Method for manufacturing resistance spot welded joint

Abstract

Provided is a method for manufacturing a resistance spot welded joint having excellent width of an appropriate current range in a post-heat treatment process, even if a high-strength steel plate having a tensile strength of 780 MPa or greater is used as a steel plate serving as a material to be joined. This method comprises a main energization process, a cooling process in a non-energization state, and a post-heat treatment process. A pressurizing force Fm (kN) in the main energization process and a pressurizing force Fc (kN) in the cooling process satisfy the relationship of the following formula (1), and a cooling time tc (ms) of the cooling process satisfies the relationship of the following formula (2).　(1): Fm +1.50 ≤ Fc ≤ Fm + 3.50 (2): 300 < tc

Description

Method for manufacturing resistance spot welded joints

[0001] This invention relates to a method for manufacturing resistance spot welding joints.

[0002] In automobile assembly lines, resistance spot welding is primarily used as a method for joining steel plates that will become 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 shear tensile strength (TSS), which is the tensile strength in the shear direction, and the cross tensile strength (CTS), which is the tensile strength in the delamination direction.

[0004] Furthermore, the application of high-strength steel plates to automobile bodies is progressing from the viewpoint of weight reduction for improved fuel efficiency and ensuring collision safety. The TSS of resistance spot welded joints tends to increase with the tensile strength of the steel plate to be joined, that is, the steel plate that serves as the base material of the resistance spot welded joint (hereinafter also referred to as the base steel plate). However, when the tensile strength of the base steel plate becomes high, especially above 780 MPa, 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 interface 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 steel plate or heat-affected zone (hereinafter also referred to as HAZ) surrounding the nugget. Interface fracture and partial plug fracture are fracture modes in which brittle fracture occurs inside the nugget. Here, a nugget is the part of the steel plate that melts and solidifies at the point of contact when an electric current is passed through overlapping steel plates in resistance spot welding. This nugget joins the steel plates together at points.

[0006] One possible cause of a decrease in CTS is the hardening of the nugget ends due to the rapid cooling of the resistance spot welded 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] Japanese Patent Publication No. 2011-67853 Japanese Patent Publication No. 2009-241086

[0009] By the way, in actual welding, for example, welding on an automobile assembly line, thousands of welds are performed on a single automobile. In this case, if the condition of the materials to be joined and the shape of the electrodes are the same at each welding point, the same heat treatment effect, that is, the CTS improvement effect, can be obtained by applying current under the same current conditions.

[0010] However, in the presence of disturbances, for example, if there are already welded points (hereinafter also referred to as pre-welded points) near the welding points on the material to be joined, or if the electrodes wear down and the contact area between the electrodes and the material to be welded changes from its initial state, the amount of charge passing through the nugget during the post-heat treatment process, i.e., the current density, will vary even under the same energizing conditions. As a result, the desired CTS improvement effect may not be obtained, leading to a decrease in yield and, consequently, a decrease in productivity.

[0011] To increase the margin for current density fluctuations due to such disturbances, it is effective to expand the range of current values ​​during the post-heat treatment process when the desired CTS improvement effect can be obtained in an ideal state without disturbances (hereinafter also referred to as the appropriate current range in the post-heat treatment process).

[0012] In the technologies described in Patent Documents 1 and 2, when high-strength steel plates, particularly those with a tensile strength of 780 MPa or higher, are used as the material to be joined, it is necessary to strictly control the current value in the post-heat treatment process. In other words, in the technologies described in Patent Documents 1 and 2, the appropriate current range in the post-heat treatment process when using high-strength steel plates with a tensile strength of 780 MPa or higher is very narrow. In recent years, there has been a growing demand for further improvements in productivity. Therefore, there is a strong need for the development of a resistance spot welding joint manufacturing method that has a wide appropriate current range in the post-heat treatment process, even when using high-strength steel plates with a tensile strength of 780 MPa or higher as the material to be joined (hereinafter also referred to as a resistance spot welding joint manufacturing method that excels in a wide appropriate current range in the post-heat treatment process).

