Method for manufacturing spot welded joint, and spot welded joint

Abstract

Provided is a method for manufacturing a spot welded joint, comprising: a regular welding step in which a regular welding current value Iw0 (kA) is supplied, so as to satisfy a predetermined condition, to a plate assembly comprising high-strength steel plates having a Ceq of at least 0.36% by mass, and a molten portion is formed thereby; a first subsequent power supply step in which a first subsequent power supply current value Iw1 is supplied for a time tw1 (ms) following a time tc1 during which power is not supplied; and a second subsequent power supply step in which, after the first subsequent power supply step, a current value Iw2 is supplied for a time tw2 following a time tc2 during which power is not supplied. The steps from the regular welding step to the final subsequent power supply step are carried out continuously in a state in which a pressurizing force FE is maintained. Further provided is a spot welded joint in which the number ratio of crystal grains having an aspect ratio of 7 or more is 50% or less in an intermediate portion in the longer-axis direction of a nugget.

Description

Spot welded joint manufacturing method and spot welded joint

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

[0002] In recent years, the automotive industry has been focusing on fuel efficiency and CO ２ To reduce emissions, there is a need to reduce the weight of vehicle bodies. At the same time, to improve collision safety, there is a need to increase the strength of vehicle body components. To meet these requirements, it is effective to use high-strength steel plates for the vehicle body and parts. Spot welding is mainly used for assembling the vehicle body and attaching parts.

[0003] Tensile strength is an important characteristic of a joint formed by spot welding a sheet assembly of overlapping steel sheets (also referred to as a "spot-welded joint" in this disclosure). The tensile strength of a spot-welded joint includes the tensile shear strength (TSS), which is measured by applying a tensile load in the shear direction, and the cross tensile strength (hereinafter sometimes referred to as "CTS" or "joint strength"), which is measured by applying a tensile load in the peel direction.

[0004] Generally, when a spot-welded joint is manufactured using high-strength steel plates, the deformability of the welded joint against the pressure of the welding electrode is low, resulting in increased stress concentration at the welded joint. Furthermore, the toughness of the welded joint is reduced due to quenching. Therefore, spot-welded joints containing high-strength steel plates tend to have low CTS, and an improvement in CTS is required.

[0005] In order to ensure the strength and toughness of spot-welded joints using sheet metal assemblies including high-strength steel sheets, a tempering current method has been proposed in which, after the main welding to form the fused part that becomes the nugget, current is passed again after a certain period of cooling, thereby tempering the nugget part and the heat-affected zone, and a post-current method has been proposed in which post-current is passed within a relatively short time after the main welding.

[0006] For example, Patent Document 1 discloses a resistance spot welding method for forming a weld by clamping two or more overlapping high-strength thin steel plates between a pair of electrodes and applying a pressure while passing a current through them, the resistance spot welding method including, in this order, a first step of passing a welding current (Im) to form a nugget, a second step of passing a current through the weld at a value equal to or less than the welding current (Im), a third step of cooling the weld, and a fourth step of passing a current through the weld at a value greater than the welding current (Im) to heat the weld to a recrystallization temperature range.

[0007] Furthermore, Patent Document 2 discloses a resistance spot welding method in which a plate assembly having a total plate thickness of t (mm), which is made up of two or more overlapping steel plates, is clamped between a pair of welding electrodes and welded by applying current while applying pressure, the method comprising a first step of forming a nugget, and a second step of cooling the welded portion by holding the welded portion without applying current while the electrodes are still applying pressure, and then applying current, the method comprising: a first step of forming a nugget; and a second step of applying current after cooling the welded portion by holding the welded portion without applying current while the electrodes are still applying pressure, the method comprising: a first step of forming a nugget, a second step of cooling the welded portion by holding the welded portion without applying current while the electrodes are still applying pressure, the first step of forming a nugget, and a second step of applying current, the method comprising: a first step of forming a nugget, a first step of forming a nugget, and a second step of applying current, the method comprising: a first step of forming a nugget, a second step of cooling the welded portion by holding the welded portion without applying current while the electrodes are still applying pressure, the method comprising: a first step of forming a nugget, a second step of cooling the welded portion by holding the welded portion without applying current, the first step of forming a nugget, and the second step of applying current, the method comprising: a first step of forming a nugget, a second step of cooling the welded portion by holding the welded portion without applying current, the second step of applying current, and the second step of applying current, the method comprising: a first step of forming a nugget, a second step of applying current, the method comprising: a first step of forming a nugget, a nugget ２ ), a resistance spot welding method is disclosed that satisfies the following equation (3): 0.05<(IB2×TB) / (IA2×TA)<1.0 (1) 20≦TB≦100 (2) 10×(t×d2) / S<Th<200×(t×d2) / S (3)

[0008] Patent Document 3 describes a spot welding method for improving the cross tensile strength of a spot welded joint, in which a plurality of steel plates including a high-strength steel plate having a tensile strength of 750 to 2500 MPa and a predetermined carbon equivalent Ceq of 0.20 to 0.55 mass % are overlapped and a fusion zone is formed by main welding, and then current application is stopped and the steel plate is cooled to form a solidified region in the fusion zone, and then a first cooling and post-current application is performed to prevent the solidified region from re-melting, and after the first cooling and post-current application process is completed, a predetermined pressing force F is applied. Ｅ(N) is maintained, and the cooling time t Ｓ (ms) The current supply is stopped, and then the post-current supply I Ｐ (kA) is the post-energization time t Ｐ A spot welding method is disclosed that includes a step of repeating the step of cooling and post-current application one or more times, in which a current is applied (ms) and a post-current is applied to prevent the solidified region from remelting.

[0009] Patent Documents 4 to 6 also disclose spot welding methods in which post-energization is performed after main energization.

[0010] Patent Document 1: Japanese Patent No. 5895430 Patent Document 2: Japanese Patent No. 5891741 Patent Document 3: Japanese Patent No. 6409470 Patent Document 4: International Publication No. 2016 / 139952 Patent Document 5: Japanese Patent Application Laid-Open No. 2018-30178 Patent Document 6: Japanese Patent Application Laid-Open No. 2013-78782

[0011] An object of the present disclosure is to provide a method for manufacturing a spot-welded joint, and a spot-welded joint, that can effectively improve the cross tensile strength of the joint compared to when a spot-welded joint is manufactured using high-strength steel plate and by performing a single post-welding current after main welding.

