US20160082543A1 - Spot-welded joint and spot welding method - Google Patents

Spot-welded joint and spot welding method Download PDF

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Publication number
US20160082543A1
US20160082543A1 US14/889,111 US201414889111A US2016082543A1 US 20160082543 A1 US20160082543 A1 US 20160082543A1 US 201414889111 A US201414889111 A US 201414889111A US 2016082543 A1 US2016082543 A1 US 2016082543A1
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Prior art keywords
energization
welding
steel plates
satisfying
post
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Inventor
Chisato Wakabayashi
Fuminori Watanabe
Seiji Furusako
Yasunobu Miyazaki
Hiroyuki Kawata
Tohru Okada
Hideki Hamatani
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FURUSAKO, SEIJI, HAMATANI, HIDEKI, KAWATA, HIROYUKI, MIYAZAKI, YASUNOBU, OKADA, TOHRU, WAKABAYASHI, CHISATO, WATANABE, FUMINORI
Publication of US20160082543A1 publication Critical patent/US20160082543A1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/10—Spot welding; Stitch welding
    • B23K11/11—Spot welding
    • B23K11/115—Spot welding by means of two electrodes placed opposite one another on both sides of the welded parts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/10—Spot welding; Stitch welding
    • B23K11/11—Spot welding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/02—Iron or ferrous alloys
    • B23K2103/04—Steel or steel alloys

Definitions

  • the present invention relates to a joint formed by overlapping a plurality of pieces of steel plates and performing spot welding on the steel plates.
  • a tensile strength is an important property.
  • a tensile strength includes a tensile shear strength (TSS) measured under a tensile load applied in a shear direction, and a cross tensile strength (CTS) measured under a tensile load applied in a peeling direction.
  • TSS tensile shear strength
  • CTS cross tensile strength
  • the CTS in a spot-welded joint formed of a plurality of pieces of steel plates each having a tensile strength of 270 MPa to 600 MPa increases, in accordance with an increase in strength of the steel plates. Therefore, a problem regarding a joint strength is difficult to occur, in the spot-welded joint formed of the steel plates each having the tensile strength of 270 MPa to 600 MPa.
  • the CTS in a spot-welded joint formed of a plurality of pieces of steel plates including at least one piece of steel plate whose tensile strength is 750 MPa or more does not increase or reduces even if the tensile strength of the steel plates increases.
  • a spot-welded joint formed of a plurality of pieces of steel plates including at least one piece of steel plate whose tensile strength is 750 MPa or more the CTS is easily reduced. This is because a stress concentration with respect to a welded portion is increased due to a lowering of ductility, and because a toughness of the welded portion is lowered since the welded portion is tempered. For this reason, an improvement of the CTS in the spot-welded joint formed of the plurality of pieces of steel plates including at least one piece of steel plate whose tensile strength is 750 MPa or more is demanded.
  • Patent Literature 1 describes a method in which main energization is finished and after a predetermined time passes, tempering energization is conducted, to thereby perform annealing on a spot-welded joint (a nugget portion and a heat-affected zone) to reduce a hardness of the joint.
  • Patent Literature 2 describes a method in which welding is performed, and after that, a welded portion is heated with high frequency to be subjected to tempering treatment.
  • Patent Literature 3 describes a method in which a nugget is formed through main welding, and then post-energization is performed with a current which is equal to or greater than a main welding current.
  • Patent Literature 4 describes a method in which spot welding is performed on steel plates each having a tensile strength of 440 MPa or more.
  • a composition of components of the steel plate is restricted to satisfy the following conditions: C ⁇ P ⁇ 0.0025; P: 0.015% or less; and S: 0.01% or less.
  • heat treatment is performed on a welded portion at 300° C. for about 20 minutes.
  • Patent Literature 5 describes a spot-welded joint formed of high-strength steel plates (tensile strength: 750 to 1850 MPa, carbon equivalent Ceq: 0.22 to 0.55 mass %) in which a microstructure of a nugget outer layer zone, and an average grain diameter and a number density of carbides in the microstructure are defined.
  • Patent Literature 6 describes a method in which spot welding is performed on steel plates each having a tensile strength of 900 to 1850 MPa, and having a plate thickness of 1.8 to 2.8 mm.