[0013] The present invention was developed to meet the above requirements and aims to provide a method for manufacturing resistance spot welded joints that offers excellent performance in widening the appropriate current range during the post-heat treatment process, 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, any numerical range expressed using "~" means a range that includes the numerical values ​​before and after "~" as the lower and upper limits, respectively, unless otherwise stated as "greater than" or "less than".

[0014] The inventors then conducted extensive research to achieve the above objectives. As a result, they discovered that the desired objectives could be achieved by performing a main energizing process, a cooling process in an unenervated state, and a post-heat treatment process, while satisfying the relationship between equations (1) and (2) described later.

[0015] This invention was completed based on the above findings and further investigations. Specifically, the gist of this invention is as follows:

[0016] 1. A method for manufacturing a resistance spot welded joint having two or more overlapping steel plates and a nugget for joining the steel plates, 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 comprises a main energizing step, a cooling step in an unenervated state, and a post-heat treatment step, wherein the applied pressure F in the main energizing step ｍ(kN) and the pressing force F in the cooling step ｃ (kN) satisfy the relationship of the following formula (1), and the cooling time t in the cooling step ｃ (ms) satisfies the relationship of the following formula (2). A method for manufacturing a resistance spot welding joint. F ｍ +1.50 ≤ F ｃ ≤ F ｍ +3.50... (1) 300 < t ｃ ... (2)

[0017] 2. In the post-heat treatment step, the current value I ｔ１ (kA) and the energization time t ｔ１ (ms) are energized, and the energization time t ｔ１ satisfies the relationship of the following formula (3). The method for manufacturing a resistance spot welding joint according to 1 above. 10 ≤ t ｔ１ [[ID=2,3]] ≤ 200... (3)<00$0193><00001$4> 3. In the post-heat treatment step, the current value I ｔ１ (kA) and the energization time t ｔ１ (kA) are energized, and then the current value is changed to the current value I ｔ２ (kA) over a transition time t ｔ２ (ms) while energizing, and the energization time t ｔ１ [[ID=,36]] satisfies the relationship of the following formula (4), and the current value I<0$00017> and the current value I ｔ２ satisfy the relationship of the following formula (5), and the transition time t<0000$19> satisfies the relationship of the following formula (6), and the energization time t ｔ１ and the transition time t ｔ２ satisfy the relationship of the following formula (7). The method for manufacturing a resistance spot welding joint according to 1 above. 0 < t ｔ１ ≤ 200... (4) 0 ≤ I ｔ２ ≤ 0.5 × I ｔ１ ... (5) 20 ≤ t ｔ２ ≤ 800... (6) 100 < t ｔ１ + t ｔ２ ... (7)

[0019] 4. In the post-heat treatment step, the current value is changed from the current value I ｔ１ (kA) to the current value Iｔ２ Transition time t to (kA) ｔ２ The current is applied while transitioning in (ms), and the current value I ｔ１ and the current value I ｔ２ The relationship in equation (5) below is satisfied, and the transition time t ｔ２ A method for manufacturing a resistance spot welding joint as described in 1 above, wherein the relationship in equation (8) is satisfied. 0 ≤ I ｔ２ ≤ 0.5 × I ｔ１ ...(5) 100 < t ｔ２ ≤ 800 ... (8)

[0020] 5. Current value I in the main energization process ｍ (kA) and the current value I in the post-heat treatment step ｔ１ A method for manufacturing a resistance spot welding joint according to any one of the above 1 to 4, wherein the relationship in equation (9) is satisfied. ｍ ≤ I ｔ１ ... (9)

[0021] 6. The high-strength steel sheet has the following composition in mass%, C: 0.05-0.50%, 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: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less, Nb: 0.100% or less, V: 0.500% or less, Ti: 0.200% or less, B: 0.0050% or less, Al: 2.00% or less, Ca: 0.005% or less, Sn: 0.200% or less, and A method for manufacturing a resistance spot welding joint according to any one of 1 to 5 above, having a component composition containing one or more Sb selected from 0.200% or less, with the remainder being Fe and unavoidable impurities.

