded joint according to one embodiment of the present 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 the manufacturing method of a resistance spot welded joint according to one embodiment of the present invention comprises two or more overlapping steel plates (base steel plates) and a nugget for joining the steel plates together. Furthermore, at least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or more. For example, as shown in Figure 1, two overlapping steel plates are prepared as materials to be joined, and high-strength steel plates are used for either the steel plate 1 positioned vertically downward or the steel plate 2 positioned vertically upward, or both. Then, in the main energizing process, the materials to be welded are sandwiched between a pair of electrodes (a vertically downward electrode 4 and a vertically upward electrode 5), and energized while pressurized to join them, thereby forming a nugget 3 on the joint surface (overlapping surface) of steel plate 1 and steel plate 2, and joining steel plate 1 and steel plate 2. Next, in the post-heat treatment process, the nugget is heat-treated by (A) primary cooling, (B) heating, (C) secondary cooling, and (D) energizing and holding to obtain a resistance spot welded joint. The materials to be joined, the main energizing process, and the post-heat treatment process will be described below.

[0022] [Materials to be joined] As described above, the materials to be joined consist 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 material to be joined, the CTS of the resistance spot weld joint may decrease, and there is a particularly strong demand to obtain a superior CTS with a shorter welding time than conventional methods. For this reason, 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 even when only steel plates with a tensile strength of less than 780 MPa are used as the materials to be joined, a superior CTS can be obtained.

[0023] The component composition of high-strength steel sheets is not particularly limited. For example, the component composition of high-strength steel sheets is, in mass%, C: 0.07 to 0.50%, Si: 0.1 to 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 An example of a component composition is one containing one or more Sb selected from 0.200% or less, with the remainder being Fe and unavoidable impurities. Hereafter, "%" in relation to component composition means "mass%" unless otherwise specified.

[0024] C: 0.07-0.50% Carbon (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%, excessive hard martensite is generated, and the number of microvoids increases. In addition, the nugget formed by resistance spot welding and the surrounding heat-affected zone (hereinafter collectively referred to as the welded area) become excessively hardened. 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: 0.1-2.0% Si is an element that contributes to improving the strength of steel. Also, since Si is a ferrite-forming element, it favorably acts on the formation of ferrite at the ends of nuggets 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: 1.0-4.5% 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%, there is a risk of embrittlement of the weld or cracking due to 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. When using steel plates with a Mn content within the above range as the material to be joined, the CTS tends to decrease, so it is particularly advantageous to apply the manufacturing method of 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 present. If the P content exceeds 0.10%, P will strongly segregate at the ends of the nuggets, 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, excessive reduction of P leads to increased costs, so the P content is preferably 0.005% or more.

[0028] S: 0.005% or less. S is an element that is inevitably present. Furthermore, S is an element that segregates at grain boundaries and embrittles the steel sheet. In addition, S is an element that forms sulfides and reduces the local deformability of the steel sheet. Therefore, the S content is preferably 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less. The lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction of S leads to increased costs, so the S content is preferably 0.001% or more.

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

[0030] O: 0.030% or less. Oxygen (O) is an element that degrades the cleanliness and toughness of steel by generating nonmetallic inclusions. Therefore, the O content is preferably 0.030% or less, 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] Furthermore, in addition to the basic elements mentioned above, the following elements (hereinafter also referred to as optional additive elements) may be optionally included in the above-mentioned component composition.

[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 amounts of Cu, Ni, and Mo lead to a deterioration of toughness. Therefore, when these elements are included, 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 strength of steel by improving hardenability. 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 included, the Cr content is preferably 1.00% or less, and 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, if Nb is included in excess, the elongation will decrease and the toughness may also deteriorate. Therefore, when Nb is included, 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 microstructure through precipitation hardening. Therefore, the V content is preferably 0.005% or more, more preferably 0.02% or more. On the other hand, if V is included in excess, the toughness of the HAZ may deteriorate. Therefore, when V is included, 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 strength of steel by improving hardenability. Therefore, the Ti content is preferably 0.003% or more, and more preferably 0.004% or more. On the other hand, if Ti is included in excess, carbides are formed, and the precipitation hardening of the carbides leads to a deterioration of toughness. Therefore, when Ti is included, the Ti content is preferably 0.20% or less, and more preferably 0.15% or less.

