Method for manufacturing spot-welded joint, and spot-welded joint

Abstract

This method for manufacturing a spot-welded joint comprises: a main welding step for forming a molten section by energizing a main welding current value Iw0 (kA) so that prescribed conditions are satisfied with respect to a plate group comprising a high-strength steel plate having a Ceq of 0.36 mass% or more; a first post-energization step in which energization is performed for a time tw1 at a first post-energization current value Iw1 after no energization for a time tc1 (ms); and a second post-energization step in which, after the first post-energization step, energization is performed for a time tw2 at a current value Iw2 after no energization for a time tc2, and a step from the main welding step to the last post-energization step is continuously performed in a state in which the pressing force FE (N) is maintained. The spot-welded joint has a number ratio of crystal grains having an aspect ratio of 7 or more at an intermediate portion of the nugget in the longitudinal axis direction of 50% or less.

Description

Technical Field

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

[0002] In recent years, in the automotive field, in order to reduce fuel consumption and CO 2 To reduce emissions, the vehicle body is required to be lightweight. In addition, to improve collision safety, the vehicle body components are required to be high-strength. In order to meet these requirements, it is effective to use high-strength steel plates for the vehicle body and components. For the assembly of the vehicle body and the installation of components, spot welding is mainly used.

[0003] Tensile strength is an important property in a joint formed by spot welding a plate group formed by overlapping a plurality of steel plates (also referred to as a "spot welded joint" in this disclosure). The tensile strength of a spot welded joint includes tensile shear stress (TSS) measured by applying a tensile load in the shear direction and cross tensile stress (hereinafter sometimes referred to as "CTS" or "joint strength") measured by applying a tensile load in the peeling direction.

[0004] Generally speaking, when a spot weld joint is made using a high-strength steel plate, the deformation capacity of the welding electrode due to pressurization is small, and the stress concentration on the weld portion increases. In addition, the toughness of the weld portion is reduced due to the quenching effect generated in the weld portion. Therefore, the CTS of the spot weld joint containing the high-strength steel plate tends to be low, and an improvement in CTS is required.

[0005] In order to ensure the strength and toughness of spot welded joints using a plate group including high-strength steel plates, the following methods have been proposed: a method of performing tempering energization to temper the nugget and heat-affected zone by performing energization again after cooling for a certain period of time after performing formal welding to form a molten portion that becomes a nugget; and a method of performing post-energization within a relatively short period of time after formal welding.

[0006] For example, Patent Document 1 discloses a resistance spot welding method, which forms a weld by applying a clamping pressure to two or more overlapping high-strength thin steel plates using a pair of electrodes while passing an electric current, and which sequentially comprises the following steps: a first step of passing a welding current (Im) to form a nugget; a second step of passing electricity to the weld at a current value less than the welding current (Im); a third step of cooling the weld; and a fourth step of passing electricity to the weld at a current value greater than the welding current (Im) to heat the weld to a recrystallization temperature range.

[0007] In addition, Patent Document 2 discloses a resistance spot welding method, which welds a plate group with a total plate thickness of t (mm) formed by overlapping two or more steel plates while clamping and pressurizing them with a pair of welding electrodes and applying electricity, and comprises the following steps: a first step of forming a nugget; and a second step of applying electricity after cooling the weld portion by holding it without applying electricity in a state of being pressurized by the electrodes, wherein the power-on time TA (ms) and the power-on current IA (kA) in the first step, and the power-on time TB (ms) and the power-on current IB (kA) in the second step satisfy equations (1) and (2), the power-on current IB (kA) is higher than the power-on current IA (kA), and the holding time Th (ms) without applying electricity in the second step is in accordance with the total plate thickness t (mm) of the plate group, the diameter d (mm) of the nugget, and the area S (mm) of the tip end portion of the welding electrode. 2 ) satisfies formula (3).

[0008] 0.05<(IB2×TB) / (IA2×TA)<1.0 (1)

[0009] 20≤TB≤100 (2)

[0010] 10×(t×d2) / S <Th<200×(t×d2) / S (3)

[0011] Patent document 3 discloses a spot welding method as a spot welding method for improving the cross tensile force of a spot welding joint, which comprises the following steps of repeating the cooling / post-energizing process once or twice or more: after a plurality of steel plates including high-strength steel plates having a tensile strength of 750 to 2500 MPa and a specified carbon equivalent Ceq of 0.20 to 0.55 mass % are overlapped and a molten portion is formed by formal welding, the energizing is stopped and the steel plates are cooled to form a solidification domain in the molten portion, and then the post-energizing is continued in a manner that the solidification domain does not re-melt, thereby performing the first cooling / post-energizing, and after the first cooling / post-energizing process is completed, the specified pressure F is maintained. E (N), stop cooling time t in a manner that meets the specified conditions S (ms) of power-on, and then the power-on current I P (kA) After the power-on time t P The current is applied for 10 ms, and the post-current is applied so that the solidification region does not remelt.

[0012] Furthermore, Patent Documents 4 to 6 also disclose spot welding methods in which post-energization is performed after main energization.

[0013] Patent Document 1: Japanese Patent No. 5895430

[0014] Patent Document 2: Japanese Patent No. 5891741

[0015] Patent Document 3: Japanese Patent No. 6409470

[0016] Patent Document 4: International Publication No. 2016 / 139952

[0017] Patent Document 5: Japanese Patent Application No. 2018-30178

[0018] Patent Document 6: Japanese Patent Application Publication No. 2013-78782 Summary of the invention

[0019] Problems to be solved by the invention

[0020] The present disclosure aims 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 a case where a spot welded joint is manufactured by performing post-energization once after main welding using a high-strength steel plate.

[0021] Means for solving problems

[0022] The gist of the present disclosure for achieving the above-mentioned object is as follows.

[0023] <1> A method for manufacturing a spot welding joint, wherein a plate group formed by overlapping a plurality of steel plates is sandwiched in a plate thickness direction by a pair of welding electrodes and energized under pressure to perform spot welding.

[0024] When the contents of C, Si, Mn, P, and S are set to [C], [Si], [Mn], [P], and [S], respectively, in terms of mass %, 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 mass % or more,

[0025] Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A)

[0026] It includes the following steps:

[0027] The following formal welding process is performed: when the arithmetic mean value of the thickness of the plurality of steel plates is set to h (mm), the plate set is welded by the pair of welding electrodes at a pressure F satisfying the following formula (B): E (N) Under the condition of pressurization, the pair of welding electrodes is energized with a welding current value I w0 (kA), a molten portion is formed in the above plate group,

[0028] 2000×h≤F E ≤4500×h(B); and

[0029] After the above-mentioned main welding process, a second post-energization process is performed.

