Method for manufacturing spot welding joint and spot welding 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

Technical Field

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

Background Art

[0002] In recent years, in the automotive field, in order to reduce fuel consumption and CO2 emissions, it has been required to reduce the weight of the vehicle body. In addition, in order to improve collision safety, it has been required to increase the strength of vehicle body members. In order to meet these requirements, it is effective to use high-strength steel sheets for vehicle bodies and parts. Spot welding is mainly used for assembling vehicle bodies and attaching parts.

[0003] In a joint formed by spot welding a stack of multiple steel sheets (also referred to as a "spot weld joint" in the present disclosure), tensile strength is an important characteristic. The tensile strength of a spot weld joint includes a tensile shear force (TSS) measured by applying a tensile load in the shear direction and a cross-tensile force (hereinafter sometimes referred to as "CTS" or "joint strength") measured by applying a tensile load in the peeling direction.

[0004] Generally, when manufacturing a spot weld joint using high-strength steel sheets, the deformability with respect to the pressure applied by the welding electrode is small, and stress concentration at the welded portion increases. In addition, the toughness of the welded portion decreases due to heat input into the welded portion. Therefore, a spot weld joint including high-strength steel sheets tends to have a low CTS, and improvement of the CTS is required.

[0005] In order to ensure the strength and toughness of a spot weld joint using a stack including high-strength steel sheets, after performing the main welding to form a molten portion serving as a nugget, after cooling for a certain period of time, energization is performed again, and a temper energization for annealing the nugget portion and the heat-affected zone is performed. Also, a method of performing post-energization in a relatively short time after the main welding has been proposed.

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

[0007] Further, Patent Document 2 discloses a resistance spot welding method in which a stack of two or more steel plates with a total plate thickness t (mm) is sandwiched between a pair of welding electrodes, and current is passed while applying pressure to perform welding. The method includes a first step of forming a nugget and a second step of passing current after cooling the welded portion by holding it without current while still applying pressure with the electrodes. The energization time TA (ms), energization current IA (kA) in the first step, energization time TB (ms), and energization current IB (kA) in the second step satisfy equations (1) and (2), the energization current IB (kA) is higher than the energization current IA (kA), and the non-energization holding time Th (ms) in the second step satisfies equation (3) in relation to the total plate thickness t (mm) of the stack, the diameter d (mm) of the nugget, and the tip area S (mm 2 ) of the welding electrode. 0.05 < (IB2 × TB) / (IA2 × TA) < 1.0 ··· (1) 20 ≤ TB ≤ 100 ··· (2) 10 × (t × d2) / S < Th < 200 × (t × d2) / S ··· (3)

[0008] In Patent Document 3, as a spot welding method for improving the cross-tensile strength of a spot weld joint, a plurality of steel plates including a high-strength steel plate with 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 molten part is formed by this welding. After that, the energization is stopped, the steel plates are cooled to form a solidification zone in the molten part, and then, the first cooling and post-energization are performed so that the solidification zone is not remelted by subsequent energization. After the process of the first cooling and post-energization is completed, a predetermined pressing force F E (N) is held, and the energization is stopped for a cooling time t S (ms) so as to satisfy a predetermined condition, and then, subsequently, a post-energization current I P (kA) is energized for a post-energization time t P (ms), and a process of performing cooling and post-energization so that the solidification zone is not remelted by subsequent energization is repeated one or more times. A spot welding method having such a process is disclosed.

[0009] Also, Patent Documents 4 to 6 disclose spot welding methods in which post-energization is performed after this 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 Unexamined Patent Application Publication No. 2018-30178 Patent Document 6: Japanese Unexamined Patent Application Publication No. 2013-78782

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present disclosure aims to provide a method for manufacturing a spot weld joint and a spot weld joint that can effectively improve the cross-tensile strength of the joint when using a high-strength steel plate and performing one post-energization after this welding to manufacture the spot weld joint, as compared with the case of manufacturing the spot weld joint by performing one post-energization.

Means for Solving the Problems

[0012] The gist of the present disclosure for achieving the above object is as follows. <1> A method for manufacturing a spot weld joint that performs spot welding by sandwiching a stack of multiple steel plates in the plate thickness direction between a pair of welding electrodes and energizing them while applying pressure, at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq represented by the following formula (A) of 0.36 mass% or more when the respective contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S] in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) when the arithmetic mean value of the plate thicknesses of the multiple steel plates is h (mm), the stack is pressurized with a pressing force F E (N) that satisfies the following formula (B) between the pair of welding electrodes, 2000×h ≤ F E ≤ 4500×h (B) a main welding step of energizing the pair of welding electrodes with a main welding current value I w0 (kA) to form a molten part in the stack and performing the main welding, after the main welding step, two post-energizing steps of energizing, including, the first post-energizing step which is the first of the two post-energizing steps is, after the main welding step, a time t c1 (ms) of suspending energization is followed by a first post-energizing current value I w1 (kA) that satisfies the following formula (D) and is energized for a time t w1 (ms) that satisfies the following formula (E), 2 ≤ t c1 ≤ 300 (C) 0.75×I w0 < I w1 < I w0 (D) t w1 > 100 (E) the second post-energizing step which is the second of the two post-energizing steps is, after the first post-energizing step, a time t that satisfies the following formula (F1) c2(ms) Following the power-off that halts the energization, a second post-energization current value I that satisfies the following (G1) formula and the following (H1) formula w2 (kA) is applied for a time t w2 (ms), and 2 ≤ t c2 ≤ 300 (F1) 0.004 × t c2 2 -0.3125 × t c2 +102 ≤ I w2 / I w0 × t w2 ≤ 0.0156 × t c2 2 -0.625 × t c2 +300 (G1) 0.75 × I w0 < I w2 (H1) The process from the main welding process to the last post-energization process is continuously performed while maintaining the pressing force F E (N) within the range that satisfies the following (B) formula, a method for manufacturing a spot weld joint. <2> The method for manufacturing a spot weld joint according to <1>, wherein the relationship between the main welding current value I w0 and the second post-energization current value I w2 satisfies the following (I) formula. <3> Including the post-energization process N times or more and 3 times or less (N is an integer of 3 or more), the nth post-energization process, which is the post-energization process after the 3rd time (n is an integer of 3 or more and N or less), after the (n - 1)th post-energization process, which is the (n - 1)th post-energization process, a time t that satisfies the following (F) formula cn (ms) Following the power-off that halts the energization, a nth post-energization current value I that satisfies the following (G) formula and the following (H) formula wn (kA) is applied for a time t wn (ms), the method for manufacturing a spot weld joint according to <1> or <2>. 2 ≤ t cn ≤ 300 (F) 0.004 × t cn 2 -0.3125 × t cn +102 ≤ I wn / I w0 × t wn ≤ 0.0156 × t cn2 -0.625×t cn +300 (G) 0.75×I w0 <I wn (H) <4> includes a spot weld portion where a plurality of overlapping steel plates are joined, At least one of the plurality of steel plates is a high-strength steel plate having a carbon equivalent Ceq represented by the following formula (A) of 0.36% by mass or more when the respective contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S] in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) 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 portion, in the middle portion in the major axis direction of the nugget, a portion having a crystal orientation difference of 15 degrees or more is defined as a grain boundary, and a spot weld joint in which the ratio of the number of grains having an aspect ratio of 7 or more is 50% or less.