[0037] B: 0.005% or less. B is an element that improves the strength of steel by improving hardenability. Therefore, the B content is preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, even if B is included in excess, the above effect will saturate. Therefore, when B is included, 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 microstructure control by refining austenite. Therefore, the Al content is preferably 0.015% or more. On the other hand, excessive Al content leads to a deterioration of toughness. Therefore, when Al is included, 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 of toughness. Therefore, when Ca is included, the Ca content is preferably 0.005% or less, and 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 included, 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 included, the Sb content is preferably 0.200% or less. There is no particular lower limit to the Sb content. From the viewpoint of obtaining the above effects sufficiently, the Sb content is preferably 0.002% or more.

[0042] The remainder of the mixture, other than the elements listed above, consists of Fe and unavoidable impurities. Furthermore, any of the optional additives listed above may be present at 0%.

[0043] Furthermore, as mentioned above, of the steel plates to be joined, at least one steel plate must be a high-strength steel plate with a tensile strength of 780 MPa or higher. The remaining steel plates may or may not be high-strength steel plates with a tensile strength of 780 MPa or higher.

[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 the base steel sheet. An example of a plating layer is a zinc plating layer. A zinc plating layer is a plating layer mainly composed of zinc (containing 50% by mass or more). The zinc plating layer can be formed on the base steel sheet by performing a zinc plating treatment in accordance with conventional methods, such as hot-dip galvanizing and electroplating. Examples of zinc plating layers include a hot-dip galvanized layer, an alloyed zinc plating layer, an electroplated zinc plating layer, a Zn-Al plating layer, and a Zn-Ni plating layer. An alloyed zinc plating layer can be formed by performing an alloying treatment after zinc plating.

[0045] The thickness of the steel sheet to be joined is preferably, for example, 0.4 to 2.2 mm. In this case, general automotive steel sheets can be used.

[0046] The number of steel plates to be joined (overlapped) is not particularly limited and may be two, three, or four. These steel plates may be of the same type or different types. The thickness of these steel plates may be the same or different. In addition, plated steel plates and unplated steel plates may be used simultaneously. When overlapping plated steel plates and unplated steel plates to form a plate assembly, for example, the plates may be overlapped so that the side of the plated steel plate with the plating layer faces the unplated steel plate.

[0047] [Main energizing process] In the main energizing process, the current value I １ The steel plates are joined together by applying current at (kA) to form a nugget. In automotive components, the nugget diameter (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 plate among the steel plates constituting the joined material. The energizing conditions for the main energizing process are not particularly limited and should be set according to the target nugget diameter (hereinafter also referred to as target nugget diameter) in accordance with conventional methods.

[0048] As an example, the current value I of the main energization process １ (kA) is preferably 3.0 to 9.0 kA. １If the amount is too small, it becomes difficult to stably obtain the target nugget diameter. On the other hand, I １ If this becomes excessive, the nugget diameter may become excessive. Also, the steel plate may melt excessively, increasing the likelihood of spatter (scattering of molten metal). Therefore, I １ The current is preferably 3.0 to 9.0 kA. １ It is more preferably 3.5 kA or higher. １ It is more preferably 8.0 kA or less. １ This may be constant or not constant during the main energization process. １ If it is not constant, １ The average value (= I from the start to the end of the main energizing process) １ Time integral value ÷ t １ ) determines whether the relationship between equations (2) and (5) described later is satisfied. Main energizing process energizing time t １ The (ms) is preferably 120 to 400 ms from the viewpoint of stably obtaining the target nugget diameter. The applied pressure (kN) in the main energizing process is preferably 2.0 to 7.0 kN from the viewpoint of stably obtaining the target nugget diameter. The applied pressure (kN) in the main energizing process is more preferably 3.0 kN or more. The applied pressure (kN) in the main energizing process is more preferably 6.5 kN or less.