[0030] In the first post-energization process, that is, the first post-energization process, after the main welding process, the welding process stops at a time t satisfying the following formula (C): c1 After the energization of (ms) and the non-energization, the first post-energization current value I satisfies the following formula (D) w1 (kA) satisfies the following equation (E) for a period of time t w1 (ms) of power on,

[0031] 2≤t c1 ≤300(C)

[0032] 0.75×I w0 w1 w0 (D)

[0033] t w1 >100(E)

[0034] In the second post-energization process, that is, the second post-energization process, after the first post-energization process, the following formula (F1) is satisfied at the time t c2 After the energization of (ms) and the non-energization, the second post-energization current value I that satisfies the following (G1) formula and the following (H1) formula is w2 (kA)Performance time t w2 (ms) of power on,

[0035] 2≤t c2 ≤300(F1)

[0036] 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)

[0037] 0.75×I w0 w2 (H1)

[0038] When the above-mentioned pressure F E (N) The steps from the main welding step to the final post-energization step are continuously performed while maintaining the range satisfying the above-mentioned formula (B). ​​​

[0039] <2> according to <1> The method for manufacturing a spot welding joint, wherein the above-mentioned formal welding current value I w0 The second current value I w2 The relationship satisfies the following formula (I).

[0040] <3> according to <1> or <2> The method for manufacturing a spot welding joint comprises the following steps: performing the post-energization step 3 to N times (N is an integer greater than 3); in the post-energization step after the third time, i.e., the nth post-energization step (n is an integer from 3 to N), after the (n-1)th post-energization step, i.e., the (n-1)th post-energization step, the welding process is stopped for a time t which satisfies the following formula (F): cn After the energization of (ms) and the non-energization, the nth subsequent energization current value I satisfies the following formula (G) and the following formula (H) wn (kA)Performance time t wn (ms) of power on.

[0041] 2≤t cn ≤300(F)

[0042] 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)

[0043] 0.75×I w0 wn (H)

[0044] <4> A spot welded joint comprises a spot welded portion formed by joining a plurality of overlapping steel plates.

[0045] When the contents of C, Si, Mn, P, and S are set to [C], [Si], [Mn], [P], and [S], respectively, in terms of mass %, 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 mass % or more,

[0046] Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A)

[0047] ​When observing the inner side of the molten boundary of the above-mentioned molten core in the cross section in the plate thickness direction passing through the center of the molten core of the above-mentioned spot welding portion, in the middle part of the long axis direction of the above-mentioned molten core, the portion with a crystal orientation difference of more than 15 degrees is used as the crystal grain boundary, and the number ratio of grains with an aspect ratio of more than 7 is less than 50%.

[0048] Effects of the Invention

[0049] According to the present disclosure, a method for manufacturing a spot welded joint and a spot welded joint can be provided that can effectively improve the cross tensile strength of the joint compared to the case where a spot welded joint is manufactured by performing post-energization once after main welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a schematic diagram showing an example of the arrangement of two steel plates and welding electrodes when resistance spot welding is started on a plate set formed by overlapping two steel plates.

[0051] Figure 2 This 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 plate set formed by stacking two steel plates.

[0052] Figure 3 This is a diagram showing an example of a current-carrying pattern in the method for manufacturing a spot welded joint according to the present disclosure.

[0053] Figure 4 This is a diagram simulating the temperature history of a welded portion obtained by the method for manufacturing a spot welded joint according to the present disclosure.

[0054] Figure 5 It is a diagram showing another example of the energization pattern of the method for manufacturing a spot welded joint according to the present disclosure.

[0055] Figure 6 This is a diagram showing an area where the aspect ratio of crystal grains is measured in a welded portion of a spot welded joint.

[0056] Figure 7 This is a diagram showing the relationship with the formula (G1) for each example in which the second post-energization condition is changed.

[0057] Figure 8 Is for through Figure 7 The joints spot welded according to the examples shown in FIG. 1 show a fine grain region of crystal grains formed near the center of the nugget. DETAILED DESCRIPTION

[0058] Hereinafter, an embodiment as an example of the present disclosure will be described.

[0059] It should be noted that, in the present disclosure, the “%” expression of the content of each element means “mass %”. In addition, in the present disclosure, the numerical range expressed by “to” means a range including the numerical values ​​recorded before and after “to” as the lower limit and the upper limit unless otherwise specified. In addition, the numerical range in which the numerical values ​​recorded before and after “to” are annotated with “exceeding” or “below” means a range not including these numerical values ​​as the lower limit or the upper limit.

[0060] In the numerical ranges recorded in stages in the present disclosure, the upper limit of a certain numerical range may also be replaced by the upper limit of other numerical ranges recorded in stages, and may also be replaced by the values ​​shown in the embodiments. In the numerical ranges recorded in stages in the present disclosure, the lower limit of a certain numerical range may also be replaced by the lower limit of other numerical ranges recorded in stages, and may also be replaced by the values ​​shown in the embodiments.

[0061] The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process can be achieved.

[0062] The inventors of the present disclosure have conducted intensive research on a method for effectively improving the strength of a spot welded joint when a plate set including a high-strength steel plate having a tensile strength of, for example, 900 MPa or more is spot welded. As a result, they found that the strength of the joint can be effectively improved by: applying a predetermined pressure F corresponding to the plate thickness of the high-strength steel plate to the plate. E (N) In a pressurized state, post-energization is performed multiple times (at least twice) after the formal welding, and in the first post-energization, the current value is made smaller than that in the formal welding, and in the second post-energization, the post-energization is performed with a current value and a power-on time that satisfy a prescribed relationship relative to the cooling time just before and the current value of the formal welding.

[0063] In addition, the steel structure of the spot welded portion manufactured by the manufacturing method of the spot welding joint disclosed in the present invention was observed, and it was found that the equiaxed grain refinement of the grains inside the weld nugget was promoted, and the morphology of the grains was different from the case where spot welding was performed only by formal welding and the case where tempering and energization were performed after formal welding.

[0064] Hereinafter, embodiments of the present disclosure will be described.

[0065] [High-strength steel plate]

[0066] First, the high-strength steel plates included in the plate group to be spot-welded in the method for producing a spot-welded joint of the present disclosure will be described.

[0067] (tensile strength)

[0068] If the tensile strength of the multiple steel plates of the plate group to be spot welded is, for example, less than 900 MPa, it is easy to obtain a high joint strength and it is difficult to cause a problem of joint strength. Therefore, in the method for manufacturing a spot welded joint disclosed in the present invention, a plate group is used in which multiple steel plates including at least one high-strength steel plate (sometimes referred to as "high-strength steel plate" in the present disclosure) with a carbon equivalent Ceq of 0.36 mass% or more are overlapped.

[0069] The plurality of steel plates constituting the plate group may all be high-strength steel plates having a carbon equivalent Ceq of 0.36 mass % or more, or at least one of the steel plates may be a high-strength steel plate having a carbon equivalent Ceq of 0.36 mass % or more, and at least one of the steel plates may be a steel plate having a carbon equivalent Ceq of less than 0.36 mass %. For example, in the case of a plate group formed by stacking three or more steel plates, at least one of the steel plates may be a high-strength steel plate, and one or two or more steel plates having a carbon equivalent Ceq of less than 0.36 mass % may be included.