  • post-energization is successively performed with a current which is 0.5 times to 0.9 times a welding current, for a time which is 0.3 times to 0.5 times a welding time.
  • Patent Literature 1 Japanese Laid-open Patent Publication No. 2002-103048
  • Patent Literature 2 Japanese Laid-open Patent Publication No. 2009-125801
  • Patent Literature 3 Japanese Laid-open Patent Publication No. 2010-115706
  • Patent Literature 4 Japanese Laid-open Patent Publication No. 2010-059451
  • Patent Literature 5 International Publication Pamphlet No. WO 2011-025015
  • Patent Literature 6 Japanese Laid-open Patent Publication No. 2011-5544
  • the present invention has an object to improve a cross tensile strength of a spot-welded joint formed of a plurality of pieces of steel plates including at least one piece of steel plate whose tensile strength is 750 MPa to 2500 MPa.
  • a spot-welded joint of the present invention is a spot-welded joint formed by overlapping a plurality of pieces of steel plates and performing spot welding on the steel plates, including a high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, being at least one piece of steel plate out of the plurality of pieces of steel plates, in which a carbon equivalent Ceq of the high-strength steel plate represented by the following expression (A) is 0.20 mass % to 0.55 mass %, and ten or more of iron-based carbides in each of which a length of a longest portion is 0.1 ( ⁇ m) or more exist in a square region whose length of one side is 10 ( ⁇ m) in which a plate thickness direction and a plate surface direction of the steel plates are set to a vertical direction and a horizontal direction, respectively, being a region within a heat-affected zone of a cross section that passes through a center of a welding mark formed on surfaces of the steel plates by the spot welding, and is cut along the plate thickness
  • [C], [Si], [Mn], [P], and [S] in the above expression (A) indicate respective contents (mass %) of C, Si, Mn, P, and S.
  • a first example of a spot welding method of the present invention is a spot welding method of overlapping a plurality of pieces of steel plates and performing spot welding on the steel plates, in which at least one piece of steel plate out of the plurality of pieces of steel plates is a high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, in which a carbon equivalent Ceq of the high-strength steel plate represented by the following expression (A) is 0.20 mass % to 0.55 mass %, the spot welding method including: performing main welding of energizing welding electrodes with a main welding current I W (kA) in a state where the overlapped plurality of pieces of steel plates are pressurized by the welding electrodes at a pressurizing force F E (N) satisfying the following expression (B); performing, after the main welding is finished, cooling after main welding of cooling the plurality of pieces of steel plates for a cooling time after main welding t S (msec) satisfying the following expression (C) while retaining the pressurizing force F
  • [C], [Si], [Mn], [P], and [S] in the above expression (A) indicate respective contents (mass %) of C, Si, Mn, P, and S, and h in the above expression (B), and the above expression (C) indicates a plate thickness of the steel plate (mm).
  • a second example of a spot welding method of the present invention is a spot welding method of overlapping a plurality of pieces of steel plates and performing spot welding on the steel plates, in which at least one piece of steel plate out of the plurality of pieces of steel plates is a high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, in which a carbon equivalent Ceq of the high-strength steel plate represented by the following expression (A) is 0.20 mass % to 0.55 mass %, the spot welding method including: performing pre-energization of energizing welding electrodes with a pre-energization current I f (kA) satisfying the following expression (C) for a pre-energization time t f (msec) satisfying the following expression (D), in a state where the overlapped plurality of pieces of steel plates are pressurized by the welding electrodes at a pressurizing force F E (N) satisfying the following expression (B); performing, after the pre-energization is finished,
  • [C], [Si], [Mn], [P], and [S] in the above expression (A) indicate respective contents (mass %) of C, Si, Mn, P, and S, and h in the above expression (B), the above expression (E), and the above expression (F) indicates a plate thickness of the steel plate (mm).
  • FIG. 1 is a diagram illustrating one example of an arrangement of two pieces of steel plates and welding electrodes when spot welding is started.
  • FIG. 2 is a diagram schematically illustrating one example of a nugget and a heat-affected zone formed by the spot welding.
  • FIG. 3 is a diagram illustrating an example of first form of an energization pattern.
  • FIG. 4 is a diagram schematically illustrating one example of an appearance in the middle of solidification of a molten zone which is solidified to be a nugget.