[0049] [Post-heat treatment process] In the post-heat treatment process, (A) primary cooling, (B) heating, (C) secondary cooling, and (D) energizing are performed in this order to heat-treat the nuggets formed in the main energizing process. Specifically, (A) primary cooling is performed to satisfy the relationship in equation (1) described later, and then (B) heating is performed by applying a high current for a short time to satisfy the relationships in equations (2) and (3) described later. This provides an effect of mitigating segregation at the ends of the nuggets, especially 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 later, and (D) energizing is performed to satisfy the relationships in equations (5) and (6) described later. This provides an effect of softening the ends of the nuggets and improving their toughness (hereinafter also referred to as the softening effect). By combining these effects, even when using high-strength steel sheets with a tensile strength of 780 MPa or more as the base material, it becomes possible to obtain excellent CTS while shortening the total time in the post-heat treatment process, particularly the cooling time and energized holding time throughout the entire post-heat treatment process.

[0050] (A) Primary cooling: In primary cooling, the state is without power and the cooling time t c１ Hold for (ms), then allow to cool for t c１ The following relationship (1) must be satisfied. Note that the no-energy state means that the current value is 0 (kA). The same applies from here on.

[0051] 40 ≤ t c１ ≦ 250...(1) t c１ If the length becomes too short, cooling will be insufficient, and in the subsequent heating (B) described later, the temperature will become excessively high, increasing the risk of the nuggets remelting. Therefore, t c１ t is 40 ms or more, preferably 50 ms or more, more preferably 60 ms or more. c１ If this becomes too long, it will lead to a longer total time in the post-heat treatment process, and consequently, a longer welding time. Therefore, t c１ The time interval 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) When heating, the current value I ２ (kA) and current is applied for a duration of t2 (ms), and the current value I ２ The following relationships (2) and (3) are satisfied for (kA) and energizing time t2 (ms), respectively.

[0053] 1.1 × I １ ≤ I ２ ≤ 1.5 × I １ ... (2) In order to obtain the above segregation relaxation effect sufficiently and achieve the intended purpose, ２ is 1.1 × I １ Preferably 1.2 × I １ That is all. On the other hand, I ２ 1.5 × I １ If it exceeds I, the nugget may remelt, leading to a decrease in joint strength. Therefore, ２ is 1.5 × I １ The following is preferably 1.4 × I １ The following applies. Note that I ２ Within the above range, (B) during the heating process, it may be constant or not.

[0054] 40 ≤ t ２ ≤ 200 ... (3) In order to obtain the above segregation relaxation effect sufficiently and achieve the intended purpose, t ２ t is 40 ms or more, preferably 50 ms or more, more preferably 60 ms or more. ２ If the welding time exceeds 200 ms, the nugget may remelt, leading to a decrease in joint strength. Furthermore, it is disadvantageous in terms of shortening the welding time. Therefore, t ２ The duration 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 segregation relaxation effect described above, (A) primary cooling and (B) heating may each be performed two or more times after the main energizing process and before the (C) secondary cooling described later. For reference, Figure 2 shows an example of the energizing pattern for the method of manufacturing a resistance spot welded joint according to one embodiment of the present invention, in which (A) primary cooling and (B) heating are each performed once. Figure 3 shows an example of the energizing pattern for the method of manufacturing a resistance spot welded joint according to one embodiment of the present invention, in which (A) primary cooling and (B) heating are each performed (repeated) two times. The conditions for each of (A) primary cooling and (B) heating 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) heating are performed two or more times each, the total cooling time for (A) primary cooling (total cooling time for each step) is preferably 1100 ms or less, more preferably 1000 ms or less, and even more preferably 900 ms or less. Similarly, the total energizing time for (B) heating (total energizing time for each step) 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) heating are performed two or more times each, the cooling time for the first (A) primary cooling is t c１ (ms), cooling time t in the kth (A) primary cooling c１－ｋ (ms) is used as notation. Similarly, the current value and energizing time during the first (B) heating are given as I ２ (kA) and t ２ (ms) is used to indicate the current value and energizing time during the kth (B) heating step, respectively. ２－ｋ (kA) and t ２－ｋThis is denoted 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. For example, n is preferably 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 order of (A) primary cooling (1st time) → (B) heating (1st time) → (A) primary cooling (2nd time) → (B) heating (2nd time) → ... (A) primary cooling (nth time) → (B) heating (nth time) → (C) secondary cooling → (D) energization holding.