[0070] In addition, the plurality of steel plates constituting the plate group in the present disclosure are not limited to flat steel plates, but may also be steel plates processed by hot stamping (hot pressing) or the like. For example, it may be a plate group formed by overlapping a plurality of steel plates processed into a three-dimensional component shape by bending or the like, or it may be a plate group formed by overlapping a flat steel plate and a steel plate processed into a component shape. It should be noted that, for example, when spot welding is performed using a steel component having a three-dimensional shape obtained by bending, welding or the like with a flat steel plate, the flat portion (plate-like portion) to be spot welded corresponds to the "steel plate" in the present disclosure.

[0071] The tensile strength of the high-strength steel sheet 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 inside the nugget are likely to occur. Therefore, the tensile strength of the high-strength steel sheet is preferably 2500 MPa or less.

[0072] It should be noted that the tensile strength (TS) of the steel plate can be measured by collecting a JIS No. 5 tensile test piece (gauge length: 50 mm, width 25 mm) from the steel plate and performing the measurement at a tensile speed of 10 mm / min in accordance with ISZ 2241:2011. When collecting from a joint, it is particularly important to avoid a portion deformed by pressing, etc. as much as possible and collect from a flat portion. It should be noted that when it is not possible to collect a JIS No. 5 tensile test piece, TS can also be measured using JIS 13B (gauge length: 50 mm, width 12.5 mm).

[0073] (Carbon equivalent Ceq)

[0074] The high-strength steel sheet in the present disclosure has a carbon equivalent Ceq represented by the following formula (A) of 0.36 mass % or more.

[0075] Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A)

[0076] In the 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 sheet, respectively.

[0077] When the carbon equivalent Ceq is 0.36 mass % or more, the tensile strength of the high-strength steel sheet can be set to, for example, 900 MPa or more.

[0078] The upper limit of the carbon equivalent Ceq is not particularly limited, but for example, when it exceeds 0.55 mass%, a high-strength steel sheet with a tensile strength exceeding 2500 MPa can be obtained, but on the other hand, it is difficult to improve the CTS of the spot welded joint. Therefore, the Ceq of the high-strength steel sheet is preferably 0.55 mass% or less.

[0079] In addition, when the plate group includes a steel plate having a tensile strength of less than 900 MPa, for example, the Ceq of the steel plate is not particularly limited and may be less than 0.36 mass %.

[0080] It should be noted that the chemical composition of the steel plate can be analyzed by any chemical analysis known to those skilled in the art, for example, by inductively coupled plasma mass spectrometry (ICP-MS). However, C and S can also be measured by combustion-infrared absorption method, and N can also be measured by inert gas melting-thermal conductivity method. These analyses are preferably performed on samples collected from steel plates according to the method of JIS G0417:1999.

[0081] (Chemical Composition)

[0082] The high-strength steel sheet in the present disclosure may have a chemical composition in which the carbon equivalent Ceq is 0.36 mass % or more. Hereinafter, a preferred chemical composition of the high-strength steel sheet in the present disclosure will be described.

[0083] C: 0.07~0.50%

[0084] C is an element that improves the tensile strength of steel. The more C content in steel, the higher the strength of the nugget. If the C content in steel is 0.07% or more, it is easy to obtain a tensile strength of 900 MPa or more. On the other hand, if the C content in steel is 0.50% or less, the reduction in the workability of the high-strength steel plate can be suppressed. Therefore, the C content of the high-strength steel plate is preferably 0.07-0.50%.

[0085] Si: 0.001~2.50%

[0086] Si is an element that improves the strength of steel by solid solution strengthening and structural strengthening. If the Si content in steel is 2.50% or more, the reduction in the workability of the steel can be suppressed. 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 the high-strength steel plate is preferably 0.001% to 2.50%.

[0087] Mn: 0.8~5.0%

[0088] Mn is an element that increases the strength of steel. If the Mn content in steel is 5.0% or less, the deterioration of the workability of the steel can be suppressed. On the other hand, if the Mn content in steel is 0.8% or more, it is easy to obtain a tensile strength of 900 MPa or more. Therefore, the Mn content of the high-strength steel plate is preferably 0.8-5.0%.

[0089] P: 0.03% or less

[0090] P is an element that embrittles the nugget. If the P content in the steel is 0.03% or more, cracking in the nugget becomes less likely to occur, and it is easy to obtain a sufficiently high joint strength. Therefore, the P content of the high-strength steel plate is preferably 0.03% or less. It should be noted that the lower limit of the P content is not limited, but from the viewpoint of suppressing the increase in manufacturing costs, the P content of the high-strength steel plate can also be 0.001% or more.

[0091] S: 0.01% or less

[0092] S is an element that embrittles the nugget. In addition, S is an element that combines with Mn to form coarse MnS and hinders the workability of steel. If the S content in the steel is 0.01% or less, it becomes difficult to produce cracks in the nugget, 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 plate is preferably 0.01% or less. It should be noted that from the viewpoint of suppressing the increase in manufacturing costs, the S content of the high-strength steel plate can also be 0.0001% or more.

[0093] N: 0.01% or less

[0094] N is an element that forms coarse nitrides and deteriorates the workability of steel. In addition, N is an element that causes pores during welding. In order to suppress the deterioration of the workability of steel and the generation of pores, the N content of the high-strength steel plate is preferably 0.01% or less. From the viewpoint of suppressing the increase in manufacturing costs, the N content of the high-strength steel plate may also be 0.0005% or more.

[0095] O: 0.01% or less

[0096] 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 content of O in the high-strength steel plate is preferably 0.01% or less. From the viewpoint of suppressing the increase in manufacturing cost, the content of O in the high-strength steel plate may be 0.0005% or more.

[0097] Al: 1.50% or less

[0098] Al is a ferrite stabilizing element and has the effect of inhibiting cementite precipitation during bainite transformation. Therefore, it is contained in order to control the steel structure. In addition, Al also functions as a deoxidizer. On the other hand, Al is easily oxidized and the workability of the steel is easily deteriorated due to the increase of inclusions. Therefore, the Al content of the high-strength steel plate is preferably 1.50% or less.

[0099] The high-strength steel sheet may selectively contain the following elements as necessary in addition to the above main elements.

[0100] Ti, Nb, V: 0.005~0.20%

[0101] Ti, Nb and V are elements that contribute to the increase of steel strength through at least any one of the following strengthening: precipitation strengthening; fine grain strengthening due to the inhibition of ferrite grain growth; and dislocation strengthening due to the inhibition of recrystallization. However, if the content of any element in steel is less than 0.005%, it is difficult to show the effect of inclusion. On the other hand, if the content in steel is less than 0.20%, it is possible to suppress the obstruction of steel workability. Therefore, the content of these elements in high-strength steel sheets is preferably 0.005-0.20%.

[0102] B: 0.0001~0.01%

[0103] 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%, it is difficult to show the effect of the inclusion. On the other hand, if the B content in the steel exceeds 0.01%, the inclusion effect is saturated. Therefore, the B content of the high-strength steel plate is preferably 0.0001 to 0.01%.