[0022] 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, the appropriate current range in the post-heat treatment process is extended, thereby increasing the margin for error in current density fluctuations due to disturbances, etc. As a result, in actual welding, for example, welding on an automobile assembly line, it becomes possible to avoid a decrease in yield and further improve productivity, which is extremely advantageous from an industrial perspective.

[0023] This is a schematic diagram illustrating the procedure for performing resistance spot welding. This is a schematic diagram showing an example of the current flow pattern (current flow pattern in the post-heat treatment process: heat treatment pattern 1) for the manufacturing method of a resistance spot welded joint according to one embodiment of the present invention. This is a schematic diagram showing an example of the current flow pattern (current flow pattern in the post-heat treatment process: heat treatment pattern 2) for the manufacturing method of a resistance spot welded joint according to one embodiment of the present invention. This is a schematic diagram showing an example of the current flow pattern (current flow pattern in the post-heat treatment process: heat treatment pattern 3) for the manufacturing method of a resistance spot welded joint according to one embodiment of the present invention.

[0024] A method for manufacturing a resistance spot welded joint according to one embodiment of the present invention will be described based on the following embodiment.

[0025] 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 that joins 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 step, the materials to be joined are sandwiched between a pair of electrodes (a vertically downward electrode 4 and a vertically upward electrode 5), and by applying current while pressurizing, a nugget 3 is formed on the joint surface (overlapping surface) of steel plate 1 and steel plate 2, and steel plate 1 and steel plate 2 are joined. Next, the nugget is cooled in the cooling step. Then, in the post-heat treatment step, the nugget is heat-treated to obtain a resistance spot welded joint. The following describes the materials to be joined, the main energizing process, the cooling process, and the post-heat treatment process.

[0026] [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 high-strength steel plate with a tensile strength of 780 MPa or more is used as the materials to be joined, the appropriate current range in the post-heat treatment process tends to become narrower, so there is a particularly strong demand for expanding the appropriate current range in the post-heat treatment process. 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. There is no particular upper limit to the tensile strength. As an example, the tensile strength is preferably 1600 MPa or less. Note that even when only steel plates with a tensile strength of less than 780 MPa are used as the materials to be joined, the effect of expanding the appropriate current range in the post-heat treatment process can be obtained.

[0027] 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: 0.05 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: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less, Nb: 0.100% or less, V: 0.500% or less, Ti: 0.200% or less, B: 0.0050% or less, Al: 2.00% or less, Ca: 0.005% or less, Sn: 0.200% or less, and Examples of component compositions include 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 percent" unless otherwise specified.

[0028] C: 0.05-0.50% Carbon (C) is an element that contributes to improving the strength of steel. If the C content is less than 0.05%, it becomes difficult to obtain the desired tensile strength. Therefore, the C content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the C content exceeds 0.50%, hard martensite is excessively generated, and the number of microvoids increases. In addition, the nugget formed by resistance spot welding and the surrounding HAZ (hereinafter collectively referred to as the weld) 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 0.45% or less.

[0029] Si: 0.1-2.0% Si is an element that contributes to improving the strength of steel. Therefore, the Si content is preferably 0.1% or more, more preferably 0.9% 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.

[0030] 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 1.5% 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 appropriate current range in the post-heat treatment process is usually narrow, so it is particularly advantageous to apply the manufacturing method of resistance spot welded joint according to one embodiment of the present invention.

[0031] P: 0.10% or less. P is an unavoidable element. 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.

[0032] S: 0.005% or less. S is an unavoidable element. If the S content exceeds 0.005%, S will strongly segregate in the center of the nugget, reducing toughness. Therefore, it becomes difficult to obtain the desired CTS. Accordingly, the S content is preferably 0.005% or less, 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Cu: 1.00% 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 0.010% 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, Ni content, and Mo content are preferably 1.00% or less, more preferably 0.80% or less.

[0037] 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.

[0038] Nb: 0.100% or less, V: 0.500% or less. Nb and V are elements that improve the strength of steel by controlling the structure through precipitation hardening. Therefore, the Nb content is preferably 0.003% or more, more preferably 0.005% or more, and the V content is preferably 0.005% or more, more preferably 0.006% or more. On the other hand, if Nb and V are included in excess, the toughness of the HAZ may deteriorate. Therefore, when these elements are included, the Nb content is preferably 0.100% or less, more preferably 0.090% or less, even more preferably 0.080% or less, and the V content is preferably 0.500% or less, more preferably 0.300% or less.