[0057] (C) Secondary cooling In secondary cooling, the state without power and the cooling time t c２ Hold for (ms), then allow to cool for t c２ The following relationship (4) must be satisfied.

[0058] 40 ≤ t c２ ≤ 250 ... (4) In order to obtain the above softening effect sufficiently and achieve the intended purpose in the current-holding described later in (D), t c２ t is 40 ms or more, preferably 50 ms or more, more preferably 60 ms or more. c２ If this becomes too long, it will lead to a longer total time in the post-heat treatment process, and consequently, a longer welding time. Therefore, t c２ The reaction time 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) Maintaining power supply: When maintaining power supply, the current value I ３ (kA) and energizing time t ３ The current is applied in (ms), and the current value I ３ (kA) and energizing time t ３ For (ms), the following relationships (5) and (6) are satisfied, respectively.

[0060] 0.2 × I １ ≤ I ３ ≤ 0.9 × I １ ... (5) During energization, the ends of the nugget are softened. This provides the softening effect described above. In order to obtain the above softening effect sufficiently and achieve the intended purpose, I ３ is 0.2 × I １The above is 0.9 × I １ The following applies: I ３ Preferably, 0.3 × I １ That's all. ３ Preferably, 0.8 × I １ The following applies. Note that I ３ Within the above range, (D) the voltage may be constant or not constant during the period of energization.

[0061] 40 ≤ t 3 ≤ 500 ... (6) In order to obtain the above softening effect sufficiently and achieve the intended purpose, t3 shall be 40 ms or more and 500 ms or less. t3 is preferably 50 ms or more, 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] Furthermore, in order to shorten the welding time, the total time of the post-heat treatment process is preferably 1200 ms or less, more preferably 1000 ms or less, and even more preferably 800 ms or less. The total time of the post-heat treatment process is the time from (A) the start of the first primary cooling to (D) the end of the energized holding.

[0063] Other than the conditions mentioned above, there are no particular limitations, and conventional methods may be followed. For example, the pressure applied in the post-heat treatment process is preferably 2.0 to 7.0 kN. The pressure applied in the post-heat treatment process is more preferably 3.0 kN or more. The pressure applied in the post-heat treatment process is more preferably 6.5 kN or less. The pressure applied in the main energizing process and the pressure applied in the post-heat treatment process may be the same or different. In addition, after the completion of (D) energizing and holding described above, pressurized holding may be performed by applying pressure to the electrodes in a de-energized state.

[0064] The operation and effects of the present invention will be described below using examples. However, the present invention is not limited to the following examples.

[0065] Steel plates having the component composition shown in Table 1 (remainder: Fe and unavoidable impurities, long side: 150 mm, short side: 50 mm) were stacked in the combinations shown in Table 2 to prepare the materials to be joined. Resistance spot welding was performed on the materials to be joined in the manner shown in Figure 1 under the conditions shown in Table 3 to manufacture resistance spot welded joints. In Table 2, "1st plate," "2nd plate," and "3rd plate" in the "Position of Steel Plate" column refer to the positions of the "1st plate," "2nd plate," and "3rd plate" from the bottom in each plate assembly, respectively. Also, "GA" in Table 2 means that the plate has an alloyed zinc plating layer as the plating layer. Furthermore, in Table 2, "Tensile Strength: 780 MPa class" means that the tensile strength is in the range of 601 to 900 MPa, "Tensile Strength: 980 MPa class" means that the tensile strength is in the range of 901 to 1110 MPa, "Tensile Strength: 1180 MPa class" means that the tensile strength is in the range of 1111 to 1400 MPa, and "Tensile Strength: 1470 MPa class" means that the tensile strength is in the range of 1401 to 1600 MPa. In Table 3, "-" means that the process in question was not performed.