[0104] Cr: 0.01~2.0%

[0105] Ni: 0.01~2.0%

[0106] Cu: 0.01~2.0%

[0107] Mo: 0.01~0.8%

[0108] Cr, Ni, Cu and Mo are elements that contribute to the improvement of the strength of steel. These elements can be used to replace part of Mn (strength-enhancing element), for example. However, if the content of any element in steel is 0.01% or more, it is easy to contribute to the improvement of strength.

[0109] Therefore, the content of these elements in the high-strength steel plate is preferably 0.01% or more. 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 the occurrence of obstacles during pickling and hot working. Therefore, the content of Cr, Ni and Cu in the high-strength steel plate is preferably 2.0% or less. In addition, the content of Mo in the high-strength steel plate is preferably 0.8% or less.

[0110] At least one of Ca, Mg and REM: 0.0001 to 1.0% in total

[0111] 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, and contribute to improving the workability of steel. If the total content of these elements in steel is 0.0001% or more, the effect of inclusion is easily manifested. On the other hand, if the total content of these elements in steel is 1.0% or less, the reduction in the workability of steel can be suppressed. Therefore, the content of these elements in high-strength steel sheets is preferably 0.0001 to 1.0% in total.

[0112] It should be noted that REM is a general term for Sc, Y and 15 elements belonging to the lanthanide series, a total of 17 elements, and REM can be added to molten steel as a mixed rare earth alloy during the steelmaking stage. In addition, elements of the lanthanide series can also be contained in a composite form.

[0113] The remainder of the high-strength steel sheet other than the above elements is Fe and impurities. It should be noted that the so-called impurities include components contained in raw materials such as ores and scraps, or components mixed in during the manufacturing process, and refer to components not intentionally contained in the steel sheet.

[0114] It should be noted that the above-mentioned Cr, Ni, Cu, Mo, B, Ti, Ni and V are all allowed to contain trace amounts below the above-mentioned lower limit as impurities. In addition, the above-mentioned lower limit of the total amount of Ca, Ce, Mg, La and REM is also allowed to contain trace amounts below the above-mentioned lower limit as impurities.

[0115] The chemical composition of the high-strength steel sheet in the present disclosure has been described above. However, when a steel sheet having a carbon equivalent Ceq of less than 0.36 mass % is included as a steel sheet constituting a sheet set, the chemical composition and tensile strength of the steel sheet are not particularly limited.

[0116] (Steel Structure)

[0117] The steel structure (steel grade) of the high-strength steel sheet is not particularly limited as long as the carbon equivalent Ceq is 0.36 mass % or more. For example, it may be any type of structure (steel grade) such as a dual-phase structure (for example, a structure containing martensite in ferrite and a structure containing bainite in ferrite), a work-induced transformation type (a structure containing retained austenite in ferrite), a quenching type (martensitic structure), or a fine crystal type (a structure mainly composed of ferrite).

[0118] Furthermore, when a steel plate having a carbon equivalent Ceq of less than 0.36 mass % is included as the steel plate constituting the plate set, the steel structure (steel type) of the steel plate is not particularly limited.

[0119] (plate thickness)

[0120] The thickness of the high-strength steel plate in the present disclosure is not particularly limited. For example, the thickness of the high-strength steel plate generally used in the body of an automobile, etc., can be cited (about 0.5 mm to 3.2 mm). However, since the stress concentration around the nugget increases with the increase in the thickness of the high-strength steel plate, the thickness of the high-strength steel plate is preferably 2.6 mm or less.

[0121] When a steel plate having a carbon equivalent Ceq of less than 0.36 mass % is included as the steel plate constituting the plate group, the plate thickness of the steel plate is not particularly limited.

[0122] 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 the steel plates may be different, or the plate thicknesses of at least two steel plates may be the same.

[0123] (Plating)

[0124] The steel sheets constituting the plate group may also have a coating formed on the surface. Examples of the coating include Zn, Zn-Fe, Zn-Ni, Zn-Al, Zn-Mg, Pb-Sn, Sn-Zn, and Al-Si. When the steel sheet has a coating on its surface, the coating may be a single layer or may have multiple layers.

[0125] As steel sheets with Zn-based coatings, for example, alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets, and electro-galvanized steel sheets can be cited. If a coating is formed on the surface of the steel sheet of the sheet group, the spot welded joint after spot welding shows excellent corrosion resistance. In the case where the coating is a zinc coating alloyed with the surface of the steel sheet, excellent corrosion resistance can be obtained, and the adhesion of the coating becomes good.

[0126] The weight per unit area of ​​the coating is not particularly limited. The coating may be formed on only one side of the steel sheet or on both sides. It should be noted that an inorganic or organic film (such as a lubricating film, etc.) may also be formed on the surface of the coating.

[0127] [Manufacturing method of spot welding joint]

[0128] Next, a method for manufacturing a spot welded joint according to the present disclosure will be described.

[0129] The method for manufacturing a spot welded joint disclosed in the present invention is to continuously perform the following steps on a plate group formed by stacking a plurality of steel plates, at least one of which is the above-mentioned high-strength steel plate: in a manner satisfying the following equations (B) to (H), a pair of welding electrodes are used to apply a pressure F E (N) A main welding step of forming a molten zone in a plate assembly under pressure; and a post-energization step of at least two times after the main welding step.

[0130] Here, the case where two steel plates including at least one of the above-mentioned high-strength steel plates are overlapped and spot welded is cited as an example for explanation. It should be noted that even when three or more steel plates including at least one high-strength steel plate are spot welded, spot welding can be performed by the same method as shown below.

[0131] Figure 1 This is a diagram schematically showing an example of the arrangement of two steel plates including at least one high-strength steel plate and welding electrodes at the start of spot welding. Figure 2 This is a diagram schematically showing an example of a nugget and a heat-affected zone (HAZ) formed by spot welding.

[0132] also, Figure 3 2A is a diagram showing an example of a current-carrying pattern when the welding electrodes are energized in the method for manufacturing a spot welded joint disclosed in the present invention. It should be noted that the current described below is the current flowing between the welding electrode 2A and the welding electrode 2B. Figure 4 This is a diagram simulating the temperature history of the welded portion obtained by the method for manufacturing a spot welded joint according to the present disclosure. Note that QuickSpot (Computational Dynamics Research Center, Inc.) can be used as software for performing simulation using heat conduction analysis.

[0133] [Formal welding process]

[0134] First, if Figure 1 As shown in FIG. 1 , a plate group is prepared in which steel plates 1A and 1B are overlapped so that the plate surfaces face each other, and a pair of welding electrodes 2A and 2B are used to sandwich the plates in the plate thickness direction. Then, a pressure F satisfying the following formula (B) is applied. E (N) Under the pressure, the welding electrodes 2A and 2B are energized with a welding current value I w0 (kA), and main welding (main current supply) is performed to form a molten zone in the plate set. It should be noted that at least one of the steel plates 1A and 1B is the above-mentioned high-strength steel plate.

[0135] 2000×h≤F E ≤4500×h(B)

[0136] Here, h (mm) refers to the arithmetic mean value of the thickness of a plurality of stacked steel plates.