[0039] Ti: 0.200% 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 0.200% or less, and more preferably 0.150% or less.

[0040] B: 0.0050% or less. B is an element that improves the strength of steel by improving hardenability. Therefore, the B content is preferably 0.0001% or more, and 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 0.0050% or less, and more preferably 0.0010% or less.

[0041] Al: 2.00% 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 2.00% or less, more preferably 1.50% or less, and even more preferably 1.20% or less.

[0042] 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.

[0043] Sn: 0.200% or less. Sn is an element that suppresses nitriding and oxidation of the steel sheet surface. Sn is also an element that stabilizes the material. On the other hand, excessive Sn content leads to a decrease in toughness. Therefore, when Sn is included, the Sn content is preferably 0.200% or less. There is no particular lower limit to the Sn content. From the viewpoint of obtaining the above effects sufficiently, the Sn content is preferably 0.002% or more.

[0044] 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.

[0045] 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%.

[0046] 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.

[0047] 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. Note that the alloyed zinc plating layer is formed by performing an alloying treatment after zinc plating.

[0048] 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.

[0049] 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.

[0050] [Main energizing process] In the main energizing process, the current value I ｍ The process involves applying current to form a nugget and joining the steel plates together. In automotive components, the nugget diameter (hereinafter also referred to as the 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 materials to be joined. The current application conditions for the main current application process are not particularly limited and should be set according to the target nugget diameter (hereinafter also referred to as the target nugget diameter) in accordance with conventional methods.

[0051] The following are the current values ​​(I ｍ , I ｔ１ , I ｔ２ The unit of each time (t) is "kA" unless otherwise specified. ｍ ,t ｃ ,t ｔ１ ,t ｔ２ The unit of each pressure (F) is "ms" unless otherwise specified. ｍ F ｃ F ｔ Unless otherwise specified, the unit of measurement is "kN".

[0052] As an example, the current value I in the main energization process ｍ (Hereafter, simply I ｍ (Also known as) is preferably 3.0 to 8.0 kA. ｍ If the amount is too small, it becomes difficult to stably obtain the target nugget diameter. On the other hand, I ｍ If this becomes excessive, the nugget diameter may become excessive. Also, the steel plate may melt excessively, increasing the likelihood of splatter (scattering of molten metal). Therefore, I ｍThe current is preferably 3.0 to 8.0 kA. ｍ It is more preferably 4.5 kA or higher. ｍ The current is more preferably 7.5 kA or less. The current value during the main energizing process may be constant or not during the main energizing process. If the current value during the main energizing process is not constant, the range of fluctuation of the current value is, for example, 0.5 kA or less. If the current value during the main energizing process is not constant, the average value (= time integral of the current value from the start to the end of the main energizing process ÷ t) is used. ｍ ) to, I ｍ Let's assume that.

[0053] Main energizing process energizing time t ｍ (Hereafter, simply t ｍ The duration (also known as) is preferably 120 to 400 ms from the viewpoint of stably obtaining the target nugget diameter.

[0054] Pressure F during the main energization process ｍ (Hereafter, simply F ｍ (Also known as) is preferably 2.0 to 7.0 kN from the viewpoint of stably obtaining the target nugget diameter. ｍ It is more preferably 3.0 kN or more. ｍ The pressure is more preferably 6.5 kN or less. The applied pressure may be limited by the capabilities of the equipment used. In this case, the applied pressure may be outside the above range as long as the target nugget diameter is obtained and the relationship in equation (1) described later is satisfied. Furthermore, the applied pressure during the main energizing process may be constant or not during the main energizing process. If the applied pressure during the main energizing process is not constant, the fluctuation range of the applied pressure is, for example, 0.1 kN or less. If the applied pressure during the main energizing process is not constant, the average value (= the time integral of the applied pressure from the start to the end of energizing in the main energizing process ÷ t) is used. ｍ ) to F ｍ The same applies to the pressure applied during the cooling and post-heat treatment processes.