[0012] The gist of the present disclosure for achieving the above object is as follows: <1> A method for manufacturing a spot-welded joint, in which a sheet set made up of a plurality of overlapping steel sheets is sandwiched between a pair of welding electrodes in the sheet thickness direction and spot-welded by passing current through the sheet set while being pressed, wherein at least one of the plurality of steel sheets is a high-strength steel sheet having a carbon equivalent Ceq of 0.36 mass % or more, as expressed by the following formula (A), where the contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S], respectively, in mass %, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) When the arithmetic mean value of the sheet thicknesses of the plurality of steel sheets is h (mm), the sheet set is welded with the pair of welding electrodes at a pressing force F that satisfies the following formula (B): Ｅ When pressurized with (N), 2000 × h ≦ F Ｅ ≦4500×h (B) The pair of welding electrodes are supplied with a main welding current value I ｗ０(kA) to perform main welding to form a fusion zone in the sheet assembly; and two post-current processes in which current is applied after the main welding process, wherein the first post-current process is a time t ｃ１ (ms) Following the non-energization period in which the energization is stopped, a first post-energization current value I that satisfies the following formula (D) is ｗ１ (kA) is the time t that satisfies the following formula (E): ｗ１ (ms) energized, 2≦t ｃ１ ≦300 (C) 0.75×I ｗ０ <I ｗ１ <I ｗ０ (D) t ｗ１ >100 (E) The second post-current application process, which is the second post-current application process, is performed after the first post-current application process, at a time t that satisfies the following formula (F1): ｃ２ Following the non-energization period in which the energization is stopped (ms), a second post-energization current value I that satisfies the following formula (G1) and the following formula (H1) is obtained. ｗ２ (kA) at time t ｗ２ (ms) energized, 2≦t ｃ２ ≦300 (F1) 0.004×t ｃ２ ２ -0.3125 x t ｃ２ +102≦I ｗ２ / I ｗ０ ×t ｗ２ ≦0.0156×t ｃ２ ２ -0.625 x t ｃ２ +300 (G1) 0.75×I ｗ０ <I ｗ２ (H1) The processes from the main welding process to the final post-current application process are performed by the pressure F Ｅ The spot-welded joint is continuously welded while maintaining the main welding current I within a range that satisfies the formula (B). ｗ０ and the second post-energization current value I ｗ２<3> The method for manufacturing a spot welded joint according to <1>, wherein the method includes 3 to N post-current processes (N is an integer of 3 or more), and the n-th post-current process (n is an integer of 3 to N) that is the third or subsequent post-current process is a time t that satisfies the following formula (F) after the (n-1)-th post-current process that is the (n-1)th post-current process. ｃｎ (ms) Following the non-energization in which the energization is stopped, the n-th subsequent energization current value I satisfies the following formula (G) and the following formula (H): ｗｎ (kA) at time t ｗｎ The method for manufacturing a spot welded joint according to <1> or <2>, wherein current is applied for 2 ≤ t (ms). ｃｎ ≦300 (F) 0.004×t ｃｎ ２ -0.3125 x t ｃｎ +102≦I ｗｎ / I ｗ０ ×t ｗｎ ≦0.0156×t ｃｎ ２ -0.625 x t ｃｎ +300 (G) 0.75×I ｗ０ <I ｗｎ (H) <4> A spot welded joint including a spot welded portion where multiple overlapping steel plates are joined, wherein at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq of 0.36 mass% or more, expressed by the following formula (A) where the contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S], respectively, in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2 [P] + 4 [S] (A) A spot welded joint, wherein, when the inside of the fusion boundary of the nugget is observed in a cross section in the plate thickness direction passing through the center of the nugget of the spot welded portion, portions in the long axis direction of the nugget where the crystal orientation difference is 15 degrees or more are considered to be grain boundaries, and the proportion of crystal grains having an aspect ratio of 7 or more is 50% or less.

[0013] According to the present disclosure, there are provided a method for manufacturing a spot-welded joint, and a spot-welded joint, which can effectively improve the cross tensile strength of the joint, compared to when a spot-welded joint is manufactured by performing a single post-current application after main welding.

[0014] 7 is a schematic diagram showing an example of the arrangement of two steel plates and a welding electrode when resistance spot welding is started on a sheet pair formed by overlapping two steel plates. FIG. 8 is a schematic diagram showing an example of a nugget and a heat-affected zone (HAZ) formed when resistance spot welding is performed on a sheet pair formed by overlapping two steel plates. FIG. 9 is a diagram showing an example of a current pattern for a method for manufacturing a spot-welded joint according to the present disclosure. FIG. 10 is a diagram showing a simulation of the temperature history of a weld by a method for manufacturing a spot-welded joint according to the present disclosure. FIG. 11 is a diagram showing another example of a current pattern for a method for manufacturing a spot-welded joint according to the present disclosure. FIG. 12 is a diagram showing a region for measuring the aspect ratio of crystal grains in a welded portion of a spot-welded joint. FIG. 13 is a diagram showing the relationship with formula (G1) for each example in which the second post-current condition is changed. FIG. 14 is a diagram showing a fine-grained region of crystal grains formed near the center of the nugget for joints spot-welded by the examples shown in FIG.

[0015] An embodiment of the present disclosure will be described below. In this disclosure, the "%" used to denote the content of each element means "mass %." In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, unless otherwise specified. In addition, when "over" or "less than" is added to a numerical value before and after "to," the numerical range does not include the numerical value as the lower or upper limit. In the numerical ranges described in stages in this disclosure, the upper limit of a certain numerical range may be replaced with the upper limit of another numerical range described in stages, or may be replaced with a value shown in an example. In the numerical ranges described in stages in this disclosure, the lower limit of a certain numerical range may be replaced with the lower limit of another numerical range described in stages, or may be replaced with a value shown in an example. The term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0016] The inventors of the present disclosure have conducted extensive research into methods for effectively improving joint strength when spot welding a sheet assembly including high-strength steel sheets having a tensile strength of, for example, 900 MPa or more to produce a spot-welded joint. As a result, it has been found that a predetermined pressing force F corresponding to the sheet thickness of the high-strength steel sheets can be effectively improved. Ｅ The present inventors have found that joint strength can be effectively improved by performing post-current treatment multiple times (at least two times) after main welding under pressure at (N), with the first post-current treatment using a smaller current value than that used in main welding, and the second post-current treatment using a current value and current duration that satisfy a predetermined relationship with the immediately preceding cooling time and the current value used in main welding. Furthermore, when the steel structure of a spot weld produced by a method for producing a spot-welded joint according to the present disclosure was observed, it was found that the grain refinement of equiaxed grains within the nugget was promoted, and that the morphology of the grains was different from that when spot welding was performed only with main welding or when tempering current or the like was performed after main welding. Hereinafter, embodiments of the present disclosure will be described.

[0017] [High-Strength Steel Plates] First, high-strength steel plates included in a plate assembly to be spot-welded in a manufacturing method for a spot-welded joint according to the present disclosure will be described.

[0018] (Tensile strength) If the tensile strengths of the multiple steel plates in the plate assembly to be spot welded are all, for example, less than 900 MPa, high joint strength is easily obtained and problems with joint strength are unlikely to occur. Therefore, in the method for manufacturing a spot-welded joint according to the present disclosure, a plate assembly is used in which multiple steel plates are overlapped, including at least one high-strength steel plate (sometimes simply referred to as a "high-strength steel plate" in the present disclosure) having a carbon equivalent Ceq of 0.36 mass % or more, as described below.