[0137] When the thickness of the two steel plates is different, the arithmetic mean of the thickness of the two steel plates (the arithmetic mean of the thickness of the steel plate 1A and the thickness of the steel plate 1B) is used as "h" in formula (B). When more than three steel plates are spot welded, for example, the sum of the thicknesses of the plurality of steel plates is calculated, and the value obtained by dividing the sum by the number of plates in the plate group is used as "h" in formula (B) above.

[0138] Regarding the arithmetic mean h of the plate thickness, when measuring the plate thickness of the steel plate before joining by spot welding, it is sufficient to measure the plate thickness of each steel plate and take the arithmetic mean as h. When overlapping the joined steel plates for measurement, since it is necessary to measure without gaps, in the case of warping and floating, it is sufficient to clamp and measure, and divide by the number of overlapping pieces to obtain h. In addition, when measuring by a welded joint, it is preferable to measure the plate thickness of a portion that is not deformed by welding as much as possible. In the case where it is impossible to measure by a vernier caliper, it can also be measured by cross-sectional observation.

[0139] The pressure F applied to the steel plates 1A and 1B by the welding electrodes 2A and 2B is E This has a significant impact on the generation of defects and cracks inside the nugget 13 and in the heat-affected zone 14. E 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 .

[0140] On the other hand, if the pressure F EIf the pressure exceeds 4500×h (N), the area where the welding electrodes 2A and 2B contact each other in the steel plates 1A and 1B will be greatly depressed. Therefore, not only the appearance will be impaired, but also the joint strength will be reduced. In addition, in order to obtain a pressure F exceeding 4500×h (N), E The welding gun (a device for applying pressure to the welding electrodes 2A and 2B and energizing them) needs to have a highly rigid robot arm. Therefore, in the present disclosure, the pressure F of the welding electrodes 2A and 2B on the steel plates 1A and 1B is E Set to "2000×h" (N) to "4500×h" (N).

[0141] Formal welding current I w0 And the formal welding power-on time t w0 (Make the actual welding current I w0 The welding current I and the time for flowing) can be set to the conditions that can form a molten portion that becomes a nugget that joins all the steel plates constituting the plate set. The welding current and the time for flowing can be similar to those used in the past to stably obtain a nugget of a desired size as the formal welding current I. w0 And the formal welding time t w0 .

[0142] Regarding the measurement of the pressure, the pressure value displayed by the welding machine can be used when it is built into the spot welding machine used, or a pressure measuring instrument such as a crystal piezoelectric sensor can be installed on the transmission shaft that becomes equal to the pressure load on the steel plate for measurement. For the current value, a welding monitor or a current monitor called a welding ammeter can be used for measurement. There is no specific measurement method, but for example, the following method can be used: insert a toroidal coil in the circuit where the current flows during welding, and read the change in the current value.

[0143] The actual welding current value I in the actual welding process w0 It is preferable to adopt a current value that can obtain a desired nugget diameter in consideration of the total plate thickness t of the plate group and the like.

[0144] Regarding the power-on time t in the actual welding process w0 For example, when the thickness of the thinnest steel plate in the plate group is set to t' (mm), the energization time t w0 It can be set to 10t'-5 to 10t'+50 cycles (the unit of time in the present disclosure is set to the number of cycles at 50 Hz) or the like.

[0145] For the plate thickness of the plate group, the target nugget diameter is 4√t' or more relative to the plate thickness t' on the thin plate side at each plate interface, which is good from the perspective of joint strength and avoiding spatter. The nugget diameter is more preferably 5√t' or more. In order to form such a nugget diameter of 5√t' or more without spattering, a gradual increase (slow rise) of 1 cycle to 80 cycles (50Hz) can also be set before the formal welding process.

[0146] It should be noted that, in the case of a slow-rise energization, the current value at the end of the slow-rise is taken as the current value I in the actual welding process. w0 (kA), during the power-on time t of the formal welding process w0 The time taken to ramp up is not included in the (ms) value. Figure 3 In the energizing mode shown in FIG, the current value is gradually increased from 0 (zero) until the current value reaches the actual welding current I w0 (kA) up to the actual welding current I w0 (kA) for formal welding.

[0147] In addition, before the actual welding process, pre-energization may be performed at a current value lower than that of the actual welding process, for example, for 2 to 80 cycles. The pre-energization may be performed in multiple stages, or a non-energization time may be provided in the middle.

[0148] Through the main welding process, a molten portion and a heat-affected zone (so-called HAZ) 14 that become a nugget 13 are formed at the current-carrying portion of the steel plate 1A and the steel plate 1B at the end of spot welding.

[0149] For spot welding equipment, conventional general spot welding equipment can be used directly. In addition, for welding electrodes, conventional welding electrodes can also be used directly. There is no particular limitation on the power supply, and an AC power supply, a DC converter, an AC converter, etc. can be used.

[0150] It should be noted that if the tip diameter of welding electrodes 2A and 2B becomes too large, the surface pressure at the tip of welding electrodes 2A and 2B decreases. The tip diameter of welding electrodes 2A and 2B is preferably about 6 mm to 8 mm.

[0151] [1st post-energization step]

[0152] After the actual welding process, as the first post-energization process (first post-energization process), the welding process stops at a time t that satisfies the following formula (C). c1 After the energization of (ms) and the non-energization, the first post-energization current value I satisfies the following formula (D) w1 (kA) satisfies the following equation (E) for a period of time t w1 (ms) of power on.

[0153] 2≤t c1 ≤300(C)

[0154] 0.75×I w0 w1 w0 (D)

[0155] t w1 >100(E)

[0156] That is, with the formal welding current I w0 After the welding electrodes 2A and 2B are energized for a predetermined time to form molten portions in the steel plates 1A and 1B, the current value is set to 0 (zero) to complete the main welding, and the main welding is continued (energizing the main welding current I w0 The pressure F E , while satisfying the time t of formula (C) c1 (ms) during which the power is stopped. Thus, the molten portion formed by the formal welding is solidified from the periphery of the molten portion (i.e., the boundary between the molten portion and other regions). It should be noted that in the present disclosure, the boundary between the molten portion and other regions is referred to as the "molten boundary".

[0157] From the official welding current I w0 Immediately after the energization is terminated, the solidification of the molten portion begins from the molten boundary. The molten portion solidifies to form a nugget 13, and a heat-affected zone 14 is formed outside the molten boundary.

[0158] After the formal welding process, the non-power-on time t c1 If the current is less than 2 ms, the end of the nugget may not solidify before the next post-power on. c1 If it exceeds 300 ms, the end of the nugget may be excessively solidified before the subsequent post-energization.

[0159] In order to avoid the post-energization of the nugget end in an insufficiently solidified state or an excessively solidified state and to properly solidify the nugget end, the non-energization time t after the actual welding process is c1 It is set to 2ms to 300ms, preferably to 40ms to 250ms.