[0055] [Cooling Process] After the above-mentioned main energization process, cooling is performed in a non-energized state. At this time, by satisfying the relationships in equations (1) and (2), an effect of expanding the appropriate current range in the post-heat treatment process can be obtained. Note that the non-energized state means that the current value is 0 kA.

[0056] F ｍ +1.50 ≤ F ｃ ≤ F ｍ +3.50 ··· (1) As a result of various experiments and studies by the inventors, while satisfying the relationship in equation (2) described later, the pressing force F ｃ (hereinafter simply referred to as F ｃ as well) is controlled within the range of (F ｍ +1.50) kN to (F ｍ +3.50) kN, and it has been found that an effect of expanding the appropriate current range in the post-heat treatment process can be obtained. Therefore, F ｃ is (F ｍ ) +1.50) kN or more, preferably (F ｍ +1.55) kN or more, more preferably (F ｍ +1.60) kN or more. F ｃ is (F ｍ +3.50) kN or less, preferably (F ｍ +3.45) kN or less, more preferably (F ｍ +3.40) kN or less.

[0057] 300 < t ｃ ··· (2) When the cooling time t ｃ (hereinafter simply referred to as t ｃ as well) in the cooling process is 300 ms or less, the effect of expanding the appropriate current range in the heat treatment process cannot be sufficiently obtained. Therefore, t ｃ is more than 300 ms, preferably 320 ms or more, more preferably 350 ms or more. The upper limit of t ｃ is not particularly limited. If t ｃ becomes too long, the construction efficiency will decrease. Therefore, t ｃ is preferably 1500 ms or less, more preferably 1000 ms or less, and even more preferably 800 ms or less.

[0058] [Post-heat treatment process] In the post-heat treatment process, heat treatment is performed on the nage formed in the main energization process to improve CTS. The conditions in the post-heat treatment process are not particularly limited, and conventional methods may be followed. From the viewpoint of sufficiently obtaining the effect of expanding the appropriate current range in the post-heat treatment process, for example, the following heat treatment patterns 1 to 3 are preferable.

[0059] (Heat treatment pattern 1) In heat treatment pattern 1, as shown in FIG. 2, the current value I ｔ１ (hereinafter simply referred to as I ｔ１ as well) and the energization time t ｔ１ (hereinafter simply referred to as t ｔ１ as well) are energized. Here, it is preferable to satisfy the relationship of the following formula (3). The current value of heat treatment pattern 1 may be constant or not constant during the energization. When the current value of heat treatment pattern 1 is not constant, the fluctuation range of the current value is, for example, 0.5 kA or less. When the current value of heat treatment pattern 1 is not constant, its average value (= time integral value of the current value from the start to the end of energization in the post-heat treatment process ÷ t [[ID=第十二]] ｔ１ )を、I ｔ１ とする。後述する熱処理パターン2についても同様である。 10 ≦ t[[ID=十六]] ｔ１ [[ID=十七]] ≦ 200 ・・・(3)

[0060] t ｔ１ is preferably 10 ms or more, more preferably 80 ms or more, from the viewpoint of obtaining the effect of improving CTS. Also, t ｔ１ is preferably 200 ms or less, more preferably 180 ms or less, still more preferably 150 ms or less, from the viewpoint of productivity.

[0061] (Heat treatment pattern 2) In heat treatment pattern 2, as shown in FIG. 3, the current value I ｔ１ { (hereinafter simply referred to as I ｔ１ as well) and the energization time t ｔ１ (hereinafter simply referred to as t ｔ１ as well) are energized, and then the current value is changed to the current value I ｔ２ (hereinafter simply referred to as I ｔ２ as well) transition time t ｔ２ (hereinafter simply referred to as t ｔ２The current is passed through the circuit while transitioning (hereinafter also referred to as transition current). Transition current is, for example, a downslope current. In transition current, the time change of the current value may be constant or not. Here, it is preferable to satisfy the following relationship (4) to (7): 0 < t ｔ１ ≦ 200 ... (4) 0 ≦ I ｔ２ ≤ 0.5 × I ｔ１ ...(5) 20 ≦ t ｔ２ ≦ 800 ... (6) 100 <t ｔ１ +t ｔ２ ... (7)

[0062] t ｔ１ From the viewpoint of obtaining an improvement effect on CTS, it is preferably greater than 0 ms, more preferably 80 ms or more. ｔ１ From the viewpoint of productivity, the time is preferably 200 ms or less, more preferably 180 ms or less, and even more preferably 150 ms or less.