  • FIG. 5 is a diagram illustrating one example of a relationship between a cooling time after main welding and a plate thickness of a steel plate.
  • FIG. 6 is a diagram illustrating a first example of a relationship between a post-energization time and a square of a value obtained by dividing a post-energization current by a main welding current.
  • FIG. 7 is a diagram illustrating, in a conceptual manner, one example of a relationship between the post-energization time and a degree of embrittlement of an outer peripheral portion of the nugget and the heat-affected zone.
  • FIG. 8 is a diagram illustrating an example of second form of an energization pattern.
  • FIG. 9 is a diagram illustrating one example of a relationship between a cooling time after pre-energization and a plate thickness of a steel plate.
  • FIG. 10 is a diagram illustrating a second example of a relationship between a post-energization time and a square of a value obtained by dividing a post-energization current by a main welding current.
  • FIG. 11A is a diagram (photograph) illustrating one example of a structure of a heat-affected zone of a welded joint obtained by unconventional welding.
  • FIG. 11B is a diagram (photograph) illustrating one example of a structure of a heat-affected zone of a welded joint obtained by conventional welding.
  • FIG. 12A is a diagram explaining one example of a precipitation condition of iron-based carbides.
  • FIG. 12B is a diagram illustrating a part of a region A in FIG. 12A in an enlarged manner.
  • the present inventors conducted earnest studies, from a metallurgical point of view and a mechanical point of view, regarding the reason why the cross tensile strength (CTS) in the spot-welded joint formed of the plurality of pieces of steel plates including at least one piece of steel plate whose tensile strength is 750 MPa to 2500 MPa cannot be sufficiently improved by the conventional technique in which the post-energization is performed after the main welding.
  • CTS cross tensile strength
  • the steel plate whose tensile strength is 750 MPa to 2500 MPa is referred to as “high-strength steel plate” according to need.
  • the nugget indicates a part of a steel plate which is melted through energization between welding electrodes and then is solidified.
  • the heat-affected zone indicates a part of a steel plate heated to a temperature equal to or more than the Ac1 point and less than a melting temperature.
  • the present inventors found out that, in order to obtain a spot-welded joint with high reliability, it is necessary to improve not only the fracture load inside the nugget but also the fracture load in the peripheral portion of the nugget.
  • the solidified region and a heat-affected zone surrounding the solidified region are retained at a high temperature for a long time.
  • a steel type is not particularly limited.
  • the steel type can employ any type such as, for example, a two-phase structure type (for example, a structure containing martensite in ferrite, or a structure containing bainite in ferrite), a strain-induced transformation type (a structure containing residual austenite in ferrite), a hardened type (a martensite structure), or a microcrystalline type (a structure essentially made of ferrite).
  • a two-phase structure type for example, a structure containing martensite in ferrite, or a structure containing bainite in ferrite
  • a strain-induced transformation type a structure containing residual austenite in ferrite
  • a hardened type a martensite structure
  • microcrystalline type a structure essentially made of ferrite
  • a spot-welded joint using the high-strength steel plate constituted by whichever steel type can suppress “reduction and fluctuation” of joint strength to realize a good fracture appearance, so that it is possible to obtain a welded joint with high reliability.
  • a steel type of a steel plate to be overlapped with the high-strength steel plate is not particularly limited as well.
  • a steel plate of a steel type different from the steel type of the high-strength steel plate can also be employed.
  • the steel plate to be overlapped with the high-strength steel plate can also be set to a mild steel plate.
  • the steel plate to be overlapped with the high-strength steel plate can also be a steel plate of a steel type which is the same as the steel type of the high-strength steel plate.
  • a tensile strength of at least one piece of steel plate (high-strength steel plate) out of a plurality of pieces of overlapped steel plates is set to 750 MPa to 2500 MPa.
  • a high joint strength is required.
  • CTS cross tensile strength
  • the tensile strength of the high-strength steel plate is less than 750 MPa, the cross tensile strength is high from the beginning, and further, a load with respect to the spot-welded joint is small. Accordingly, a problem regarding a deterioration of fracture appearance in a welded portion and the joint strength is hard to occur. Therefore, the tensile strength of the high-strength steel plate is set to 750 MPa or more.