Advantages of the Invention

[0013] According to the present disclosure, there are provided a method for manufacturing a spot weld joint capable of effectively improving the cross-tensile strength of the joint as compared with the case of manufacturing the spot weld joint by performing one post-current application after the welding, and a spot weld joint.

Brief Description of the Drawings

[0014]

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Mode for Carrying Out the Invention

[0015] Hereinafter, an embodiment which is an example of the present disclosure will be described. In the present disclosure, the “%” display of the content of each element means “mass %”. Also, in the present disclosure, the numerical range represented by using “~” means a range including the numerical values described before and after “~” as the lower limit value and the upper limit value, unless otherwise specified. Also, before and after “~” When the numerical values described before and after are attached with “more than” or “less than”, the numerical range means a range that does not include these numerical values as the lower limit value or the upper limit value. In the numerical ranges described stepwise in the present disclosure, the upper limit value of a certain stepwise numerical range may be replaced with the upper limit value of the numerical range described in other stepwise descriptions, or may be replaced with the value shown in the examples. In the numerical ranges described stepwise in the present disclosure, the lower limit value of a certain stepwise numerical range may be replaced with the lower limit value of the numerical range described in other stepwise descriptions, or may be replaced with the value shown in the examples. Also, the term “step” includes not only an independent step but also this term as long as the intended purpose of the step is achieved even when it cannot be clearly distinguished from other steps.

[0016] The inventors of the present disclosure have intensively studied a method for effectively improving the joint strength when manufacturing a spot weld joint by spot welding a stack of steel sheets including a high-strength steel sheet having a tensile strength of, for example, 900 MPa or more. As a result, under a predetermined pressing force F E (N), after performing multiple (at least two) post-weld energizations after this welding, the current value is made smaller than that of this welding in the first post-weld energization, and in the second post-weld energization, the post-weld energization is performed with a current value and an energization time that satisfy a predetermined relationship with the immediately preceding cooling time and the current value of this welding, and it has been found that the joint strength can be effectively improved. In addition, when observing the steel structure of the spot weld portion manufactured by the method for manufacturing a spot weld joint according to the present disclosure, it has been found that the equiaxed grain refinement of the crystal grains inside the nugget is promoted, and the crystal grain form is different from the case where spot welding is performed only by this welding or the case where temper energization or the like is performed after this welding. Hereinafter, embodiments of the present disclosure will be described.

[0017] [High-strength steel sheet] First, the high-strength steel sheet included in the stack of steel sheets on which spot welding is performed in the method for manufacturing a spot weld joint according to the present disclosure will be described.

[0018] (Tensile strength) If the tensile strength of each of the plurality of steel sheets of the stack of steel sheets to be spot welded is, for example, less than 900 MPa, it is easy to obtain a high joint strength, and problems with the joint strength are less likely to occur. Therefore, in the method for manufacturing a spot weld joint according to the present disclosure, a stack of a plurality of steel sheets including at least one high-strength steel sheet (which may be simply referred to as a "high-strength steel sheet" in the present disclosure) having a carbon equivalent Ceq of 0.36 mass% or more is used.

[0019] All of the plurality of steel plates constituting the plate stack may be high-strength steel plates with a carbon equivalent Ceq of 0.36 mass% or more, or at least one may be a high-strength steel plate with a carbon equivalent Ceq of 0.36 mass% or more, and at least one may be a steel plate with a carbon equivalent Ceq of less than 0.36 mass%. For example, in the case of a plate stack formed by laminating three or more steel plates, at least one may be a high-strength steel plate, and one or more steel plates with a carbon equivalent Ceq of less than 0.36 mass% may be included. Further, in the present disclosure, the plurality of steel plates constituting the plate stack are not limited to flat steel plates, and may also be steel plates processed by hot stamping (hot pressing) or the like. For example, a plate stack formed by laminating a plurality of steel plates processed into a three-dimensional component shape by bending or the like may be used, or a plate stack formed by laminating a flat steel plate and a steel plate processed into a component shape may be used. In addition, for example, when spot welding is performed using a steel member having a three-dimensional shape by processing a flat steel plate by processing such as bending or welding, the flat portion (plate-like portion) where the spot welding is performed corresponds to the "steel plate" in the present disclosure.

[0020] The tensile strength of the high-strength steel plate is preferably 900 MPa or more, and the upper limit is not particularly limited. 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. In addition, for the measurement of the tensile strength (TS) of the steel plate, a JIS No. 5 tensile test piece (gauge length: 50 mm, width 25 mm) is taken from the steel plate, and at a tensile speed of 10 mm / min J IS It may be carried out in accordance with Z 2241:2011. When taking from the joint, avoid as much as possible the parts deformed by pressing or the like, and take from a flat part. If a JIS No. 5 tensile test piece cannot be taken, the TS may be measured as JIS13B (gauge length: 50 mm, width 12.5 mm).

[0021] (Carbon equivalent Ceq) The high-strength steel plate in the present disclosure has a carbon equivalent Ceq represented by the following formula (A) of 0.36 mass% or more. Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S] (A) In formula (A), [C], [Si], [Mn], [P], and [S] are each The contents (mass%) of C, Si, Mn, P, and S contained in the high-strength steel plate. If the carbon equivalent Ceq is 0.36 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, for example, if it exceeds 0.55 mass%, a high-strength steel plate having a tensile strength exceeding 2500 MPa can be obtained, but it is difficult to improve the CTS of a spot-welded joint. Therefore, it is preferable that the Ceq of the high-strength steel plate is 0.55 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 by a method conforming to JIS G0417:1999.