[0063] I ｔ２ From the viewpoint of obtaining an improvement effect on CTS, preferably 0.5 × I ｔ１ More preferably, 0.45 × I ｔ１ The following applies. Also, I ｔ２ Preferably, it is 0 or more, more preferably 0.01 × I ｔ１ That's all.

[0064] t ｔ２ From the viewpoint of obtaining an improvement effect on CTS, it is preferably 20 m or more, more preferably 60 ms or more, and even more preferably 100 ms or more. ｔ２ From the viewpoint of productivity, the time is preferably 800 ms or less, more preferably 780 ms or less, and even more preferably 750 ms or less.

[0065] t ｔ１ +t ｔ２ From the viewpoint of obtaining an improvement effect on CTS, it is preferably more than 100 ms.

[0066] (Heat treatment pattern 3) In heat treatment pattern 3, as shown in Figure 4, the current value is current value I ｔ１ (Hereafter, simply I ｔ１(Also known as) Current value I ｔ２ (Hereafter, simply I ｔ２ (Also known as) Transition time t ｔ２ (Hereafter, simply t ｔ２ The current is applied while transitioning (also known as transition current application). Transition current application is, for example, a downslope current application. In transition current application, the amount of change in the current value over time may be constant or not. In heat treatment pattern 3, I ｔ１ No retention is performed (that is, t ｔ１ = 0). Here, it is preferable to satisfy, for example, the following relationships (5) and (8): 0 ≤ I ｔ２ ≤ 0.5 × I ｔ１ ...(5) 100 < t ｔ２ ≤ 800 ... (8)

[0067] I ｔ２ From the viewpoint of obtaining an improvement effect on CTS, preferably 0.5 × I ｔ１ More preferably, 0.45 × I ｔ１ The following applies. Also, I ｔ２ Preferably, it is 0 or more, more preferably 0.01 × I ｔ１ That's all.

[0068] t ｔ２ From the viewpoint of obtaining an improvement effect on CTS, it is preferably more than 100 ms. Also, t ｔ２ From the viewpoint of productivity, the time is preferably 800 ms or less, more preferably 780 ms or less, and even more preferably 750 ms or less.

[0069] Other than the above conditions, there are no particular limitations, and the usual law should be followed. For example, I ｔ１ It is preferable that the following relationship (9) is satisfied. Also, I ｔ１ Preferably, 2.0 × I ｍ The following applies: I ｍ ≤ I ｔ１ ... (9)

[0070] Furthermore, the pressure F applied during the post-heat treatment process. ｔ (Hereafter, simply F ｔ (Also known as) Heat treatment patterns 1 to 3 are, for example, F ｃ It should be the same as F.ｔ is, F ｃ You can change it from there.

[0071] 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.

[0072] 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. Then, resistance spot welding was performed on the materials to be joined under the conditions shown in Table 3 and in the manner shown in Figure 1 to manufacture resistance spot welded joints. In this process, during the post-heat treatment step, I ｔ１ The value was varied in 0.1 kA increments. Note that the value of I listed in Table 1 is also shown. ｔ１ In the evaluation of CTS described later, I ｔ１ It is within the range of F. ｔ is, F ｃ This was treated the same as above. Also, in Table 2, "1st sheet" and "2nd sheet" in the "Position of Steel Plate" column refer to the positions of the "1st sheet" and "2nd sheet" from the bottom in each plate assembly, respectively.

[0073] 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 diameter was 5.5√t or less for all of them.

[0074] Then, the wide range of the appropriate current in the post-heat treatment process was evaluated according to the following procedure.