[0160] In the first post-energization step, the time t that satisfies the above formula (C) is stopped. c1 After the energization of (ms) and the non-energization, the first post-energization current value I that satisfies the above formula (D) is w1 (kA) satisfies the above equation (E) for a period of time t w1 That is, the first post-energization current value I w1 Less than the official welding current value I of the official welding process w0 ​​, and greater than 0.75×I w0 , the first post-power-on time t w1 The time is set to be longer than 100ms. Preferably, the first post-energization is performed under such conditions, so that Figure 4 As shown in FIG. , at least a portion of the nugget is remelted without crossing the melting boundary formed in the main welding process, so that the non-power-on time t after the main welding process is c1 The γ phase (fcc crystal structure) formed in the nugget by cooling is transformed into the δ phase (bcc crystal structure).

[0161] From the viewpoint of remelting the nugget and shortening the overall spot welding time, the first post-energization time t w1 It is preferably set to 110 ms to 1000 ms, and more preferably set to 150 ms to 800 ms.

[0162] It should be noted that if the pressure F in the first post-energization step is E Directly maintain the pressure F in the actual welding process E However, the non-power-on time t c1 The pressure F E The pressure F in the actual welding process is set within the range satisfying the above formula (B). E Different pressure F E .

[0163] [Second post-energization process]

[0164] The method for manufacturing a spot welded joint disclosed herein performs at least one post-energization step as the second and subsequent post-energization steps after the first post-energization step.

[0165] 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, the time t that satisfies the following formula (F1) is stopped. c2 After the energization of (ms) and the non-energization, the second post-energization current value I that satisfies the following formula (G1) is w2 (kA) After the second power-on time t w2 (ms) of power on.

[0166] 2≤t c2 ≤300(F1)

[0167] 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)

[0168] It should be noted that the presence or absence of phase change caused by each post-energization process and the position of the phase change can be as follows: Figure 8 As shown in , the cross section near the center of the nugget is observed by EBSD, and the fine grain area of ​​the grains before and after each post-energization process is judged. After the molten portion that becomes the nugget is formed by formal energization, it is phase-transformed at appropriate positions by post-energization twice or more that satisfies the conditions of the present disclosure, thereby forming a nugget with a number ratio of grains with an aspect ratio of 7 or more being 50% or less near the center of the nugget as described below.

[0169] After the first post-energization process, the non-energized time t c2 If the current is less than 2 ms, the end of the nugget may not solidify before the second post-energization step. c2 If it exceeds 300 ms, the end of the nugget may be excessively solidified before the first post-energization step.

[0170] In order to avoid the second post-energization in a state where the end of the nugget is insufficiently solidified or excessively solidified and to appropriately solidify the end of the nugget, the non-energization time t after the first post-energization step is c2 It is set to 2ms to 300ms, preferably 60ms to 250ms.

[0171] In the second post-energization process, the nugget is cooled and the non-energized time t c2 And it becomes γ phase (fcc crystal structure). w2 And the second power-on time t w2 The actual welding current value I corresponding to the actual welding process w0 and the non-energized time t of the second post-energization process c2 The condition of the above formula (G1) is satisfied and the power is turned on later, so that Figure 4 As shown in , the nugget is transformed into the δ phase (bcc crystal structure) again. As a result, the equiaxed graining of the nugget progresses, and the CTS can be improved.

[0172] In addition, in the method for manufacturing a spot welding joint disclosed in the present invention, the actual welding current value I w0 The first post-current value I w1 In the same way, according to the current value I in the second post-energization process w2 Greater than 0.75×I w0 , the official welding current value Iw0 The second post-current value I w2 The current is supplied so that the relationship between (kA) and (kA) satisfies the following formula (H1).

[0173] 0.75×I w0 w2 (H1)

[0174] At the official welding current value I w0 The second post-current value I w2 If the relationship of the above formula (H1) is not satisfied, sufficient heat input cannot be given even if power is supplied for a long time, and the position where equiaxed graining occurs in the nugget remains at the center of the nugget or does not change, so the CTS improvement effect cannot be obtained.

[0175] Furthermore, the actual welding current value I w0 The second post-current value I w2 The relationship preferably satisfies the following formula (I).

[0176] I w2 >I w0 (I)

[0177] Figure 5 This is an example of a current supply pattern that satisfies the formula (I). w0 Compared with the second increase in the current value I w2 , which can reliably promote equiaxed graining within the nugget and improve CTS.

[0178] After the second power is turned on, the pressure F is maintained. E When the post-energization process is completed twice while stopping the energization, the pressure F applied by the welding electrodes 2A and 2B is E Released from the stacked steel plates 1A, 1B (plate group). It should be noted that after the second post-energization step, it is preferable to provide a so-called holding time in which only pressurization is performed and no electricity is applied. The holding time is preferably 5 cycles (50 Hz) or more.

[0179] It should be noted that if the pressure F in the second post-energization step is E The pressure F in the main welding process is also maintained immediately after the first post-energization process. E However, the non-power-on time t c2 The pressure F E The pressure F in the actual welding process is set within the range satisfying the above formula (B). E Different pressure F E That is, from the main welding process to the end of the final post-energization process, as long as the pressure F is applied,​E It may be constant or variable within the range satisfying the formula (B).

[0180] A slow-down may be provided after the second post-energization step. The slow-down can further improve the characteristics of the spot weld by utilizing the effects of reducing cracking due to brittleness of the liquid metal, reducing pores, and suppressing delayed fracture.

[0181] It should be noted that, in the case of slow-down energization, the current value at the start of the slow-down is set to the second post-energization current value I in the second post-energization process. w2 (kA), the second post-energization time t in the second post-energization process w2 The time taken for ramp down is not included in the value (ms).

[0182] By performing resistance spot welding including the above-mentioned steps on a plate group formed by overlapping a plurality of steel plates including at least one high-strength steel plate, the CTS can be greatly improved compared to the case where resistance spot welding is performed with a single current. In addition, according to the present disclosure, the joint strength can be effectively improved with a short welding time compared to the conventional tempering current.

[0183] Here, the effect of improving CTS by applying power in a manner that satisfies the above-mentioned formula (G1) in the second post-power-on process is described. For a plate group consisting of two overlapping high-strength steel plates having the chemical composition (unit: mass %, remainder: Fe and impurities), plate thickness and tensile strength (TS) shown in the following Table 1, spot welding is performed by applying the formal welding power and post-power-on under the conditions shown in Table 2 to produce a welded joint. For the obtained welded joint, CTS is measured by the method according to JIS Z3137; 1999. It should be noted that in Table 2, "cooling" refers to the non-power-on time (cooling time) when the power is stopped.

[0184] [Table 1]

[0185]

[0186] [Table 2]

[0187]

[0188] Test Example 0 was performed until the first post-energization after the main welding was energized. Using the CTS of the joint of Test Example 0 as a reference, the CTS improvement rates of the joints of Test Examples 1 to 5 performed until the second post-energization are shown in Table 2. When the CTS improvement rate relative to the first post-energization (Test Example 0) exceeded 10%, it was judged that there was a CTS improvement effect, and when it was less than 10%, it was judged that there was no CTS improvement effect.