[0023] (chemical composition) The high-strength steel plate according to the present disclosure may have any chemical composition so long as the carbon equivalent Ceq is 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: 0.07-0.50% C is an element that increases the tensile strength of steel. The higher the C content in the steel, the higher the strength of the nugget can be enhanced. When the C content in the steel is 0.07% or more, a tensile strength of 900 MPa or more is likely to be obtained. On the other hand, if the C content in the steel is 0.50% or less, the deterioration of 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 - 0.50%.

[0025] Si: 0.001 - 2.50% Si is an element that increases the strength of steel by solid solution strengthening and structure strengthening. When the Si content in the steel is 2.50% or more, the deterioration of the workability of the steel can be suppressed. On the other hand, if the Si content in the steel is 0.001% or more, it is easy to manufacture industrially and technically. Therefore, the Si content of the high-strength steel plate is preferably 0.001% - 2.50%.

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

[0027] P: 0.03% or less P is an element that embrittles the nugget. When the P content in the steel is 0.03% The following it is difficult for cracks to occur in the nugget, and a sufficiently high joint strength is likely to be obtained. Therefore, the P content of the high-strength steel plate is preferably 0.03% or less. Although the lower limit value of the P content is not limited, from the perspective of suppressing the increase in manufacturing cost, 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. Also, S is an element that combines with Mn to form coarse MnS and inhibits the workability of steel. If the S content in the steel is 0.01% or less, cracks in the nugget are less likely to occur, and it is easy to obtain a sufficiently high joint strength. Furthermore, the workability of the steel is improved. Therefore, the S content of the high-strength steel sheet is preferably 0.01% or less. From the viewpoint of suppressing an increase in manufacturing cost, the S content of the high-strength steel sheet 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. Also, N is an element that causes blowholes during welding. To suppress the deterioration of the workability of steel and the generation of blowholes, the N content of the high-strength steel sheet is preferably 0.01% or less. From the viewpoint of suppressing an increase in manufacturing cost, the N content of the high-strength steel sheet 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 the deterioration of the workability of steel, the O content of the high-strength steel sheet is preferably 0.01% or less. From the viewpoint of suppressing an increase in manufacturing cost, the O content of the high-strength steel sheet may be 0.0005% or more.

[0031] Al: 1.50% or less Al is a ferrite-stabilizing element and has effects such as suppressing cementite precipitation during bainite transformation. For this reason, it is contained for controlling the steel structure. Also, Al functions as a deoxidizer. On the other hand, Al is easily oxidized, and the workability of steel is likely to deteriorate due to an increase in inclusions. Therefore, the Al content of the high-strength steel sheet is preferably 1.50% or less.

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

[0033] Ti, Nb, V: 0.005 - 0.20% Ti, Nb, and V are elements that contribute to the increase in the strength of steel by at least one of precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. However, if the content of any of these elements in the steel is less than 0.005%, the containing effect is difficult to appear. On the other hand, if the content in the steel is 0.20% or less, the inhibition of the workability of the steel can be suppressed. Therefore, the content of these elements in the high-strength steel sheet is preferably 0.005 to 0.20% for all.

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

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

[0036] Therefore, the content of these elements in the high-strength steel sheet is preferably 0.01% or more for all. On the other hand, if the content of Cr, Ni, and Cu in the steel is 2.0% or less, when the content of Mo in the steel exceeds 0.8%, it is possible to suppress problems occurring during pickling or hot working. Therefore, the content of Cr, Ni, and Cu in the high-strength steel sheet is preferably 2.0% or less. Also, the content of Mo in the high-strength steel sheet is preferably 0.8% or less.

[0037] At least one of Ca, Mg, and REM: a total of 0.0001 - 1.0% Ca, Mg, and REM (rare earth metal) are elements that contribute to improving the workability of steel by reducing the size of the oxide after deoxidation and the size of sulfides present in the hot-rolled steel sheet. If the total content of these elements in the steel is 0.0001% or more, the containing effect is likely to appear. On the other hand, when the total content of these elements in the steel is 1.0% or less, a decrease in the workability of the steel is suppressed. Therefore, the total content of these elements in the high-strength steel sheet is preferably 0.0001 - 1.0%.

[0038] Note that REM is a general term for a total of 17 elements including Sc, Y, and 15 elements belonging to the lanthanoid series. REM can be added to the molten steel as mischmetal at the steelmaking stage. Also, elements of the lanthanoid series may be contained in combination.

[0039] The remainder other than the above elements in the high-strength steel sheet is Fe and impurities. Note that impurities are exemplified by 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. Note that for the aforementioned Cr, Ni, Cu, Mo, B, Ti, Ni, and V, it is allowed to contain trace amounts less than the above lower limit as impurities. Also, for Ca, Ce, Mg, La, and REM, it is allowed to contain trace amounts less than the above lower limit of their total amount as impurities.

[0040] As described above, the chemical composition of the high-strength steel sheet in the present disclosure has been explained. However, when the steel sheet having a carbon equivalent Ceq of less than 0.36 mass% is included as the steel sheet constituting the sheet group, the chemical composition and tensile strength of the steel sheet 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 the carbon equivalent Ceq is 0.36% by mass or more. For example, two-phase structure types (for example, a structure containing martensite in ferrite, a structure containing bainite in ferrite), processing-induced transformation types (a structure containing retained austenite in ferrite), quenched types (martensite structure), fine crystal types (ferrite-based structure), etc., any type of structure (steel type) may be used.

[0042] When the steel plate having a carbon equivalent Ceq of less than 0.36% by mass is included as the steel plate constituting the plate group, the steel structure (steel type) of the steel plate is not particularly limited either.

[0043] (Plate thickness) The plate thickness of the high-strength steel plate in the present disclosure is not particularly limited. For example, the plate thickness (0.5 mm to 3.2 mm) of the high-strength steel plate generally used for automobile bodies and the like can be mentioned. However, since the stress concentration around the nugget increases as the plate thickness of the high-strength steel plate increases, the plate thickness of the high-strength steel plate is preferably 2.6 mm or less.