  • the tensile strength of the high-strength steel plate exceeds 2500 MPa, the suppression of “reduction and fluctuation” of the joint strength becomes difficult. Further, in accordance with this, it becomes difficult to suppress the deterioration of the fracture appearance in the welded portion, and to suppress an occurrence of a defect or a crack inside the nugget. Therefore, the tensile strength of the high-strength steel plate is set to 2500 MPa or less.
  • a tensile strength of a steel plate to be overlapped with the high-strength steel plate is not particularly limited as well.
  • the steel plate to be overlapped with the high-strength steel plate can also be set to a high-strength steel plate whose tensile strength is 750 MPa to 2500 MPa, and it can also be set to a steel plate whose tensile strength is less than 750 MPa.
  • the steel plate is a steel member used in the automobile field and the like, the tensile strength thereof may be selected in accordance with the steel member to be used.
  • a plate thickness of the high-strength steel plate is not particularly limited.
  • a plate thickness (0.5 mm to 3.2 mm) of a high-strength steel plate used in general for a vehicle body or the like of an automobile suffices.
  • the plate thickness of the high-strength steel plate is preferably 2.6 mm or less.
  • a plate thickness of a steel plate to be overlapped with the high-strength steel plate is not particularly limited. It is also possible that plate thicknesses of a plurality of pieces of steel plates to be overlapped are mutually different. For example, when three pieces or more of steel plates are overlapped, plate thicknesses of the respective three pieces or more of steel plates may also be different from one another. It is only required that at least one piece of steel plate out of the three pieces or more of steel plates is the high-strength steel plate, and the other steel plates may also be mild steel plates. Further, when three pieces or more of steel plates are overlapped, plate thicknesses of at least two pieces of steel plates may also be the same. Note that generally, a thickness of a steel plate is 6 mm or less.
  • a carbon equivalent Ceq of the high-strength steel plate represented by the following expression (1) is preferably within a range of 0.20 mass % to 0.55 mass %. If the carbon equivalent Ceq is less than 0.20 mass %, it is not possible to obtain a tensile strength of equal to or more than 750 MPa, which is the lower limit value of the tensile strength of the high-strength steel plate described above. On the other hand, it is not preferable that the carbon equivalent Ceq exceeds 0.55 mass %, since the tensile strength exceeds 2500 MPa, which is the upper limit value of the tensile strength of the high-strength steel plate described above.
  • the Ceq of a steel plate to be overlapped with the high-strength steel plate can take any value.
  • [C], [Si], [Mn], [P], and [S] indicate respective contents (mass %) of C, Si, Mn, P, and S.
  • composition of components capable of securing the tensile strength (750 MPa to 2500 MPa) of the high-strength steel plate described above is only required to select a composition of components capable of securing the tensile strength (750 MPa to 2500 MPa) of the high-strength steel plate described above.
  • the composition of components of the high-strength steel plate is preferably the following composition of components. Note that in the description hereinbelow, % means mass %.
  • C is an element which increases a tensile strength of steel. It is possible that the higher a C content in the steel is, the higher a strength of a nugget becomes. However, if the C content in the steel is less than 0.07 mass %, it is difficult to obtain a tensile strength of 750 MPa or more. On the other hand, if the C content in the steel exceeds 0.45 mass %, a workability of the high-strength steel plate is lowered. Therefore, the C content in the high-strength steel plate is preferably 0.07 mass % to 0.45 mass %.
  • Si is an element which increases a strength of steel by solid solution strengthening and structure strengthening. However, if a Si content in the steel exceeds 2.50 mass %, the workability of the steel is lowered. Meanwhile, it is technically difficult to reduce the Si content in the steel to less than 0.001 mass % industrially. Therefore, the Si content in the high-strength steel plate is preferably 0.001 mass % to 2.50 mass %.
  • Mn is an element which increases a strength of steel.
  • a Mn content in the steel exceeds 5.0 mass %, the workability of the steel deteriorates.
  • the Mn content in the steel is less than 0.8 mass %, it is difficult to obtain a tensile strength of 750 MPa or more. Therefore, the Mn content in the high-strength steel plate is preferably 0.8 mass % to 5.0 mass %.
  • the P content in the high-strength steel plate is preferably 0.03 mass % or less. Note that it is not preferable, in terms of cost, to reduce the P content in the steel to less than 0.001 mass %. Therefore, the P content in the high-strength steel plate is preferably 0.001 mass % or more. However, it is also possible to set the P content in the high-strength steel plate to less than 0.001 mass %.