[0066] Here, a C-gun (servo spot welding gun) with a servo motor-driven pressurized DC power supply was used as the resistance welding machine. Both electrodes (electrode 4 on the lower vertical side and electrode 5 on the upper vertical side) were DR-type electrodes made of chromium copper with a tip diameter of 6 mm and a tip radius of curvature of 40 mm. The nugget diameters were in the range of 3.0√t to 5.5√t. The pressing force during the main energizing process and the pressing force during the post-energing process were both kept constant (3.5 kN).

[0067] Then, we evaluated whether it was possible to achieve both a shorter post-heat treatment process and excellent CTS (hereinafter also referred to as the compatibility between shorter welding time and excellent CTS) according to the following procedure. The evaluation results are shown in Table 3.

[0068] [Compatibility of Shorter Welding Time and Excellent CTS] Using the resistance spot welded joints manufactured as described above, a cross tensile test was performed in accordance with the procedure specified in JIS Z 3137:1999, and the CTS was measured. Then, the compatibility of shorter welding time and excellent CTS was evaluated based on the total time of the post-heat treatment process during the manufacture of the above resistance spot welded joints and the measured CTS, according to the following criteria. Pass A (Excellent): Total post-heat treatment time is 800 ms or less, and CTS is 3.4 kN (JIS Class A) or higher. Pass B (Very Good): Total post-heat treatment time is more than 800 ms and 1000 ms or less, and CTS is 3.4 kN (JIS Class A) or higher. Pass C (Good): Total post-heat treatment time is more than 1000 ms and 1200 ms or less, and CTS is 3.4 kN (JIS Class A) or higher. Fail (Poor): CTS is less than 3.4 kN (JIS Class A), and / or total post-heat treatment time is more than 1200 ms.

[0069]

[0070]

[0071]

[0072] In all of the inventive examples, even when using high-strength steel plates with a tensile strength of 780 MPa or more as the material to be joined, it was possible to achieve both shorter welding times and excellent CTS (Cold Threshold Strength). On the other hand, in all of the comparative examples, it was not possible to achieve both shorter welding times and excellent CTS.

[0073] 1, 2 Steel plate 3 Nugget 4, 5 Electrode

Claims

1. A method for manufacturing a resistance spot welding joint having two or more overlapping steel plates and a nugget for joining the steel plates together, wherein at least one of the steel plates is a high-strength steel plate with a tensile strength of 780 MPa or more, and the method includes a main energization step and a post-heat treatment step. In the main energization step, energization is performed at a current value I1 (kA) to form the nugget. In the post-heat treatment step, (A) primary cooling, (B) heating, (C) secondary cooling, and (D) energization holding are performed. In the (A) primary cooling, it is held in a non-energized state and a cooling time t c１ (ms), and the cooling time t c１ satisfies the relationship of the following formula (1). In the (B) heating, energization is performed at a current value I ２ (kA) and an energization time t2 (ms), and the current value I ２ (kA) and the energization time t2 (ms) respectively satisfy the relationships of the following formulas (2) and (3). In the (C) secondary cooling, it is held in a non-energized state and a cooling time t c２ (ms), and the cooling time t c２ satisfies the relationship of the following formula (4). In the (D) energization holding, energization is performed at a current value I ３ (kA) and an energization time t ３ (ms), and the current value I ３ (kA) and the energization time t ３ (ms) respectively satisfy the relationships of the following formulas (5) and (6). A method for manufacturing a resistance spot welding joint. 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. The method for manufacturing 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 energizing process and before the (C) secondary cooling.

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

4. The high-strength steel sheet has the following composition in mass%, C: 0.07-0.50%, Si: 0.1-2.0%, 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 A method for manufacturing a resistance spot welding joint according to any one of claims 1 to 3, having a component composition containing one or more Sb selected from 0.200% or less, with the remainder being Fe and unavoidable impurities.

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