[0044] When the steel plate having a carbon equivalent Ceq of less than 0.36% by mass is included as the steel plate constituting the plate group, the plate thickness of the steel plate is not particularly limited either. In addition, the plate thicknesses of the plurality of steel plates constituting the plate group may be the same or different from each other. For example, when three or more steel plates are stacked, the plate thicknesses of each of the steel plates may be different, or the plate thicknesses of at least two steel plates may be the same.

[0045] (Plating) The steel plate constituting the plate group may have a plating layer formed on its surface. Examples of the type of the plating layer include Zn-based, Zn-Fe-based, Zn-Ni-based, Zn-Al-based, Zn-Mg-based, Pb-Sn-based, Sn-Zn-based, Al-Si-based, and the like. When the steel plate has a plating layer on its surface, the plating layer may be a single layer or may have a multi-layer plating layer.

[0046] Examples of steel sheets provided with a Zn-based plating layer include alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets, and electro-galvanized steel sheets. If a plating layer is formed on the surface of the steel sheets in the sheet stack, the spot weld joint after spot welding exhibits excellent corrosion resistance. When the plating layer is a zinc plating layer alloyed with the surface of the steel sheet, excellent corrosion resistance is obtained, and the adhesion of the paint becomes good.

[0047] The plating 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. Note that an inorganic or organic film (for example, a lubricating film, etc.) may be formed on the surface layer of the plating layer.

[0048] [Method for manufacturing a spot weld joint] Next, a method for manufacturing a spot weld joint according to the present disclosure will be described. The method for manufacturing a spot weld joint according to the present disclosure applies a pressing force F E (N) to a stack of a plurality of steel sheets, at least one of which is the high-strength steel sheet described above, and forms a molten portion in the stack of steel sheets in a pressed state by a pair of welding electrodes, and after this main welding step, at least two post-welding energization steps are continuously performed.

[0049] Here, a case where two steel sheets including at least one of the above-described high-strength steel sheets are overlapped and spot-welded will be described as an example. Note that even in the case of spot-welding three or more steel sheets including at least one high-strength steel sheet, spot welding can be performed in the same manner as the method shown below. FIG. 1 is a diagram schematically showing an example of the arrangement of two steel sheets including at least one high-strength steel sheet and welding electrodes at the start of spot welding. FIG. 2 is a diagram schematically showing an example of a nugget and a heat-affected zone (HAZ) formed by spot welding. FIG. 3 is a diagram showing an example of a current application pattern when applying current to the welding electrodes in the method for manufacturing a spot welding joint according to the present disclosure. Note that the current described below is the current flowing between the welding electrode 2A and the welding electrode 2B. Further, FIG. 4 is a diagram obtained by simulating the temperature history of the welded portion by the method for manufacturing a spot welding joint according to the present disclosure. Note that QuickSpot (Computational Mechanics Research Center, Inc.) can be used as software for performing the simulation by heat conduction analysis.

[0050] [This Welding Step] First, as shown in FIG. 1, a plate stack in which the steel plates 1A and 1B are overlapped such that their plate surfaces face each other is prepared, and the plate stack is sandwiched in the plate thickness direction by a pair of welding electrodes 2A and 2B. Then, while applying a pressing force F E (N) that satisfies the following formula (B), a welding current value I w0 (kA) is applied to the welding electrodes 2A and 2B to perform this welding (this current application) for forming a molten portion in the plate stack. Note that at least one of the steel plates 1A and 1B is the high-strength steel plate described above. 2000×h ≦ F E ≦ 4500×h (B) Here, h (mm) means the arithmetic average value of the plate thicknesses of the plurality of overlapped steel plates. When the plate thicknesses of the two steel plates are different, the arithmetic average value of the plate thicknesses of the two steel plates (the arithmetic average value of the plate thickness of the steel plate 1A and the plate thickness of the steel plate 1B) is used as "h" in the formula (B). When spot-welding a plurality of three or more steel plates, for example, the sum of the plate thicknesses of the plurality of steel plates is obtained, and the value obtained by dividing the sum by the number of plate stacks is used as "h" in the formula (B).

[0051] When measuring the thickness of steel plates before they are joined by spot welding, the arithmetic mean value h of the plate thickness can be calculated 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 any warping or floating, the plate thickness should be clamped or otherwise measured, and the result should be divided by the number of overlapping plates to obtain h. When measuring from a welded joint, it is preferable to measure the plate thickness at a point that is as free of deformation due to welding as possible. If it is not possible to measure with a caliper or the like, it may 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 in the inside of 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 applied pressure F E If the applied pressure F exceeds "4500×h" (N), the area of ​​the steel plates 1A and 1B where the welding electrodes 2A and 2B contact each other will be significantly dented. This will not only impair the appearance, but also reduce the strength of the joint. E In order to obtain this, the welding gun (a device that applies a pressure to the welding electrodes 2A and 2B and energizes them) needs to have a highly rigid robot arm. Therefore, in the present disclosure, the pressure F of the welding electrodes 2A and 2B against the steel plates 1A and 1B is E shall be greater than or equal to "2000×h" (N) and less than or equal to "4500×h" (N).

[0054] 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 the condition for forming the fusion zone that will become the nugget that joins all the steel plates that make up the plate assembly. The welding current and current flow time that are the same as those conventionally used to stably obtain a nugget of the required size are set as the welding current I w0 and the actual welding time t w0can be adopted as.

[0055] When measuring the pressing force, if it is built in the spot welder to be used, the displayed pressing value of the welder may be used, or a pressure gauge such as a quartz piezoelectric sensor may be attached to a transmission shaft equivalent to the pressing load on the steel plate for measurement. Regarding the current value, it can be measured using a current monitor called a weld checker or a welding current meter. Although the measurement method is not specified, for example, a method of inserting a toroidal coil into the circuit through which current flows during welding and reading the change in the current value may be used.

[0056] The welding current value I in this welding process w0 Preferably, a current value that can obtain a desired nugget diameter is adopted in consideration of the total plate thickness t of the plate assembly, etc. The energization time t in this welding process w0 For example, when the plate thickness of the thinnest steel plate in the plate assembly is t’ (mm), the energization 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).