[0189] In addition, each joint was cut in the plate thickness direction in a manner passing through the center of the nugget, and the grains near the center of the nugget were observed by electron beam backscatter diffraction (EBSD). The observation results are shown in FIG. Figure 7 It should be noted that the portion with a crystal orientation difference of 15 degrees or more is regarded as a crystal grain boundary. The area surrounded by the dotted square is the fine grain area formed in the first post-energization, and the area surrounded by the solid square is the fine grain area formed in the second post-energization. From Table 2 and Figure 7 The results shown in are estimated as follows.

[0190] Test Example 1 Due to I w2 / I w0 ×t w2 The value is lower than the lower limit of formula (G1), the heat input is insufficient, and the fine particles generated in the second post-energization are very close to the center of the weld nugget, which is the position reached after the crack progresses violently in the joint strength test, so it does not help to improve the joint strength.

[0191] Test Examples 2 to 4 Due to I w2 / I w0 ×t w2 The value of satisfies formula (G1) and is an appropriate heat input. The fine particles generated in the second post-energization are at a certain distance from the center of the weld nugget and are located on the inner side compared to the outermost side of the fine particles generated in the first post-energization, so it effectively acts to improve the toughness.

[0192] Test Example 5 Due to I w2 / I w0 ×t w2 The value exceeds the upper limit of formula (G1), and the fine particles generated in the second post-energization exceed the fine particle region generated in the first post-energization. Therefore, the fine particles generated in the first post-energization disappear, and a high joint strength improvement effect is not obtained.

[0193] (Post-energization process after the third time)

[0194] The method for manufacturing a spot welding joint disclosed in the present invention may also include directly maintaining the pressure F after the second post-energization step. E In the state of , no power supply and post-power supply are further repeated alternately, and the post-power supply process is performed 3 to N times (N is an integer greater than 3) including the above-mentioned first post-power supply process and the second post-power supply process. The post-power supply process after the third time has the same relationship as the second post-power supply process, that is, after the n-1th (n is an integer from 3 to N) post-power supply process, the time t that stops satisfying the following (F) formula cn After the energization of (ms) and the non-energization, the nth subsequent energization current value I satisfies the following formula (G) and formula (H) wn(kA) After the nth power-on time t wn (ms) of power on.

[0195] In the second and subsequent post-energization steps, i.e., the nth post-energization step (n is an integer from 3 to N), after the (n-1)th post-energization step, the time t that satisfies the following formula (F) is stopped. cn After the energization of (ms) and the non-energization, the nth subsequent energization current value I satisfies the following formula (G) wn (kA) After the nth power-on time t wn (ms) of power on.

[0196] 2≤t cn ≤300(F)

[0197] 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)

[0198] In the nth post-energization step, it is preferred that the phase change occurs at an inner side than the outermost position where the phase change occurs in the (n-1)th post-energization step.

[0199] When the post-energization process is performed three or more times, a slow-down process may be provided after the last post-energization process. The slow-down process can further improve the characteristics of the spot weld by utilizing the effects of reducing cracking due to the brittleness of the liquid metal, reducing pores, and suppressing delayed fracture.

[0200] It should be noted that when ramping down is performed after the last post-power-on step, the current value at the start of ramping down is set as the power-on current value (kA) in the last post-power-on step, and the time spent on ramping down is not included in the post-power-on time (ms) of the last post-power-on step.

[0201] It is believed that, similar to the second post-energization process, the post-energization process after the third time is repeated in a manner that satisfies the formula (F), the formula (G) and the formula (H) after no energization, so that the equiaxed graining progresses through the phase transformation from the γ phase (fcc crystal structure) to the δ phase (bcc crystal structure) in the nugget (inside the molten boundary), thereby further improving the CTS. However, the more the post-energization process is increased, the longer the welding time becomes. Therefore, the post-energization process is preferably set to 2 to 4 times, more preferably 2 to 3 times, and particularly preferably 2 times.

[0202] <Spot welding joint>

[0203] According to the manufacturing method of the spot welding joint disclosed in the present invention, the following spot welding joint can be manufactured. That is, the spot welding joint disclosed in the present invention includes a spot welding portion formed by joining a plurality of overlapping steel plates, and when the contents of C, Si, Mn, P, and S are set to [C], [Si], [Mn], [P], and [S] respectively in mass %, 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 mass % or more, and when the inside of the melting boundary of the molten nugget is observed in a cross section in the plate thickness direction passing through the center of the molten nugget of the spot welding portion, the portion with a crystal orientation difference of 15 degrees or more in the middle part of the long axis direction of the molten nugget is used as a crystal grain boundary, and the number ratio of grains with an aspect ratio of 7 or more is 50% or less.

[0204] Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A)

[0205] The spot welded joint disclosed in the present invention can be manufactured in a shorter welding time than when a plate set including high-strength steel plates having a Ceq of 0.36 mass % or more is subjected to tempering and energization after main welding, and can have higher joint strength than a spot welded joint subjected to spot welding only in a main welding process.

[0206] (Method for measuring the ratio of the number of crystal grains having an aspect ratio of 7 or more)

[0207] Figure 6 This is a diagram showing the area where the aspect ratio of the grains is measured in the weld of the spot welded joint. For the cross section in the plate thickness direction passing through the center of the weld nugget of the spot weld, the crystal orientation inside the melting boundary of the weld nugget is measured using EBSD analysis. Figure 6 As shown in the figure, the measurement field is 0.5 mm to the left and right of the imaginary line L2 in the plate thickness direction passing through the center of the nugget, the position of the plate interface L1, and the middle part M of the melting boundary of a steel plate. It should be noted that the steel structure of the nugget is roughly symmetrical in the upper and lower parts with the plate interface L1 as the boundary. Therefore, as long as Figure 6 The middle portion M of one plate side may be measured as shown in FIG.

[0208] Then, in the measurement field of view, the grain boundaries that are regarded as lath block grain boundaries and original austenite grain boundaries with a crystal orientation difference of more than 15 degrees are depicted, and the aspect ratio of each grain is calculated. The maximum length of the grain is taken as the major diameter, and the interval between two parallel lines parallel to the major diameter direction and connected to the grain at the widest interval is taken as the minor diameter, and the aspect ratio (major diameter / minor diameter) is calculated. Regarding the size of the field of view, since the size of the grains varies depending on the sample, it is not limited, but it can be observed in a size containing more than 100 grains. In the above-mentioned field of view, the value of the aspect ratio is calculated for more than 100 grains, and the proportion of grains with an aspect ratio of more than 7 is calculated.

[0209] The spot welded joint of the present disclosure has a ratio of 50% or less of grains having an aspect ratio of 7 or more as measured above. That is, for a plate set including the above-mentioned high-strength steel plate, by performing a post-energization process two or more times under the above-mentioned conditions after formal welding, equiaxed graining in the nugget can be promoted, and a spot welded joint can be obtained in which the ratio of grains having an aspect ratio of less than 7 is more than half (50%). Such a spot welded joint can exhibit a high CTS compared to a case in which the ratio of the number of grains having an aspect ratio of 7 or more exceeds 50%.

[0210] It should be noted that the ratio of crystal grains having an aspect ratio of 7 or more is preferably 40% or less, and more preferably 30% or less.