[0019] The multiple steel plates constituting the plate assembly may all be high-strength steel plates having a carbon equivalent Ceq of 0.36% by mass or more, or at least one steel plate may be a high-strength steel plate having a carbon equivalent Ceq of 0.36% by mass or more and at least one steel plate having a carbon equivalent Ceq of less than 0.36% by mass. For example, in the case of a plate assembly in which three or more steel plates are stacked, at least one of the steel plates may be a high-strength steel plate, and one or more steel plates having a carbon equivalent Ceq of less than 0.36% by mass may be included. Furthermore, in the present disclosure, the multiple steel plates constituting the plate assembly are not limited to flat steel plates, and may be steel plates processed by hot stamping (hot pressing) or the like. For example, the plate assembly may be a plate assembly in which multiple steel plates processed into a three-dimensional part shape by bending or the like are stacked, or a plate assembly in which a flat steel plate and a steel plate processed into a part shape are stacked. For example, when spot welding is performed using a steel member having a three-dimensional shape obtained by processing a flat steel plate such as bending or welding, the flat portion (plate-shaped portion) on which spot welding is performed corresponds to the "steel plate" in this disclosure.

[0020] The tensile strength of the high-strength steel plate is preferably 900 MPa or more, and there is no particular upper limit. However, if the tensile strength is too high, defects and cracks are likely to occur inside the nugget. Therefore, the tensile strength of the high-strength steel plate is preferably 2500 MPa or less. The tensile strength (TS) of the steel plate can be measured by taking a JIS No. 5 tensile test piece (gauge length: 50 mm, width: 25 mm) from the steel plate and performing the measurement at a tension speed of 10 mm / min in accordance with ISZ 2241:2011. When taking a test piece from a joint, the test piece should be taken from a flat area, avoiding areas that have been deformed by pressing or the like. If a JIS No. 5 tensile test piece cannot be taken, the TS may be measured using a JIS 13B test piece (gauge length: 50 mm, width: 12.5 mm).

[0021] (Carbon Equivalent Ceq) The high-strength steel plate according to the present disclosure has a carbon equivalent Ceq of 0.36% by mass or more, as expressed by the following formula (A): Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) In formula (A), [C], [Si], [Mn], [P], and [S] represent the contents (mass%) of C, Si, Mn, P, and S contained in the high-strength steel plate, respectively. If the carbon equivalent Ceq is 0.36% by mass or more, the tensile strength of the high-strength steel plate can be, for example, 900 MPa or more. The upper limit of the carbon equivalent Ceq is not particularly limited. However, if it exceeds 0.55% by mass, for example, a high-strength steel plate having a tensile strength exceeding 2500 MPa can be obtained, but it is difficult to improve the CTS of spot-welded joints. Therefore, the Ceq of the high-strength steel plate is preferably 0.55% by mass or less. When a sheet assembly includes a steel sheet having a tensile strength of less than 900 MPa, the Ceq of the steel sheet is not particularly limited and may be less than 0.36 mass%.

[0022] The chemical composition of the steel sheet may be analyzed by any chemical analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. These analyses may be performed on samples taken from the steel sheet in accordance with JIS G0417:1999.

[0023] (Chemical Composition) The high-strength steel plate according to the present disclosure may have a chemical composition that provides a carbon equivalent Ceq of 0.36 mass % or more. A preferred chemical composition of the high-strength steel plate according to the present disclosure will be described below.

[0024] C: 0.07 to 0.50% C is an element that increases the tensile strength of steel. The higher the C content in steel, the higher the strength of the nugget. If the C content in steel is 0.07% or more, a tensile strength of 900 MPa or more is easily obtained. On the other hand, if the C content in steel is 0.50% or less, a decrease in the workability of the high-strength steel plate can be suppressed. Therefore, the C content of the high-strength steel plate is preferably 0.07 to 0.50%.

[0025] Si: 0.001 to 2.50% Si is an element that increases the strength of steel through solid solution strengthening and microstructural strengthening. If the Si content in steel is 2.50% or more, it is possible to suppress a decrease in the workability of the steel. On the other hand, if the Si content in steel is 0.001% or more, it is easy to manufacture industrially and technically. Therefore, the Si content of high-strength steel plate is preferably 0.001% to 2.50%.

[0026] Mn: 0.8 to 5.0% Mn is an element that increases the strength of steel. When the Mn content in steel is 5.0% or less, deterioration of the workability of the steel can be suppressed. On the other hand, when the Mn content in steel is 0.8% or more, a tensile strength of 900 MPa or more is easily obtained. Therefore, the Mn content of high-strength steel plate is preferably 0.8 to 5.0%.

[0027] P: 0.03% or less P is an element that embrittles the nugget. When the P content in steel is 0.03% or more, cracks are less likely to occur in the nugget, and sufficiently high joint strength is more likely to be obtained. Therefore, the P content of the high-strength steel plate is preferably 0.03% or less. Note that there is no lower limit for the P content, but from the viewpoint of suppressing an increase in manufacturing costs, the P content of the high-strength steel plate may be 0.001% or more.

[0028] S: 0.01% or less S is an element that embrittles the nugget. Furthermore, S is an element that combines with Mn to form coarse MnS, impairing the workability of steel. If the S content in steel is 0.01% or less, cracks are less likely to occur in the nugget, making it easier to obtain sufficiently high joint strength. Furthermore, the workability of the steel is improved. Therefore, the S content of the high-strength steel plate is preferably 0.01% or less. Note that, from the viewpoint of suppressing an increase in manufacturing costs, the S content of the high-strength steel plate may be 0.0001% or more.

[0029] N: 0.01% or less N is an element that forms coarse nitrides and deteriorates the workability of steel. N is also an element that causes blowholes during welding. In order to suppress deterioration of the workability of steel and the occurrence of blowholes, the N content of the high-strength steel plate is preferably 0.01% or less. From the viewpoint of suppressing an increase in manufacturing costs, the N content of the high-strength steel plate may be 0.0005% or more.

[0030] O: 0.01% or less O is an element that forms oxides and deteriorates the workability of steel. From the viewpoint of suppressing deterioration of the workability of steel, the O content of the high-strength steel plate is preferably 0.01% or less. From the viewpoint of suppressing an increase in manufacturing costs, the O content of the high-strength steel plate may be 0.0005% or more.

[0031] Al: 1.50% or less Al is a ferrite stabilizing element and has the effect of suppressing cementite precipitation during bainite transformation. Therefore, it is contained to control the steel structure. Al also functions as a deoxidizer. On the other hand, Al is easily oxidized, and an increase in inclusions tends to deteriorate the workability of the steel. Therefore, the Al content of high-strength steel sheets is preferably 1.50% or less.

[0032] In addition to the above-mentioned main elements, the high-strength steel sheet may selectively contain the following elements as necessary.

[0033] Ti, Nb, V: 0.005 to 0.20% Ti, Nb, and V are elements that contribute to increasing the strength of steel through at least one of precipitation strengthening, grain refinement due to the suppression of ferrite grain growth, and dislocation strengthening due to the suppression of recrystallization. However, if the content of any of these elements in steel is less than 0.005%, the effect of including them is unlikely to be realized. On the other hand, if the content in steel is 0.20% or less, the workability of the steel can be prevented from being impaired. Therefore, the content of each of these elements in high-strength steel sheets is preferably 0.005 to 0.20%.

[0034] B: 0.0001 to 0.01% B is an element that controls the steel structure and strengthens the steel. However, if the B content in the steel is less than 0.0001%, the effect of adding it is difficult to be realized. On the other hand, if the B content in the steel exceeds 0.01%, the effect of adding it becomes saturated. Therefore, the B content of high-strength steel sheets is preferably 0.0001 to 0.01%.