  • S is an element which causes embrittlement of a nugget. Further, S is an element which is bonded to Mn to form coarse MnS, thereby hindering the workability of steel. If a S content in the steel exceeds 0.01 mass %, a crack in the nugget is apt to occur, which makes it difficult to obtain a sufficiently high joint strength. Further, the workability of the steel is lowered. Therefore, the S content in the high-strength steel plate is preferably 0.01 mass % or less. Note that it is not preferable, in terms of cost, to reduce the S content in the steel to less than 0.0001 mass %. Therefore, the S content in the high-strength steel plate is preferably 0.0001 mass % or more. However, it is also possible to set the S content in the high-strength steel plate to less than 0.0001 mass %.
  • N is an element which forms a coarse nitride to deteriorate the workability of steel. Further, N is an element which causes a generation of blowhole at a time of welding. If a N content in the steel exceeds 0.01 mass %, the deterioration of the workability of steel and the generation of blowhole are caused prominently. Therefore, the N content in the high-strength steel plate is preferably 0.01 mass % or less. Note that it is not preferable, in terms of cost, to reduce the N content in the steel to less than 0.0005 mass %. Therefore, the N content in the high-strength steel plate is preferably 0.0005 mass % or more. However, it is also possible to set the N content in the high-strength steel plate to less than 0.0005 mass %.
  • the O content in the high-strength steel plate is preferably 0.01 mass % or less. Note that it is not preferable, in terms of cost, to reduce the O content in the high-strength steel plate to less than 0.0005 mass %. Therefore, the O content in the high-strength steel plate is preferably 0.0005 mass % or more. However, it is also possible to set the O content in the high-strength steel plate to less than 0.0005 mass %.
  • Al is a ferrite stabilizing element and exhibits an effect such as a suppression of precipitation of cementite during a bainite transformation. Accordingly, Al is contained for controlling a steel structure. Further, Al also functions as a deoxidizer. On the other hand, Al is easily oxidized. If an Al content exceeds 1.00 mass %, inclusions increase, resulting in that the deterioration of the workability of steel is apt to occur. Therefore, the Al content in the high-strength steel plate is preferably 1.00 mass % or less.
  • the high-strength steel plate may selectively contain the following elements according to need, other than the above-described main elements.
  • Ti, Nb, and V are elements which contribute to an increase in a strength of steel by at least any one of precipitation strengthening, fine grain strengthening by a suppression of growth of a ferrite crystal grain, and dislocation strengthening by a suppression of recrystallization.
  • a content of any of the elements in steel is less than 0.005 mass %, the effect of adding the elements is difficult to be exhibited.
  • the content of each of these elements in the steel exceeds 0.20 mass %, the workability of the steel is hindered. Therefore, it is preferable that the contents of these elements in the high-strength steel plate are respectively 0.005 mass % to 0.20 mass %.
  • B is an element which strengthens steel by controlling a steel structure.
  • a B content in the steel is less than 0.0001 mass %, the effect of adding the element is difficult to be exhibited.
  • the B content in the steel exceeds 0.01 mass %, the effect of adding the element is saturated. Therefore, the B content in the high-strength steel plate is preferably 0.0001 mass % to 0.01 mass %.
  • Cr, Ni, Cu, and Mo are elements which contribute to an improvement of strength of steel. These elements can be used in place of a part of Mn (strength improving element), for example. However, if a content of any of the elements in the steel is less than 0.01 mass %, no contribution is made for improving the strength.
  • the contents of these elements in the high-strength steel plate are respectively 0.01 mass % or more.
  • the content of each of Cr, Ni, and Cu in the steel exceeds 2.0 mass %, and if the Mo content in the steel exceeds 0.8 mass %, problems sometimes occur at a time of pickling or hot working. Therefore, it is preferable that the content of each of Cr, Ni, and Cu in the high-strength steel plate is 2.0 mass % or less. Further, it is preferable that the Mo content in the high-strength steel plate is 0.8 mass % or less.
  • Ca, Ce, Mg, and REM are elements which contribute to an improvement of the workability of steel by reducing a size of an oxide after deoxidation or a size of a sulfide existing in a hot-rolled steel plate.