[0057] Regarding the plate thickness of the plate assembly, from the viewpoints of joint strength and prevention of scattering, it is preferable to aim for a nugget diameter of 4√t’ or more with respect to the plate thickness t’ on the thin plate side at each plate interface. More preferably, the nugget diameter is 5√t’ or more. In order to form such a nugget diameter of 5√t’ or more without generating scattering, a gradual increase (up slope) of 1 cycle to 80 cycles (50 Hz) may be set before this welding process. When performing up slope energization, the current value at the end of the up slope is taken as the current value I w0 (kA) in this welding process, and the energization time t w0 (ms) in this welding process does not include the time taken for the up slope. In the energization pattern shown in FIG. 3, until the current value reaches the welding current I w0 (kA), the current value is up-sloped from 0 (zero), and welding is performed with the welding current I w0 (kA). Before this welding process, pre - energization may be performed at a current value lower than that of this welding process, for example, for 2 to 80 cycles. This pre - energization may be multi - stage energization, or a non - energization time may be provided in the middle.

[0058] By this welding process, a molten part that becomes a nugget 13 and a heat - affected zone (so - called HAZ) 14 are formed at the energized part between the steel plate 1A and the steel plate 1B at the end of spot welding.

[0059] For spot - welding equipment, conventional general spot - welding equipment can be used as it is. Also, for welding electrodes and the like, conventional welding electrodes can be used as they are. The power source is not particularly limited, and an AC power source, a DC inverter, an AC inverter, etc. can be used.

[0060] Note that if the tip diameters of the welding electrodes 2A and 2B become too large, the surface pressure at the tips of the welding electrodes 2A and 2B will decrease. The tip diameters of the welding electrodes 2A and 2B are preferably about 6 mm to 8 mm.

[0061] [First post - energization process] After this welding process, as the first post - energization process (the first post - energization process), after a non - energization period in which the energization is stopped for a time t c1 (ms) that satisfies the following formula (C), a first post - energization current value I w1 (kA) that satisfies the following formula (D) is energized for a time t w1 (ms) that satisfies the following formula (E). 2 ≤ t c1 ≤ 300 (C) 0.75×I w0 < I w1 < I w0 (D) t w1 > 100 (E)

[0062] That is, after energizing the welding current I w0 to the welding electrodes 2A and 2B for a predetermined time to form a molten part in the steel plates 1A and 1B, and then making the current value zero after the welding is completed, immediately after the welding (when the welding current I w0 is being energized), the pressing force FE While maintaining the same, for a time t c1 (ms) that satisfies equation (C), power supply is suspended. This causes the molten part formed by this welding to solidify from the outer periphery of the molten part (i.e., the boundary with other regions of the molten part). In the present disclosure, the boundary between the molten part and other regions is referred to as the "molten boundary".

[0063] This welding current I w0 Immediately after the energization of ends, the solidification of the molten part starts from the molten boundary. As the molten part solidifies, the nugget 13 is formed, and a heat-affected zone 14 is formed outside the molten boundary.

[0064] After this welding process, if the non-energization time t c1 is less than 2 ms, there is a risk that the nugget end will not solidify before the subsequent post-energization. On the other hand, if the non-energization time t c1 exceeds 300 ms, there is a risk that the nugget end will solidify too much before the subsequent post-energization. To avoid post-energization in a state where the solidification of the nugget end is insufficient or overly solidified and to appropriately proceed with the solidification of the nugget end, the non-energization time t c1 after this welding process is set to be 2 ms or more and 300 ms or less, and preferably 40 ms or more and 250 ms or less.

[0065] In the first post-energization process, following the non-energization where power supply is suspended for a time t c1 (ms) that satisfies equation (C), a first post-energization current value I w1 (kA) that satisfies equation (D) is energized for a time t w1 (ms) that satisfies equation (E). That is, the first post-energization current value I w1 is smaller than the welding current value I w0 of this welding process and larger than 0.75×I w0 , and the first post-energization time t w1 is set to be a time exceeding 100 ms. By performing the first post-energization under such conditions, as shown in FIG. 4, at least a part of the nugget part is remelted without exceeding the molten boundary formed in this welding process, and the non-energization time t c1It is preferable to transform the γ-phase (fcc crystal structure) formed in the plunger part into the δ-phase (bcc crystal structure) by cooling.

[0066] The first post-energization time t in the first post-energization process w1 is preferably 110 ms or more and 1000 ms or less, and more preferably 150 ms or more and 800 ms or less, from the viewpoints of remelting of the plunger part and shortening of the time for the entire spot welding.

[0067] In addition, the pressing force F in the first post-energization process E is preferably maintained as the pressing force F in this welding process E for work efficiency. However, the pressing force F c1 in the non-energization time t E may be a different pressing force F E from the pressing force F E in this welding process within the range satisfying the above formula (B).

[0068] [Second Post-energization Process] The method for manufacturing a spot weld joint according to the present disclosure performs at least one post-energization process as the second and subsequent post-energization processes after the first post-energization process. After the first post-energization process (the first post-energization process), in the second post-energization process (the second post-energization process), after the first post-energization process, energization is stopped for a time t c2 (ms) satisfying the following formula (F1), and then the second post-energization current value I w2 (kA) satisfying the following formula (G1) is energized for the second post-energization time t w2 (ms). 2 ≦ t c2 ≦ 300 (F1) 0.004 × t c2 2 -0.3125 × t c2 +102 ≦ I w2 / I w0 × t w2 ≦ 0.0156 × t c2 2 -0.625 × t c2 +300 (G1)

[0069] In addition, the presence or absence of the occurrence of a phase transformation and the position of the phase transformation due to each post - energization process can be determined by observing a cross - section near the center of the nugget by EBSD as shown in FIG. 8 and by the fine - grain regions of the crystal grains before and after each post - energization process. After forming a molten portion that becomes a nugget by the main energization, by performing phase transformations at appropriate positions by post - energization two or more times that satisfy the conditions of the present disclosure, a nugget can be formed in which the percentage of the number of grains having an aspect ratio of 7 or more is 50% or less near the center of the nugget as described later.

[0070] After the first post - energization process, the non - energization time t c2 is less than 2 ms, there is a risk that the nugget end will not solidify before the second post - energization process. On the other hand, when the non - energization time t c2 exceeds 300 ms, there is a risk that the nugget end will solidify too much before the 2 post - energization process. To avoid the second post - energization in a state where the solidification of the nugget end is insufficient or in a state where it is excessively solidified and to appropriately advance the solidification of the nugget end, the non - energization time t c2 after the first post - energization process is set to 2 ms or more and 300 ms or less, and preferably 60 ms or more and 250 ms or less.