[0211] Example

[0212] Hereinafter, the method for manufacturing a resistance spot welding joint disclosed in the present invention will be described by way of examples. It should be noted that the method for manufacturing a resistance spot welding joint disclosed in the present invention is not limited to these examples.

[0213] Steel plates having the chemical composition (unit: mass %, balance: Fe and impurities), plate thickness, and tensile strength (TS) shown in the following Table 3 were prepared. In addition, in each table in the examples, underlined elements are outside the scope of the present disclosure.

[0214] [Table 3]

[0215]

[0216] The steel plates were combined and resistance spot welded to produce welded joints under the conditions (plate combination, pressure, current conditions, etc.) shown in Table 4. The CTS of the obtained welded joints was measured.

[0217] Regarding the CTS of the joint obtained by spot welding the three overlapping plate groups in No. 24, the CTS was measured at the interface with the steel plate (Ceq: 0.36 mass % or more) used as the object of the present disclosure, that is, the interface between the steel plate B and the steel plate C. It should be noted that the steel plate D of the plate group in No. 24 is prepared for the plate group including high-strength steel plates to which the method for manufacturing a spot welded joint of the present disclosure can be applied, and is not underlined.

[0218] In addition, in the case of a joint of a plate group formed by overlapping three steel plates of the same type in No. 25, since the CTSs of the interfaces between the two plates become the same value, the CTS of any interface can be measured, but the CTS is measured by cross-tensioning at the interface that becomes the upper side during welding.

[0219] For each plate set, the CTS (reference CTS) of the welded joint was measured after the main welding current was applied to the first post-current application under the conditions in each number. The increase rate compared to the reference CTS was calculated, and the case exceeding 10% was judged to have an effect of improving the joint strength.

[0220] [Table 4]

[0221]

[0222] No. 1 to No. 9 were conducted to the second post-energization after the main welding energization, but they did not satisfy any of the equations (A) to (H) in the present disclosure, and no CTS improvement effect was obtained. It should be noted that although the proportion of grains with an aspect ratio of 7 or more in No. 1 was less than 50%, the Ceq of the steel plate C constituting the plate group was low, and even in the joint conducted to the first post-energization, the CTS was high, so the CTS improvement effect brought about by the second post-energization was not obtained.

[0223] No. 14 is an example in which only the main welding current was applied.

[0224] No. 15 is an example of a long period of post-energization (so-called tempering energization) at a relatively small current value for tempering after the formal welding energization. Although the CTS improvement effect is obtained, the non-energization process time after the formal welding energization also needs to be extended, and it is difficult to say that the CTS is effectively improved as a spot welding process as a whole. It should be noted that the CTS improvement rate of Nos. 14 and 15 is calculated by comparing with the CTS of the welded joint that was performed until the first post-energization after the formal welding energization under the conditions of No. 16 using the same plate group (two steel plates B).

[0225] Although Nos. 18 to 22 performed the second post-energization after the main welding energization, they did not satisfy any of the equations (A) to (H1) in the present disclosure, and did not obtain the CTS improvement effect.

[0226] On the other hand, the numbers classified as Examples all satisfy the formulas (A) to (H) in the present disclosure and obtain the CTS improvement effect.

[0227] Note that No. 13 is an example in which the third post-energization was performed and the CTS improvement rate was the highest. In Table 4, the non-energization, current value, and time in the third post-energization are separately recorded in the column of the second post-energization for convenience.

[0228] 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 document, patent application, and technical standard were specifically and individually described.

[0229] Explanation of symbols

[0230] 1A, 1B steel plate

[0231] 2A, 2B welding electrodes

[0232] 13Nugget

[0233] 14Heat affected zone (HAZ)

Claims

1. A method for manufacturing a spot welding joint, wherein a plate group formed by overlapping a plurality of steel plates is sandwiched in a plate thickness direction by a pair of welding electrodes and energized under pressure to perform spot welding, When the contents of C, Si, Mn, P, and S are set to [C], [Si], [Mn], [P], and [S], respectively, in terms of mass %, 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 mass % or more, Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A) It includes the following steps: The following formal welding process is performed: when the arithmetic mean value of the thicknesses of the plurality of steel plates is set to h, the plate set is welded by the pair of welding electrodes at a pressure F satisfying the following formula (B): E Under the pressure, the pair of welding electrodes is energized with a welding current value I w0 , a molten portion is formed in the plate set, h is in mm, F E The unit is N, I w0 The unit is kA, 2000×h≤F E ≤4500×h(B); and After the main welding process, a post-energization process is performed twice. In the first post-energization process, that is, the first post-energization process, after the main welding process, the welding process stops at a time t satisfying the following formula (C): c1 After the energization of the non-energization, the first post-energization current value I satisfies the following (D) formula w1 The time t that satisfies the following formula (E) w1 Power on, t c1 The unit is ms, I w1 The unit is kA, t w1 The unit is ms, 2≤t c1 ≤300(C) 0.75×I w0 <I w1 <I w0 (D) t w1 >100(E) In the second post-energization process, that is, the second post-energization process, after the first post-energization process, the following formula (F1) is satisfied at the time t c2 After the energization of the non-energization, the second post-energization current value I satisfies the following (G1) formula and the following (H1) formula w2 Time t w2 Power on, t c2 The unit is ms, I w2 The unit is kA, t w2 The unit is ms, 2≤t c2 ≤300(F1) 0.004×t c2 2 -0.3125×t c2 +102≤I w2 / AND w0 ×t w2 ≤0.0156×t c2 2 -0.625×t c2 +300(G1)0.75×I w0 w2 (H1)​ When the pressure F E The steps from the main welding step to the final post-energization step are continuously performed while maintaining the range satisfying the above-mentioned formula (B).

2. The method for manufacturing a spot welding joint according to claim 1, wherein: The formal welding current value I w0 The second post-current value I w2 The relationship satisfies the following formula (I): I w2 >I w0 (I)。 3. The method for manufacturing a spot welding joint according to claim 1 or claim 2, comprising performing the post-energization step 3 to N times, wherein N is an integer greater than or equal to 3, and in the post-energization step after the third time, i.e., the nth post-energization step, after the (n-1)th post-energization step, i.e., the (n-1)th post-energization step, the welding process stops at a time t satisfying the following formula (F): cn After the energization of the non-energization, the nth subsequent energization current value I satisfies the following formula (G) and the following formula (H). wn Time t wn The power supply is, n is an integer from 3 to N, t cn The unit is ms, I wn The unit is kA, t wn The unit is 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)。 4. A spot welded joint comprising a spot weld formed by joining a plurality of overlapping steel plates, When the contents of C, Si, Mn, P, and S are set to [C], [Si], [Mn], [P], and [S], respectively, in terms of mass %, 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 mass % or more, Ceq=[C]+[Si] / 30+[Mn] / 20+2[P]+4[S](A) When observing the inner side of the molten boundary of the molten core in a cross section in the plate thickness direction passing through the center of the molten core of the spot weld, in the middle part of the long axis direction of the molten core, the portion with a crystal orientation difference of 15 degrees or more is taken as a crystal grain boundary, and the number ratio of grains with an aspect ratio of 7 or more is less than 50%.

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