[0035] Cr: 0.01 to 2.0% Ni: 0.01 to 2.0% Cu: 0.01 to 2.0% Mo: 0.01 to 0.8% Cr, Ni, Cu, and Mo are elements that contribute to improving the strength of steel. These elements can be used, for example, to replace part of Mn (a strength-improving element). However, any of these elements is likely to contribute to improving strength if their content in steel is 0.01% or more.

[0036] Therefore, the contents of these elements in the high-strength steel plate are preferably 0.01% or more. On the other hand, if the contents of Cr, Ni, and Cu in the steel are 2.0% or less, problems occurring during pickling or hot working can be suppressed when the Mo content in the steel exceeds 0.8%. Therefore, the contents of Cr, Ni, and Cu in the high-strength steel plate are preferably 2.0% or less. Furthermore, the Mo content in the high-strength steel plate is preferably 0.8% or less.

[0037] At least one of Ca, Mg, and REM: 0.0001 to 1.0% in total Ca, Mg, and REM (rare earth metal) are elements that reduce the size of oxides after deoxidation and the size of sulfides present in hot-rolled steel sheets, thereby contributing to improving the workability of steel. If the total content of these elements in steel is 0.0001% or more, the effects of inclusion are likely to be realized. On the other hand, if the total content of these elements in steel is 1.0% or less, a decrease in the workability of the steel is suppressed. Therefore, the total content of these elements in high-strength steel sheets is preferably 0.0001 to 1.0%.

[0038] REM is a collective term for 17 elements, including Sc, Y, and 15 elements belonging to the lanthanoid series, and REM can be added to molten steel as misch metal during steelmaking. Furthermore, REM may contain a composite of elements from the lanthanoid series.

[0039] The remainder of the high-strength steel sheet other than the above elements is Fe and impurities. The impurities include, for example, components contained in raw materials such as ore and scrap, or components mixed in during the manufacturing process, and refer to components that are not intentionally contained in the steel sheet. The aforementioned Cr, Ni, Cu, Mo, B, Ti, Ni, and V are each permitted to be contained as impurities in trace amounts less than the lower limit values. Ca, Ce, Mg, La, and REM are also permitted to be contained as impurities in trace amounts less than the lower limit values ​​of their total amounts.

[0040] The chemical composition of the high-strength steel plate in the present disclosure has been described above. However, when a steel plate having a carbon equivalent Ceq of less than 0.36 mass% is included as a steel plate constituting a plate assembly, the chemical composition and tensile strength of the steel plate are not particularly limited.

[0041] (Steel Structure) The steel structure (steel type) of the high-strength steel plate is not particularly limited as long as it has a carbon equivalent Ceq of 0.36 mass % or more. For example, any type of structure (steel type) may be used, such as a two-phase structure (e.g., a structure containing martensite in ferrite, or a structure containing bainite in ferrite), a deformation-induced transformation structure (a structure containing retained austenite in ferrite), a hardened structure (martensite structure), or a fine crystal structure (a structure mainly composed of ferrite).

[0042] Furthermore, when a steel plate having a carbon equivalent Ceq of less than 0.36 mass % is included as a steel plate constituting a sheet assembly, the steel structure (steel type) of the steel plate is not particularly limited.

[0043] (Thickness) The thickness of the high-strength steel plate in the present disclosure is not particularly limited. For example, the thickness may be approximately the same as that of high-strength steel plates commonly used for automobile bodies, etc. (0.5 mm to 3.2 mm). However, since stress concentration around the nugget increases with increasing thickness of the high-strength steel plate, the thickness of the high-strength steel plate is preferably 2.6 mm or less.

[0044] When the steel plates constituting the plate assembly include steel plates having a carbon equivalent Ceq of less than 0.36 mass%, the thickness of the steel plates is not particularly limited. Furthermore, the thicknesses of the steel plates constituting the plate assembly may be the same or different from each other. For example, when three or more steel plates are stacked, the thicknesses of the steel plates may be different, or at least two of the steel plates may have the same thickness.

[0045] (Plating) The steel sheets constituting the sheet assembly may have a plating layer formed on their surfaces. Examples of the type of plating layer include Zn-based, Zn—Fe-based, Zn—Ni-based, Zn—Al-based, Zn—Mg-based, Pb—Sn-based, Sn—Zn-based, and Al—Si-based. When a plating layer is formed on the surface of the steel sheet, the plating layer may be a single layer or multiple plating layers.

[0046] Examples of steel sheets with a Zn-based plating layer include alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets, and electrogalvanized steel sheets. If a plating layer is formed on the surface of a steel sheet in a sheet assembly, the spot-welded joint after spot welding will exhibit excellent corrosion resistance. If the plating layer is an alloyed galvanized layer on the surface of the steel sheet, excellent corrosion resistance is obtained and paint adhesion is also good.

[0047] The coating weight of the plating layer is not particularly limited. The plating layer may be formed on only one side of the steel sheet or on both sides. An inorganic or organic coating (e.g., a lubricating coating) may be formed on the surface of the plating layer.

[0048] [Method for manufacturing a spot welded joint] Next, a method for manufacturing a spot welded joint according to the present disclosure will be described. The method for manufacturing a spot welded joint according to the present disclosure involves applying a pressure F to a sheet assembly made up of multiple overlapping steel sheets, at least one of which is the high-strength steel sheet described above, using a pair of welding electrodes so as to satisfy the following formulas (B) to (H): Ｅ In this case, a main welding process for forming a fusion zone in the plate assembly is carried out under pressure at (N), and at least two post-welding processes are carried out after the main welding process.

[0049] Here, an example will be described in which two overlapping steel plates, including at least one high-strength steel plate, are spot-welded. Note that spot welding can be performed using a method similar to the method described below even when three or more steel plates, including at least one high-strength steel plate, are spot-welded. FIG. 1 is a diagram schematically illustrating an example of the arrangement of two steel plates, including at least one high-strength steel plate, and welding electrodes when spot welding is initiated. FIG. 2 is a diagram schematically illustrating an example of a nugget and heat-affected zone (HAZ) formed by spot welding. FIG. 3 is a diagram illustrating an example of a current flow pattern when current is applied to welding electrodes in a manufacturing method for a spot-welded joint according to the present disclosure. Note that the current described below refers to the current flowing between welding electrode 2A and welding electrode 2B. Furthermore, FIG. 4 is a diagram illustrating a simulation of the temperature history of a welded joint produced by a manufacturing method for a spot-welded joint according to the present disclosure. Note that QuickSpot (Computational Mechanics Research Center, Inc.) can be used as software for performing simulations using heat conduction analysis.

[0050] [Main welding process] First, as shown in Fig. 1, a sheet set is prepared by overlapping steel sheets 1A and 1B with their sheet surfaces facing each other, and the sheets are sandwiched between a pair of welding electrodes 2A and 2B in the sheet thickness direction. Then, a pressure F that satisfies the following formula (B) is applied. Ｅ (N) and the actual welding current value I ｗ０ (kA) is applied to the welding electrodes 2A and 2B to perform main welding (main current application) to form a fusion zone in the sheet assembly. At least one of the steel sheets 1A and 1B is the high-strength steel sheet described above. 2000 × h≦F Ｅ ≦4500×h (B) Here, h (mm) means the arithmetic mean value of the thicknesses of the overlapping steel plates. When the thicknesses of the two steel plates are different, the arithmetic mean value of the thicknesses of the two steel plates (the arithmetic mean value of the thicknesses of Steel Plate 1A and Steel Plate 1B) is used as "h" in formula (B). When spot welding three or more steel plates, for example, the sum of the thicknesses of the multiple steel plates is calculated, and the value obtained by dividing this sum by the number of plates in the plate set is used as "h" in formula (B).