[0071] In the second post - energization process, with the non - energization time t c2 , the nugget part is cooled to become a γ phase (fcc crystal structure). Then, by performing post - energization under the condition that the second post - energization current value I w2 and the second post - energization time t w2 satisfy the above formula (G1) with respect to the main welding current value I w0 of this welding process and the non - energization time t c2 of the second post - energization process, the nugget part is phase - transformed again into a δ phase (bcc crystal structure) as shown in FIG. 4. As a result, the equiaxed grain refinement of the nugget part progresses, and the CTS can be improved.

[0072] Also, in the method for manufacturing a spot - welded joint according to the present disclosure, similar to the relationship between the main welding current value I w0 and the first post - energization current value I w1 , the current value I w2 in the second post - energization process is 0.75×Iw0 Increase it so that the main welding current value I w0 and the second post - energization current value I w2 (kA) satisfy the following formula (H1) during energization. 0.75×I w0 <I w2 (H1) The main welding current value I w0 and the second post - energization current value I w2 If they do not satisfy the relationship of the above formula (H1), even if energization is performed for a long time, sufficient heat input cannot be given, and the position where equiaxed grain refinement in the nugget occurs remains at the center of the nugget or does not change, so the effect of improving CTS cannot be obtained.

[0073] Furthermore, the relationship between the main welding current value I w0 and the second post - energization current value I w2 preferably satisfies the following formula (I). I w2 >I w0 (I) Figure 5 shows an example of an energization pattern that satisfies formula (I). In this way, by making the second post - energization current value I w0 larger than the main welding current value I w2 equiaxed grain refinement in the nugget can be surely promoted, and CTS can be improved.

[0074] After the second post - energization, when stopping energization while maintaining the applied pressure F E as it is and finishing the post - energization process in two times, release the applied pressure F E by the welding electrodes 2A, 2B from the overlapped steel plates 1A, 1B (plate set). In addition, it is preferable to provide a so - called holding time after the second post - energization process where only pressure is applied without energization. The holding time is desirably 5 cycles (50 Hz) or more.

[0075] In addition, if the applied pressure F E in the second post - energization process is also maintained as it is the applied pressure F E in the main welding process following the first post - energization process, it is preferable in terms of work efficiency. However, for the applied pressure F c2 during the non - energization time t EWithin the range that satisfies the above formula (B), the pressing force F in this welding process E may be a pressing force F different from that E That is, the pressing force F from this welding process until the end of the last post - energization process E may be constant or may vary as long as it is within the range that satisfies the formula (B).

[0076] A down - slope may be provided after the second post - energization process. Due to the down - slope, the characteristics of the spot - welded portion can be further improved by the effects of reducing cracks caused by liquid metal embrittlement, reducing blowholes, and suppressing delayed fracture. When performing down - slope energization, the current value at the start of the down - slope is set to the second post - energization current value I w2 (kA) in the second post - energization process, and the second post - energization time t w2 (ms) in the second post - energization process does not include the time taken for the down - slope.

[0077] By performing resistance spot welding consisting of the above - described respective processes on a stack of a plurality of steel plates including at least one high - strength steel plate, the CTS can be significantly improved compared to the case of performing resistance spot welding with single energization. Further, according to the present disclosure, the joint strength can be effectively improved with a shorter welding time compared to conventional temper energization, etc.

[0078] Here, the effect of improving the CTS by performing the second post - energization process while satisfying the above formula (G1) will be described. For a stack of two high - strength steel plates having the chemical composition (unit: mass%, balance: Fe and impurities), plate thickness, and tensile strength (TS) shown in Table 1 below, spot welding was performed by performing this welding energization and post - energization under the conditions shown in Table 2, and a welded joint was manufactured. For the obtained welded joint, the CTS was measured by a method conforming to JIS Z 3137; 1999. In Table 2, "cool" means the non - energization time (cooling time) when the energization is paused.

[0079]

Table 1

[0080]

Table 2

[0081] In Test Example 0, after the main welding energization, it was carried out until the first post-energization. Based on the CTS of the joint in Test Example 0, the CTS improvement rates of the joints in Test Examples 1 to 5 carried out until the second post-energization are shown in Table 2. Those with a CTS improvement rate exceeding 10% with respect to one-time post-energization (Test Example 0) were judged to have a CTS improvement effect "yes", and those with 10% or less were judged to have no CTS improvement effect. Also, each joint was cut in the plate thickness direction so as to pass through the center of the nugget, and the crystal grains near the center of the nugget were observed by electron backscatter diffraction (EBSD), and the observation results are shown in Fig. 7. In addition, the portion where the crystal orientation difference is 15 degrees or more was defined as the grain boundary. The region surrounded by the dotted square is the fine grain region formed by the first post-energization, and the region surrounded by the solid square is the fine grain region formed by the second post-energization. From the results shown in Table 2 and Fig. 7, the following is inferred.

[0082] In Test Example 1, I w2 / I w0 ×t w2 The value of is below the lower limit of the (G1) formula, the heat input is insufficient, the fine grains generated by the second post-energization are quite close to the center of the nugget, and it is the position reached after significant crack propagation in the joint strength test, so it does not contribute to the improvement of joint strength. In Test Examples 2 to 4, I w2 / I w0 ×t w2 The value of satisfies the (G1) formula, is an appropriate heat input, the fine grains generated by the second post-energization are at a certain distance from the center of the nugget, and are inside the outermost of the fine grains generated by the first post-energization, so it effectively works to improve toughness. In Test Example 5, I w2 / I w0 ×t w2 The value of exceeds the upper limit of the (G1) formula, and the fine grains generated by the second post-energization exceed the fine grain region generated by the first post-energization, so the fine grains generated by the first post-energization have disappeared, and a high joint strength improvement effect cannot be obtained.