  • contents of these elements in the steel are less than 0.0001 mass % in total, the effect of adding the elements is difficult to be exhibited.
  • the contents of these elements in the steel exceed 1.0 mass % in total, the workability of the steel is reduced. Therefore, it is preferable that the contents of these elements in the high-strength steel plate are 0.0001 mass % to 1.0 mass % in total.
  • REM is an element which belongs to a lanthanoide series
  • REM and Ce can be added as misch metals to molten steel in a stage of steelmaking.
  • elements of the lanthanoide series may be contained compositely.
  • a balance other than the respective elements described above in the high-strength steel plate may be constituted of Fe and inevitable impurities. Note that regarding any one of Cr, Ni, Cu, Mo, B, Ti, Ni, and V described above, containing a very small amount less than the above lower limit values as impurities is tolerated. Further, regarding Ca, Ce, Mg, La, and REM, containing a very small amount less than the above lower limit values of the total amounts thereof as impurities is tolerated.
  • composition of components of the high-strength steel plate and a composition of components of a steel plate to be overlapped with the high-strength steel plate may employ any composition of components.
  • a plating layer may be formed on a surface of the high-strength steel plate. Further, it is also possible that a plating layer is formed on a surface of a steel plate to be overlapped with the high-strength steel plate.
  • the plating layer there can be cited, for example, a Zn base, a Zn—Fe base, a Zn—Ni base, a Zn—Al base, a Zn—Mg base, a Pb—Sn base, a Sn—Zn base, an Al—Si base, and the like.
  • the high-strength steel plate including a Zn-based plating layer there can be cited, for example, an alloyed hot-dip galvanized steel plate, a hot-dip galvanized steel plate, an electrogalvanized steel plate, and the like.
  • the plating layer is formed on the surface of the high-strength steel plate, a spot-welded joint exhibits an excellent corrosion resistance. If the plating layer is a galvanized layer alloyed on the surface of the high-strength steel plate, an excellent corrosion resistance is obtained, and further, an adhesiveness of coating material becomes good.
  • a weight of the plating layer is not particularly limited as well. It is preferable to set a weight of the plating layer on one surface of the high-strength steel plate to 100 g/m 2 or less. If the weight of the plating layer on one surface of the high-strength steel plate exceeds 100 g/m 2 , the plating layer may hinder the welding.
  • the plating layer may be formed on only one surface or both surfaces of the high-strength steel plate. Note that an inorganic or organic coating film (such as, for example, a lubricating coating film) or the like may be formed on a surface layer of the plating layer. Conditions same as the conditions regarding the plating layer described above are applied to a steel plate to be overlapped with the high-strength steel plate.
  • FIG. 1 is a diagram illustrating one example of an arrangement of two pieces of steel plates including at least one piece of high-strength steel plate and welding electrodes when spot welding is started.
  • steel plates 1 A and 1 B are overlapped so that their plate surfaces face each other.
  • the overlapped steel plates 1 A and 1 B are sandwiched by welding electrodes 2 A and 2 B from up and down directions, and by applying a required pressurizing force, the welding electrodes 2 A and 2 B are energized.
  • FIG. 2 is a diagram schematically illustrating one example of a nugget and a heat-affected zone formed by the spot welding.
  • FIG. 3 is a diagram illustrating an example of first form of an energization pattern when the energization is performed on the welding electrodes. Note that in this case, in order to simplify the explanation, a case where two pieces of steel plates including at least one piece of high-strength steel plate are spot-welded, is cited as an example. However, as described above, even in a case where three pieces or more of steel plates including at least one piece of high-strength steel plate are spot-welded, it is possible to conduct the spot welding through a method same as a method to be described below.
  • the steel plates 1 A and 1 B, and the welding electrodes 2 A and 2 B are arranged in a manner as illustrated in FIG. 1 . Further, when energization is performed in an energization pattern illustrated in FIG. 3 , for example, a nugget 3 is formed at a boundary between the steel plates 1 A and 1 B, as illustrated in FIG. 2 . Further, a heat-affected zone 4 is formed in a periphery of the nugget 3 . Note that at least one of the steel plates 1 A and 1 B is the above-described high-strength steel plate.
  • a current to be described below indicates a current which flows between the welding electrode 2 A and the welding electrode 2 B.