[0051] When measuring the thickness of steel plates before joining by spot welding, the arithmetic mean value h of the plate thickness can be determined by measuring the thickness of each steel plate and taking the arithmetic mean. When measuring overlapping steel plates to be joined, it is necessary to measure without any gaps, so if there is warping or lifting, the plate should be clamped or otherwise measured, and the result can be divided by the number of overlapping plates to obtain h. Furthermore, when measuring from a welded joint, it is desirable to measure the plate thickness at a location that is as free from deformation due to welding as possible. If it is not possible to measure using a vernier caliper or the like, it can also be measured by observing the cross section.

[0052] The pressure F applied to the steel plates 1A and 1B by the welding electrodes 2A and 2B Ｅ The applied pressure F has a large effect on the occurrence of defects and cracks inside the nugget 13 and in the heat-affected zone 14. Ｅ However, if it is less than “2000×h” (N), it becomes difficult to suppress the occurrence of defects and cracks inside the nugget 13 and in the heat-affected zone 14 .

[0053] On the other hand, the pressure F Ｅ If the applied pressure F exceeds "4500 × h" (N), the area of ​​the steel plates 1A, 1B where the welding electrodes 2A, 2B come into contact will be significantly depressed. This will not only impair the appearance but also reduce the joint strength. Ｅ In order to obtain this, the welding gun (a device that applies a pressure to welding electrodes 2A, 2B and energizes them) needs to have a highly rigid robot arm. Therefore, in the present disclosure, the pressure F of welding electrodes 2A, 2B on steel plates 1A, 1B is Ｅ is set to be equal to or greater than "2000×h" (N) and equal to or less than "4500×h" (N).

[0054] Actual welding current I ｗ０ and the actual welding current time t ｗ０ (The actual welding current I ｗ０ The welding current I is set to a value equivalent to the welding current and current flow time conventionally used to stably obtain a nugget of a required size. ｗ０ and the actual welding time t ｗ０ It can be adopted as.

[0055] The applied pressure can be measured using the displayed applied pressure value of the spot welding machine if it is built into the machine, or by attaching a pressure measuring instrument such as a quartz piezoelectric sensor to the transmission shaft, which is equivalent to the applied pressure load on the steel sheet. The current value can be measured using a current monitor called a weld checker or welding ammeter. There is no specific measurement method, but one method is to insert a toroidal coil into the circuit through which current flows during welding and read the change in the current value.

[0056] The actual welding current value I ｗ０ It is preferable to select a current value that can obtain a desired nugget diameter, taking into consideration the total plate thickness t of the plate assembly, etc. ｗ０ For example, when the thickness of the thinnest steel plate in the plate assembly is t' (mm), the current application time t ｗ０ can be 10t'-5 to 10t'+50 cycles (in this disclosure, the unit of time is the number of cycles at 50 Hz), etc.

[0057] With respect to the thickness of the plate assembly, it is preferable to aim for a nugget diameter of 4√t' or more at the interface of each plate, where t' is the thickness of the thinner plate, from the viewpoint of joint strength and avoidance of expulsion. A nugget diameter of 5√t' or more is even more desirable. In order to form such a nugget diameter of 5√t' or more without causing expulsion, a gradual increase (upslope) of 1 cycle to 80 cycles (50 Hz) may be set before the actual welding process. When upslope current is applied, the current value at the end of the upslope is set to the current value I in the actual welding process. ｗ０ (kA), and the welding time t ｗ０ In the current pattern shown in FIG. 3, the current value is the actual welding current I ｗ０ The current value is increased from 0 (zero) until the actual welding current I ｗ０ (kA) for main welding. Also, before the main welding process, pre-energization may be performed, for example, for 2 to 80 cycles, at a current value lower than that of the main welding process. This pre-energization may be multi-staged, or may include a period of no current flow in between.

[0058] By this welding process, a fusion zone and a heat affected zone (so-called HAZ) 14 that will become a nugget 13 at the end of spot welding are formed in the current-carrying portion between the steel plates 1A and 1B.

[0059] The spot welding equipment can be any conventional spot welding equipment. The welding electrodes can also be any conventional welding electrodes. The power source is not particularly limited, and an AC power source, a DC inverter, an AC inverter, or the like can be used.

[0060] If the tip diameter of welding electrodes 2A, 2B becomes too large, the surface pressure at the tip of welding electrodes 2A, 2B decreases. The tip diameter of welding electrodes 2A, 2B is preferably about 6 mm to 8 mm.

[0061] [First Post-Welding Step] After the main welding step, a first post-welding step (first post-welding step) is performed, which is a time t ｃ１ (ms) Following the non-energization period in which the energization is stopped, a first post-energization current value I that satisfies the following formula (D) is ｗ１ (kA) is the time t that satisfies the following formula (E): ｗ１ (ms) Power is applied. 2≦t ｃ１ ≦300 (C) 0.75×I ｗ０ <I ｗ１ <I ｗ０ (D) t ｗ１ >100 (E)

[0062] That is, the actual welding current I ｗ０ After the welding electrodes 2A and 2B are energized for a predetermined time to form molten parts on the steel plates 1A and 1B, the current value is set to 0 (zero) and the actual welding is completed. ｗ０ When the power is on, the applied pressure F Ｅ While keeping it as it is, the time t ｃ１ (ms). This causes the molten zone formed by the actual welding to solidify from the outer periphery of the molten zone (i.e., the boundary between the molten zone and other regions). In this disclosure, the boundary between the molten zone and other regions is referred to as the "molten boundary."

[0063] Actual welding current I ｗ０Immediately after the current flow is completed, solidification of the molten portion begins from the fusion boundary. The molten portion solidifies to form a nugget 13, and a heat-affected zone 14 is formed outside the fusion boundary.

[0064] After the main welding process, the non-energizing time t ｃ１ If the non-energization time t is less than 2 ms, there is a risk that the nugget edge will not solidify before the subsequent post-energization. ｃ１ If the time t exceeds 300 ms, the nugget edge may be excessively solidified before the subsequent post-current application. In order to avoid post-current application when the nugget edge is insufficiently or excessively solidified and to appropriately solidify the nugget edge, the no-current application time t after the main welding process is set to ｃ１ is set to 2 ms or more and 300 ms or less, and preferably set to 40 ms or more and 250 ms or less.

[0065] In the first post-current application step, the time t ｃ１ (ms) Following the non-energization in which the energization is stopped, the first post-energization current value I ｗ１ (kA) is the time t that satisfies the above formula (E). ｗ１ (ms) energization. That is, the first post-energization current value I ｗ１ is the main welding current value I ｗ０ and is smaller than 0.75 × I ｗ０ and the first post-energization time t ｗ１ By performing the first post-current application under these conditions, as shown in FIG. 4, at least a part of the nugget portion is remelted without crossing the fusion boundary formed in the main welding process, and the non-current application time t ｃ１ It is preferable that the gamma phase (fcc crystal structure) formed in the nugget portion is transformed into a delta phase (bcc crystal structure) by cooling.