[0083] (After the third and subsequent post - energization processes) In the method for manufacturing a spot - welded joint according to the present disclosure, following the second post - energization process, with the pressing force F E held as it is, no - energization and post - energization may be alternately repeated further, and the post - energization process may be performed 3 or more times and N or less times (N is an integer of 3 or more). In the post - energization processes after the third time, all have the same relational expressions as in the second post - energization process, that is, after the (n - 1) - th post - energization process (n is an integer from 3 to N), after no - energization for a time t cn (ms) that satisfies the following (F) formula, following which, a current value I wn (kA) of the n - th post - energization that satisfies the following (G) and (H) formulas is applied for the n - th post - energization time t wn (ms). In the n - th post - energization process, which is the post - energization process after the second time (n is an integer from 3 to N), after no - energization for a time t cn (ms) that satisfies the following (F) formula after the (n - 1) - th post - energization process, a current value I wn (kA) that satisfies the following (G) formula is applied for the n - th post - energization time t wn (ms). 2≦t cn ≦300 (F) 0.004×t cn 2 -0.3125×t cn +102≦I wn / I w0 ×t wn ≦0.0156×t cn 2 -0.625×t cn +300 (G) 0.75×I w0 <I wn (H) In the n - th post - energization process, it is preferable to cause a phase transformation inside the outermost position where a phase transformation occurred in the (n - 1) - th post - energization process.

[0084] When the post - energization process is performed three or more times, a down - slope may be provided after the last post - energization process. By means of the down - slope, the characteristics of the spot - welded part can be further improved due to the effects of reducing cracks caused by liquid - metal embrittlement, reducing blowholes, and suppressing delayed fracture. In addition, when performing down - slope energization after the last post - energization process, the current value at the start of the down - slope is taken as the energization current value (kA) in the last post - energization process, and the post - energization time (ms) of the last post - energization process does not include the time taken for the down - slope.

[0085] Similar to the second post - energization process, for the third and subsequent post - energization processes, by repeating post - energization following non - energization so as to satisfy the formulas (F), (G), and (H), it is considered that equiaxed grain refinement progresses due to the phase transformation from the γ phase (fcc crystal structure) to the δ phase (bcc crystal structure) inside the nugget (inside the fusion boundary), and thus the CTS is further improved. However, the welding time becomes longer as the number of post - energization processes increases. Therefore, the number of post - energization processes is preferably two or more and four or less, more preferably two or more and three or less, and particularly preferably two.

[0086] <Spot - welded joint> According to the method for manufacturing a spot - welded joint according to the present disclosure, the following spot - welded joint can be manufactured. That is, the spot - welded joint according to the present disclosure includes a spot - welded part where a plurality of stacked steel plates are joined. Among the plurality of steel plates, at least one steel plate is a high - strength steel plate with a carbon equivalent Ceq represented by the following formula (A) being 0.36 mass% or more when the respective contents of C, Si, Mn, P, and S in mass% are [C], [Si], [Mn], [P], and [S] respectively. When observing the inside of the fusion boundary of the nugget in the cross - section in the plate - thickness direction passing through the center of the nugget of the spot - welded part, the portion with a crystal orientation difference of 15 degrees or more in the middle part in the major - axis direction of the nugget is regarded as the grain boundary, and the percentage of the number of 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 weld joint according to the present disclosure can be manufactured in a shorter welding time compared to the case where temper energization is performed after performing this welding on a plate assembly including a high-strength steel plate with Ceq of 0.36 mass% or more, and can have a higher joint strength compared to a spot weld joint spot-welded only by this welding process.

[0088] (Method for measuring the proportion of crystal grains with an aspect ratio of 7 or more) FIG. 6 is a diagram showing a region for measuring the aspect ratio of crystal grains in the welded portion of the spot weld joint. For a cross-section in the plate thickness direction passing through the center of the nugget of the spot weld portion, the crystal orientation inside the fusion boundary of the nugget is measured using EBSD analysis. As shown in FIG. 6, an intermediate portion M including the position of the virtual line L2 0.5 mm to the left and right in the plate thickness direction passing through the center of the nugget and the fusion boundary of one steel plate at the plate interface L1 is taken as the measurement field of view. Since the steel structure of the nugget is substantially vertically symmetric with the plate interface L1 as the boundary, it is sufficient to measure the intermediate portion M on one plate side as shown in FIG. 6. Then, grain boundaries with a crystal orientation difference of 15 degrees or more, which are block grain boundaries and prior austenite grain boundaries, are drawn in the measurement field of view, and the aspect ratio of each crystal grain is calculated. The maximum length of the crystal grain is taken as the major axis, and the aspect ratio (major axis / minor axis) is calculated with the distance between two parallel lines parallel to the major axis direction and in contact with the crystal grain at the widest interval 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 may be observed in a size including 100 or more grains. In the above field of view, the aspect ratio values are obtained for 100 or more grains, and the proportion of grains with an aspect ratio of 7 or more is calculated.

[0089] The spot weld joint according to the present disclosure has a ratio of grains with an aspect ratio value of 7 or more, as measured above, of 50% or less. That is, for the plate assembly including the high-strength steel plate described above, after this welding, by performing the post-electrification process two or more times under the above-described conditions, equiaxed grain refinement in the nugget portion is promoted, and a spot weld joint is obtained in which the ratio of grains with an aspect ratio of less than 7 is half (50%) or more. Such a spot weld joint can exhibit a higher CTS than when the percentage of grains with an aspect ratio of 7 or more exceeds 50%. Note that the ratio of grains with an aspect ratio value of 7 or more is preferably 40% or less, and more preferably 30% or less.

Example

[0090] Hereinafter, the manufacturing method of the resistance spot weld joint according to the present disclosure will be described by way of examples. Note that the manufacturing method of the resistance spot weld joint according to the present disclosure is not limited to these examples.

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

[0092]

Table 3

[0093] Each steel plate was combined and resistance spot welding was performed under the conditions (plate assembly, pressing force, energization conditions, etc.) shown in Table 4 to manufacture a weld joint. The CTS of the obtained weld joint was measured. For the joint obtained by spot welding a three-layer plate assembly in No. 24, the CTS was measured at the interface with the steel plate (Ceq: 0.36 mass% or more) targeted in the present disclosure, that is, at the interface between steel plate B and steel plate C. Note that steel plate D in 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 the spot weld joint according to the present disclosure can be applied, and no underline is attached.

[0094] In the case of the joint of a stack of three steel plates of the same type at No. 25, since the CTS values of the two plate interfaces are equal, the CTS of either interface may be measured. However, cross tension was performed on the interface that was on the upper side during welding to measure the CTS.