[0075] [Evaluation of the wide range of appropriate currents in the post-heat treatment process] Using each resistance spot welded joint 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. The wide range of appropriate currents in the post-heat treatment process was then evaluated according to the following criteria. Pass (Excellent): The appropriate current range (the range of resistance spot welded joints in the post-heat treatment process that can be obtained with a CTS of 3.4 kN (JIS Class A) or higher) ｔ１The difference between the upper and lower limits is 1.5 kA or more: Fail (defective): The appropriate current range is less than 1.5 kA.

[0076]

[0077]

[0078]

[0079] In all of the inventive examples, even when using high-strength steel plates with a tensile strength of 780 MPa or more as the steel plates to be joined, the wide range of the appropriate current in the post-heat treatment process was excellent. On the other hand, in all of the comparative examples, the wide range of the appropriate current in the post-heat treatment process was not sufficient.

[0080] 1, 2 Steel plate 3 Nugget 4, 5 Electrode

Claims

1. A method for manufacturing a resistance spot welded joint having two or more overlapping steel plates and a nugget for joining the steel plates, 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 comprises a main energizing step, a cooling step in an unenervated state, and a post-heat treatment step, wherein the applied pressure F in the main energizing step is applied. ｍ (kN) and the pressure F applied during the cooling process. ｃ (kN) satisfies the following relationship (1), and the cooling time t in the cooling process ｃ A method for manufacturing a resistance spot welded joint such that (ms) satisfies the following relationship (2). F ｍ +1.50 ≤ F ｃ ≤ F ｍ +3.50...(1) 300 < t ｃ ... (2) 2. In the post-heat treatment step, an electric current value I ｔ１ (kA) and a current application time t ｔ１ (ms) are applied, and the current application time t ｔ１ satisfies the relationship of the following formula (3). The method for manufacturing a resistance spot welding joint according to claim 1. 10 ≤ t ｔ１ ≤ 200... (3) 3. In the aforementioned post-heat treatment process, the current value I ｔ１ (kA) and energizing time t ｔ１ The current is applied at (kA), and then the current value is set to current value I ｔ２ Transition time t to (kA) ｔ２ The current is applied while transitioning in (ms), and the current application time t ｔ１ The relationship in equation (4) below is satisfied, and the current value I ｔ１ and the current value I ｔ２ The relationship in equation (5) below is satisfied, and the transition time t ｔ２ The relationship in equation (6) is satisfied, and the energizing time t ｔ１ and the transition time t ｔ２ A method for manufacturing a resistance spot welded joint according to claim 1, wherein the relationship in equation (7) is satisfied. 0 < t ｔ１ ≦ 200 ... (4) 0 ≦ I ｔ２ ≤ 0.5 × I ｔ１ ...(5) 20 ≦ t ｔ２ ≦ 800 ... (6) 100 <t ｔ１ +t ｔ２ ... (7) 4. In the aforementioned post-heat treatment step, the current value is set to current value I ｔ１ (kA) to current value I ｔ２ Transition time t to (kA) ｔ２ The current is applied while transitioning in (ms), and the current value I ｔ１ and the current value I ｔ２ The relationship in equation (5) below is satisfied, and the transition time t ｔ２ A method for manufacturing a resistance spot welded joint according to claim 1, wherein the relationship in equation (8) is satisfied. ｔ２ ≤ 0.5 × I ｔ１ ...(5) 100 < t ｔ２ ≤ 800 ... (8) 5. Current value I in the main energization process ｍ (kA) and the current value I in the post-heat treatment step ｔ１ A method for manufacturing a resistance spot welding joint according to any one of claims 1 to 4, wherein the relationship in equation (9) is satisfied. ｍ ≤ I ｔ１ ... (9) 6. The high-strength steel sheet has the following composition in mass%, C: 0.05-0.50%, 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: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less, Nb: 0.100% or less, V: 0.500% or less, Ti: 0.200% or less, B: 0.0050% or less, Al: 2.00% or less, Ca: 0.005% or less, Sn: 0.200% or less, and A method for manufacturing a resistance spot welding joint according to any one of claims 1 to 5, wherein the component composition contains one or more Sb selected from 0.200% or less, with the remainder being Fe and unavoidable impurities.

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