[0066] First post-energization time t in the first post-energization step ｗ１ From the viewpoint of remelting the nugget portion and shortening the overall time for spot welding, the welding time is preferably set to 110 ms or more and 1000 ms or less, and more preferably set to 150 ms or more and 800 ms or less.

[0067] The pressure F in the first post-current application step Ｅis the applied pressure F in this welding process Ｅ However, it is preferable in terms of work efficiency to maintain the non-energized time t ｃ１ Pressure F at Ｅ The applied pressure F in this welding process is set within a range that satisfies the above formula (B). Ｅ and a different pressure F Ｅ It may also be possible to use the following.

[0068] [Second Post-Current Passage Step] In the method for manufacturing a spot welded joint according to the present disclosure, at least one post-current pass is performed as a second or subsequent post-current pass after the first post-current pass. In the second post-current pass (second post-current pass) after the first post-current pass, a time t ｃ２ (ms) Following the non-energization in which the energization is stopped, a second post-energization current value I that satisfies the following formula (G1) is ｗ２ (kA) during the second post-energization time t ｗ２ (ms) Power is applied. 2≦t ｃ２ ≦300 (F1) 0.004×t ｃ２ ２ -0.3125 x t ｃ２ +102≦I ｗ２ / I ｗ０ ×t ｗ２ ≦0.0156×t ｃ２ ２ -0.625 x t ｃ２ +300 (G1)

[0069] The occurrence of phase transformation due to each post-current passing step and the position of the phase transformation can be determined by observing the cross section near the center of the nugget by EBSD and examining the fine grain regions of the crystal grains before and after each post-current passing step, as shown in Fig. 8. After forming a fusion zone that becomes the nugget by this current passing, by performing two or more post-current passings that satisfy the conditions of the present disclosure to cause phase transformation at appropriate positions, it is possible to form a nugget in which the proportion of grains with an aspect ratio of 7 or more near the center of the nugget is 50% or less, as will be described later.

[0070] After the first post-energization step, non-energization time t ｃ２ If the non-energization time t is less than 2 ms, the nugget edge may not solidify before the second post-current application step. ｃ２If the time t t exceeds 300 ms, the nugget edge may be oversolidified before the first post-current application step. In order to avoid the second post-current application when the nugget edge is insufficiently or excessively solidified and to appropriately promote solidification of the nugget edge, the non-current application time t ｃ２ is set to 2 ms or more and 300 ms or less, and preferably set to 60 ms or more and 250 ms or less.

[0071] In the second post-energization step, the non-energization time t ｃ２ The nugget portion is cooled to form a γ phase (fcc crystal structure). ｗ２ and the second post-energization time t ｗ２ is the actual welding current value I ｗ０ and the non-energizing time t of the second post-energizing step ｃ２ However, by performing post-energization under the condition that satisfies the above formula (G1), the nugget portion undergoes phase transformation again to the δ phase (bcc crystal structure) as shown in Fig. 4. This promotes refinement of equiaxed grains in the nugget portion, and can improve the CTS.

[0072] In addition, in the manufacturing method of the spot welded joint according to the present disclosure, the actual welding current value I ｗ０ and the first post-energization current value I ｗ１ Similarly to the relationship between the current value I ｗ２ is 0.75 x I ｗ０ The actual welding current value I ｗ０ and the second post-energization current value I ｗ２ The current is applied so that the relationship between the current and the current (kA) satisfies the following formula (H1): 0.75×I ｗ０ <I ｗ２ (H1) Actual welding current value I ｗ０ and the second post-energization current value I ｗ２ However, if the relationship of the above formula (H1) is not satisfied, sufficient heat input cannot be provided even if current is passed for a long time, and the position in the nugget where equiaxed grain refinement occurs remains in the center of the nugget or does not change, so the effect of improving CTS cannot be obtained.

[0073] Furthermore, the actual welding current value I ｗ０ and the second post-energization current value I ｗ２ It is preferable that the relationship satisfies the following formula (I): ｗ２>I ｗ０ (I) Figure 5 shows an example of a current pattern that satisfies the formula (I). ｗ０ The second post-energization current value I ｗ２ By increasing the value, the refinement of equiaxed grains in the nugget is reliably promoted, and the CTS can be improved.

[0074] After the second post-energization, the pressure F Ｅ When the current is stopped while the welding electrodes 2A and 2B are held in place and the post-current process is completed in two steps, the pressure F is applied from the overlapped steel plates 1A and 1B (plate assembly). Ｅ After the second post-current application step, it is preferable to provide a so-called holding time during which only pressure is applied without current application. The holding time is preferably 5 cycles (50 Hz) or more.

[0075] The pressure F in the second post-current application step Ｅ Also, the pressure F in the main welding process following the first post-current application process Ｅ However, it is preferable in terms of work efficiency to maintain the non-energized time t ｃ２ Pressure F at Ｅ The applied pressure F in this welding process is set within a range that satisfies the above formula (B). Ｅ and a different pressure F Ｅ That is, the pressure F may be set to F from the main welding step to the end of the final post-welding step. Ｅ may be constant or may vary as long as it is within a range that satisfies formula (B).

[0076] A downslope may be provided after the second post-current process. The downslope can further improve the properties of the spot weld by reducing cracking due to liquid metal embrittlement, reducing blowholes, and suppressing delayed fracture. When downslope current is used, the current value at the start of the downslope is set to the second post-current current value I in the second post-current process. ｗ２ (kA), and the second post-current application time t ｗ２ (ms) does not include the time taken for downslope.

[0077] By performing resistance spot welding consisting of the above-mentioned steps on a sheet assembly in which multiple steel sheets, including at least one high-strength steel sheet, are overlapped, the CTS can be significantly improved compared to resistance spot welding performed with a single current. Furthermore, according to the present disclosure, joint strength can be effectively improved in a shorter welding time compared to conventional tempering current, etc.

[0078] Here, the effect of improving the CTS by conducting the second post-energization process so as to satisfy the above formula (G1) will be described. A plate assembly was formed by stacking two high-strength steel plates having the chemical compositions (unit: mass %, balance: Fe and impurities), plate thicknesses, and tensile strengths (TS) shown in Table 1 below, and spot welding was performed by applying a main welding current and a post-energization current under the conditions shown in Table 2 to produce a welded joint. The CTS of the obtained welded joint was measured using a method in accordance with JIS Z 3137;1999. In Table 2, "cool" refers to the time during which no current was applied (cooling time) when current application was stopped.