[0095] For each stack of plates with each number, after the main welding energization under the conditions for each number, the CTS (reference CTS) of the welded joint up to the first post-energization was measured. The increase rate was calculated for comparison with this reference CTS, and those exceeding 10% were judged to have an effect of improving the joint strength.

[0096]

Table 4

[0097] For Nos. 1 to 9, after the main welding energization, up to the second post-energization was carried out, but none of them satisfied any one of the formulas (A) to (H) in the present disclosure, and no CTS improvement effect was obtained. Note that for No. 1, the ratio of grains with an aspect ratio of 7 or more was 50% or less, but the Ceq of the steel plate C constituting the stack of plates was low, and the CTS was high even for the joint up to the first post-energization. Therefore, no CTS improvement effect was obtained by the second post-energization. No. 14 is an example where only the main welding energization was carried out. No. 15 is an example where after the main welding energization, for tempering, post-energization (so-called tempering energization) with a relatively small current value for a long time was carried out. Although a CTS improvement effect was obtained, it is necessary to also lengthen the non-energization process time after the main welding energization, and it is difficult to say that the CTS is effectively improved for the entire spot welding process. Note that the CTS increase rates for Nos. 14 and 15 were calculated in comparison with the CTS of the welded joint up to the first post-energization after the main welding energization under the conditions of No. 16 using the same stack of plates (two steel plates B). For Nos. 18 to 22, after the main welding energization, up to the second post-energization was carried out, but none of them satisfied any one of the formulas (A) to (H1) in the present disclosure, and no CTS improvement effect was obtained.

[0098] On the other hand, all the numbers classified into the examples satisfy the formulas (A) to (H) in the present disclosure, and the CTS improvement effect has been obtained. Note that No. 13 is an example in which the energization is performed up to the third post-energization, and the CTS improvement rate is the highest. In Table 4, the non-energization, current value, and time in the third post-energization are described separately in the column of the second post-energization 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 described in this specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually recited.

Explanation of Reference Numerals

[0100] 1A, 1B Steel plates 2A, 2B Welding electrodes 13 Nugget 14 Heat affected zone (HAZ)

Claims

1. A method for manufacturing a spot weld joint, comprising sandwiching a stack of multiple steel plates in the plate thickness direction between a pair of welding electrodes, applying pressure, and energizing in the pressurized state to perform spot welding, wherein at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq represented by the following formula (A) of 0.36% by mass or more, when the respective contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S] in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) When the arithmetic mean value of the thicknesses of the plurality of steel plates is h (mm), the plate stack is pressurized with a pressing force F Ｅ (N) in a pressurized state by the pair of welding electrodes, 2000 × h ≤ F Ｅ ≤ 4500 × h (B) Apply the present welding current value I ｗ０ (kA) to the pair of welding electrodes and perform the present welding to form a molten part in the plate assembly in the present welding step, after the main welding process, there are two post-energizing processes of energizing, and it includes The first post - energization process, which is the first of the above - mentioned post - energization processes, after the main welding process, has a power - off period of time t (ms) that satisfies the following formula (C), followed by energization with a first post - energization current value I (kA) that satisfies the following formula (D) for a time t (ms) that satisfies the following formula (E). ｃ１ After the power - off period that satisfies the following formula (C) for a time t (ms) after the main welding process, the first post - energization current value I ｗ１ (kA) is energized for a time t ｗ１ (ms) and 2 ≤ t ｃ１ ≤ 300 (C) 0.75 × I ｗ０ < I ｗ１ < I ｗ０ (D) t ｗ１ > 100 (E) The second post-energization process, which is the second post-energization process, after the first post-energization process, the time t that satisfies the following (F1) formula ｃ２ (ms) After the power-off that pauses the energization, the second post-energization current value I that satisfies the following (G1) formula and the following (H1) formula ｗ２ (kA), for a time t ｗ２ (ms) energize, 2 ≤ t ｃ２ ≤ 300 (F1) 0.004 × t ｃ２ ２ −0.3125 × t ｃ２ +102 ≤ I ｗ２ / I ｗ０ × t ｗ２ ≤ 0.0156 × t ｃ２ ２ −0.625 × t ｃ２ +300 (G1) 0.75 × I ｗ０ <I ｗ２ (H1) The steps from the said main welding process to the last post-current application process are continuously carried out while maintaining the said pressing force F Ｅ (N) within the range satisfying the said formula (B), a method for manufacturing a spot weld joint.

2. The base welding current value I ｗ０ and the second post-welding current value I ｗ２ The method for manufacturing a spot weld joint according to claim 1, wherein the relationship therebetween satisfies the following formula (I). I ｗ２ > I ｗ０ (I)

3. Including the post-energization process three or more times and N or less times (N is an integer of 3 or more), the nth post-energization process (n is an integer of 3 or more and N or less), which is the post-energization process after the third time, is a time t that satisfies the following formula (F) after the (n - 1)th post-energization process, which is the (n - 1)th post-energization process. ｃｎ After the non-energization period of suspending energization for a time t (ms) that satisfies the following formula (F), the nth post-energization current value I ｗｎ (kA) that satisfies the following formula (G) and the following formula (H) is energized for a time t ｗｎ (ms). The method for manufacturing a spot welding joint according to claim 1 or claim 2. 2 ≤ t ｃｎ ≤ 300 (F) 0.004 × t ｃｎ ２ −0.3125 × t ｃｎ +102 ≤ I ｗｎ / I ｗ０ × t ｗｎ ≤ 0.0156 × t ｃｎ ２ −0.625 × t ｃｎ +300 (G) 0.75 × I ｗ０ < I ｗｎ (H)

4. including a spot weld portion where multiple stacked steel plates are joined, at least one of the multiple steel plates is a high-strength steel plate having a carbon equivalent Ceq represented by the following formula (A) of 0.36% by mass or more, when the respective contents of C, Si, Mn, P, and S are [C], [Si], [Mn], [P], and [S] in mass%, Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2[P] + 4[S] (A) 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 portion, in the middle portion in the major axis direction of the nugget, a portion having a crystal orientation difference of 15 degrees or more is defined as a grain boundary, and the percentage of the number of grains having an aspect ratio of 7 or more is 50% or less, a spot weld joint

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