[0079]

[0080]

[0081] For Test Example 0, the first post-welding current was applied after the main welding current. Table 2 shows the CTS improvement rates of Test Examples 1 to 5, which were applied up to the second post-welding current, based on the CTS of the joint for Test Example 0. A CTS improvement rate of more than 10% relative to one post-welding current (Test Example 0) was judged to have a CTS improvement effect, while a rate of 10% or less was judged to have no CTS improvement effect. Each joint was also cut in the thickness direction through the center of the nugget, and the grains near the center of the nugget were observed using electron backscatter diffraction (EBSD). The results are shown in Figure 7. The areas with a crystal orientation difference of 15 degrees or more were considered to be grain boundaries. The area surrounded by the dotted square is the fine-grain region formed by the first post-welding current, and the area surrounded by the solid square is the fine-grain region formed by the second post-welding current. The following is inferred from the results shown in Table 2 and Figure 7.

[0082] Test Example 1 is I ｗ２ / I ｗ０ ×t ｗ２The value of is below the lower limit of the formula (G1), the heat input is insufficient, and the fine grains generated by the second post-current application are quite close to the nugget center, which is a position that the crack reaches after considerable crack growth in the joint strength test, so it does not contribute to improving the joint strength. ｗ２ / I ｗ０ ×t ｗ２ The value of satisfies the formula (G1), the heat input is appropriate, and the fine grains generated by the second post-current are at a certain distance from the nugget center and are located inside the outermost fine grains generated by the first post-current, which effectively improves toughness. ｗ２ / I ｗ０ ×t ｗ２ The value of exceeds the upper limit of the formula (G1), and the fine grains generated by the second post-current flow exceed the fine grain region generated by the first post-current flow. Therefore, the fine grains generated by the first post-current flow disappear, and a high joint strength improvement effect cannot be obtained.

[0083] (Third and subsequent post-current application steps) In the method for manufacturing a spot welded joint according to the present disclosure, after the second post-current application step, a pressing force F Ｅ While maintaining the state as it is, non-energization and post-energization may be alternately repeated, and the post-energization process may be performed 3 to N times (N is an integer of 3 or more) including the first post-energization process and the second post-energization process described above. The third and subsequent post-energization processes all satisfy the same relational expression as that of the second post-energization process, that is, after the n-1th post-energization process (n is an integer of 3 or more and N or less), the time t ｃｎ Following the non-energization period in which the energization is stopped (ms), the n-th subsequent energization current value I satisfies the following formulas (G) and (H): ｗｎ (kA) is the nth post-current application time t ｗｎ The second or subsequent post-current application process, that is, the nth post-current application process (n is an integer of 3 or more and N or less), is performed after the (n-1)th post-current application process for a time t that satisfies the following formula (F): ｃｎ (ms) Following the non-energization in which the energization is stopped, the nth subsequent energization current value I satisfies the following formula (G): ｗｎ (kA) is the nth post-current application time t ｗｎ (ms) Power is applied. 2≦t ｃｎ ≦300 (F) 0.004×t ｃｎ ２ -0.3125 x t ｃｎ+102≦I ｗｎ / I ｗ０ ×t ｗｎ ≦0.0156×t ｃｎ ２ -0.625 x t ｃｎ +300 (G) In the n-th post-current applying step, it is preferable to cause the phase transformation to occur inside the outermost position where the phase transformation occurred in the (n-1)-th post-current applying step.

[0084] When the post-current application process is performed three or more times, a downslope may be provided after the final post-current application process. The downslope can further improve the properties of the spot weld by reducing cracking due to liquid metal embrittlement, reducing blowholes, and suppressing delayed fracture. Note that even when a downslope application process is performed after the final post-current application process, the current value at the start of the downslope is taken as the current value (kA) for the final post-current application process, and the post-current application time (ms) for the final post-current application process does not include the time required for the downslope.

[0085] As with the second post-current process, the third and subsequent post-current processes are repeated with no current applied so as to satisfy the formulas (F), (G), and (H). This is thought to promote equiaxed grain refinement due to the phase transformation from the γ phase (fcc crystal structure) to the δ phase (bcc crystal structure) within the nugget (inside the fusion boundary), thereby further improving the CTS. However, the more post-current processes are applied, the longer the welding time becomes. Therefore, the number of post-current processes is preferably two to four times, more preferably two to three times, and particularly preferably two times.

[0086] <Spot-welded joint> The manufacturing method for a spot-welded joint according to the present disclosure can manufacture the following spot-welded joint. That is, the spot-welded joint according to the present disclosure includes a spot weld formed by joining multiple overlapping steel plates, and at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq of 0.36 mass % or more, as expressed by the following formula (A) where the contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S], respectively, in mass %. When observing the inside of the fusion boundary of the nugget in a cross section in the plate thickness direction passing through the center of the nugget at the spot weld, portions in the middle of the nugget in the longitudinal direction where the crystal orientation difference is 15 degrees or more are defined as grain boundaries, and the proportion of crystal grains with an aspect ratio of 7 or more is 50% or less. Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A)

[0087] The spot-welded joint according to the present disclosure can be produced in a shorter welding time than when a sheet assembly including a high-strength steel sheet having a Ceq of 0.36 mass % or more is subjected to main welding and then tempering current is applied, and can have higher joint strength than a spot-welded joint spot-welded only by the main welding process.

[0088] (Method for Measuring the Proportion of Crystal Grains with an Aspect Ratio of 7 or More) Figure 6 shows the area in a spot-welded joint where the aspect ratio of crystal grains is measured. EBSD analysis is used to measure the crystal orientation inside the fusion boundary of the nugget on a cross section of the spot-welded joint in the sheet thickness direction passing through the center of the nugget. As shown in Figure 6, the measurement field is set to 0.5 mm on either side of a virtual line L2 in the sheet thickness direction passing through the center of the nugget, and to an intermediate portion M including the position of the sheet interface L1 and the fusion boundary of one steel sheet. Because the steel structure of the nugget is nearly symmetrical above and below the sheet interface L1, it is sufficient to measure the intermediate portion M on one sheet side, as shown in Figure 6. Then, grain boundaries with a crystal orientation difference of 15 degrees or more, which are considered to be block grain boundaries and prior austenite grain boundaries, are drawn in the measurement field, and the aspect ratio of each crystal grain is calculated. The aspect ratio (major axis / minor axis) is calculated by defining the maximum length of the crystal grain as the major axis and the distance between two parallel lines parallel to the major axis direction and tangent to the crystal grain at the widest distance as the minor axis. The size of the field of view is not limited because the size of the grains varies depending on the sample, but it is sufficient to observe a size that includes 100 or more grains. In the above field of view, the aspect ratio values ​​of 100 or more grains are obtained, and the proportion of grains with an aspect ratio of 7 or more is calculated.

[0089] In the spot-welded joint according to the present disclosure, the proportion of grains having an aspect ratio of 7 or more measured as described above is 50% or less. That is, by performing two or more post-energization processes under the conditions described above after main welding on a sheet assembly including the high-strength steel sheet, equiaxed grain refinement in the nugget portion is promoted, and a spot-welded joint is obtained in which the proportion of grains having an aspect ratio of less than 7 is half (50%) or more. Such a spot-welded joint can exhibit a higher CTS than a spot-welded joint in which the proportion of grains having an aspect ratio of 7 or more exceeds 50%. The proportion of grains having an aspect ratio of 7 or more is preferably 40% or less, more preferably 30% or less.

[0090] Hereinafter, the manufacturing method of a resistance spot welded joint according to the present disclosure will be described with reference to examples. However, the manufacturing method of a resistance spot welded joint according to the present disclosure is not limited to these examples.

[0091] Steel sheets were prepared having the chemical compositions (unit: mass%, balance: Fe and impurities), thicknesses, and tensile strengths (TS) shown in Table 3 below. Note that in each table in the examples, underlines indicate that the results are outside the scope of the present disclosure.

[0092]

[0093] The steel plates were combined and resistance spot welded under the conditions (plate assembly, pressure, current conditions, etc.) shown in Table 4 to produce welded joints. The CTS of the resulting welded joints was measured. The CTS of the joint in No. 24, in which a three-ply plate assembly was spot welded, was measured at the interface with the steel plate (Ceq: 0.36 mass% or more) that is the target of the present disclosure, i.e., the interface between steel plate B and steel plate C. Steel plate D of the plate assembly in No. 24 was prepared for use in a plate assembly including a high-strength steel plate to which the manufacturing method of a spot-welded joint according to the present disclosure can be applied, and is not underlined.

[0094] In addition, in the case of the joint of the plate assembly in which three steel plates of the same type are stacked, as shown in No. 25, the CTS values ​​of the two plate interfaces are equivalent, so the CTS of either interface can be measured, but the CTS was measured by performing cross tension on the interface that was on the upper side at the time of welding.

[0095] For each plate assembly, the CTS (standard CTS) of the welded joint was measured after the main welding current was applied under the conditions for each number, up to the first post-welding current. The rate of increase was calculated by comparing with this standard CTS, and those exceeding 10% were judged to have an effect of improving joint strength.

[0096]

[0097] Although Nos. 1 to 9 were subjected to the main welding current and then the second post-current, they did not satisfy any one of the formulas (A) to (H) in the present disclosure, and none of them achieved an improvement in CTS. Note that in No. 1, the proportion of grains with an aspect ratio of 7 or more was 50% or less, but the Ceq of the steel sheet C constituting the sheet assembly was low, and the CTS was high even in the joint that had been subjected to the first post-current, so the CTS improvement effect of the second post-current was not achieved. No. 14 is an example in which only the main welding current was performed. No. 15 is an example in which a long post-current was performed (so-called tempering current) at a relatively low current value after the main welding current for tempering. Although an improvement in CTS was achieved, the non-current process time after the main welding current was also required to be long, and it is difficult to say that the CTS of the spot welding process as a whole was effectively improved. The CTS improvement rates of Nos. 14 and 15 were calculated by comparing them with the CTS of welded joints in which the same plate combination (two steel plates B) was used under the conditions of No. 16, and in which main welding current was applied up to the first post-current application. Nos. 18 to 22 were subjected to main welding current up to the second post-current application, but did not satisfy any of the formulas (A) to (H1) in the present disclosure, and no CTS improvement effect was obtained in any of them.

[0098] On the other hand, all of the numbers classified as examples satisfy the formulas (A) to (H) in the present disclosure, and an improvement in CTS was achieved. Note that number 13 is an example in which up to the third post-energization was performed, and it has the highest CTS improvement rate. In Table 4, the no-energization, current value, and time for the third post-energization are listed separately in the second post-energization column for convenience.

[0099] The disclosure of Japanese Patent Application No. 2022-125100, filed on August 4, 2022, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated herein by reference.

[0100] 1A, 1B Steel plate 2A, 2B Welding electrode 13 Nugget 14 Heat-affected zone (HAZ)

Claims

1. A method for manufacturing a spot welded joint, in which a plate assembly made of a plurality of overlapping steel plates is sandwiched between a pair of welding electrodes in the plate thickness direction and spot welded by passing current through it while applying pressure, wherein at least one of the plurality of steel plates is a high-strength steel plate having a carbon equivalent Ceq expressed by the following formula (A) of 0.36 mass% or more, where the contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S], respectively, in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) When the arithmetic mean value of the plate thicknesses of the plurality of steel plates is h (mm), the plate assembly is welded with the pair of welding electrodes at a pressure F that satisfies the following formula (B): Ｅ When pressurized with (N), 2000 × h ≦ F Ｅ ≦4500×h (B) The pair of welding electrodes are supplied with a main welding current value I ｗ０ (kA) to perform main welding to form a fusion zone in the sheet assembly; and two post-current processes in which current is applied after the main welding process, wherein the first post-current process is a time t ｃ１ (ms) Following the non-energization period in which the energization is stopped, a first post-energization current value I that satisfies the following formula (D) is ｗ１ (kA) is the time t that satisfies the following formula (E): ｗ１ (ms) energized, 2≦t ｃ１ ≦300 (C) 0.75×I ｗ０ <I ｗ１ <I ｗ０ (D) t ｗ１ >100 (E) The second post-current application process, which is the second post-current application process, is performed after the first post-current application process, at a time t that satisfies the following formula (F1): ｃ２ Following the non-energization period in which the energization is stopped (ms), a second post-energization current value I that satisfies the following formula (G1) and the following formula (H1) is obtained. ｗ２ (kA) at time t ｗ２ (ms) energized, 2≦t ｃ２ ≦300 (F1) 0.004×t ｃ２ ２ -0.3125 x t ｃ２ +102≦I ｗ２ / I ｗ０ ×t ｗ２ ≦0.0156×t ｃ２ ２ -0.625 x t ｃ２ +300 (G1) 0.75×I ｗ０ <I ｗ２ (H1) The processes from the main welding process to the final post-current application process are performed by the pressure F Ｅ (N) is maintained within a range that satisfies the formula (B).

2. The actual welding current value I ｗ０ and the second post-energization current value I ｗ２ The method for manufacturing a spot welded joint according to claim 1, wherein the relationship between the above and the above satisfies the following formula (I): ｗ２ >I ｗ０ (I) 3. The post-current application process is included 3 or more times and N or less times (N is an integer of 3 or more), and the nth post-current application process (n is an integer of 3 or more and N or less) which is the third or later post-current application process is a time t that satisfies the following formula (F) after the (n-1)th post-current application process which is the (n-1)th post-current application process. ｃｎ (ms) Following the non-energization in which the energization is stopped, the n-th subsequent energization current value I satisfies the following formula (G) and the following formula (H): ｗｎ (kA) at time t ｗｎ The method for manufacturing a spot welded joint according to claim 1 or 2, wherein current is passed through the welding joint for 2 ≤ t (ms). ｃｎ ≦300 (F) 0.004×t ｃｎ ２ -0.3125 x t ｃｎ +102≦I ｗｎ / I ｗ０ ×t ｗｎ ≦0.0156×t ｃｎ ２ -0.625 x t ｃｎ +300 (G) 0.75×I ｗ０ <I ｗｎ (H) 4. A spot welded joint comprising a spot weld where multiple overlapping steel plates are joined, wherein at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq of 0.36 mass% or more, as expressed by the following formula (A) where the contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S], respectively, in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) A spot welded joint, wherein, when observing the inside of the fusion boundary of the nugget in a cross section in the plate thickness direction passing through the center of the nugget of the spot weld, the portions in the middle part of the long axis direction of the nugget where the crystal orientation difference is 15 degrees or more are considered to be crystal grain boundaries, and the proportion of crystal grains with an aspect ratio of 7 or more is 50% or less.