  • a current value is gradually increased (up-sloped) from 0 (zero) until when it reaches a value of a main welding current I W (kA). Further, main welding is performed under a state where the current value is set to the value of the main welding current I W (kA). When the main welding is finished, the current value is set to 0 (zero), and a state where the current value is 0 (zero) is retained for a cooling time after main welding (solidification time) t S (msec).
  • the current value is set to a value of a post-energization current I P (kA), and a state where the current value is the value of the post-energization current I P (kA) is retained for a post-energization time t P (msec), thereby performing post-energization.
  • the current value is set to 0 (zero).
  • a retention time t H (msec) indicated in FIG. 3 corresponds to a time of retaining a pressurizing force F E (N) after the post-energization is finished, as will be described later.
  • the current value is not gradually increased (up-sloped) from 0 (zero) until when it reaches the value of the main welding current I W (kA), and the current value is immediately set to the value of the main welding current I W (kA).
  • the energization with the main welding current I W is performed while pressurizing the overlapped plurality of pieces of steel plates by the welding electrodes 2 A and 2 B at the pressurizing force F E satisfying the following expression (2).
  • the pressurizing force F E exceeds “3920 ⁇ h” (N)
  • a welding gun a device which performs energization by applying a pressurizing force to the welding electrodes 2 A and 2 B
  • the pressurizing force F E of the welding electrodes 2 A and 2 B with respect to the steel plates 1 A and 1 B is set to not less than “1960 ⁇ h” (N) nor more than “3920 ⁇ h” (N).
  • the diameters of the tips of the welding electrodes 2 A and 2 B are respectively about 6 mm to 8 mm.
  • h indicates a plate thickness of a steel plate (mm). Plate thicknesses of two pieces of steel plates are sometimes different (in an example illustrated in FIG. 2 , plate thicknesses of the steel plates 1 A and 1 B are sometimes different). In this case, it is only required to use an arithmetic average value of the plate thicknesses of the two pieces of steel plates (an arithmetic average value of the plate thickness of the steel plate 1 A and the plate thickness of the steel plate 1 B), as “h” in the above expression (2).
  • the welding electrodes 2 A and 2 B are energized with the main welding current I W while pressurizing the steel plates 1 A and 1 B at the above-described pressurizing force F E , to thereby perform the main welding.
  • the main welding current I W and a main welding time are not particularly limited. It is only required to employ a welding current and an energization time which are nearly the same as a welding current and an energization time conventionally employed for stably obtaining a nugget with a required size, as the main welding current I W and the main welding time.
  • a square root of an average value in the main welding time of values each of which being a square of the main welding current in the main welding time (specifically, an effective value of the main welding current), or a maximum value of the main welding current, can be employed as the main welding current I W .
  • a spot welding equipment As a spot welding equipment, a conventional spot welding equipment commonly used can be used as it is. Further, regarding welding electrodes and the like, it is also possible to use conventional welding electrodes as they are.
  • a power supply is not limited in particular as well, and an AC power supply, a DC inverter, an AC inverter, or the like can be used.
  • a molten zone is solidified from an outer periphery of the molten zone (specifically, a boundary of the molten zone with another region), to thereby form a shell-shaped solidified region having an unsolidified region remained inside thereof.
  • the boundary of the molten zone with the other region is referred to as a melting boundary according to need.
  • FIG. 4 is a diagram schematically illustrating one example of an appearance in the middle of solidification of the molten zone which is solidified to be a nugget.
  • the unsolidified region 6 is solidified to form a nugget.
  • post-energization is started when the unsolidified region 6 exists.
  • the cooling time after main welding t S determines a width (length in a plate surface direction) of the solidified region 5 at the time of starting the post-energization.
  • a martensite transformation occurs in a process of performing cooling for the cooling time after main welding t S after the main welding.
  • an apparent martensite transformation temperature increases.
  • auto-temper automatic tempering
  • the toughness of the heat-affected zone 4 is improved by later-described post-energization.
  • the heat-affected zone 4 is required to be formed of an austenite single phase.
  • the cooling time after main welding t S has to be set to 300 (msec) or less.
  • the cooling time after main welding t S exceeds 300 (msec)
  • a temperature is lowered to enlarge the solidified region 5 . Therefore, the post-energization for a long time has to be performed for obtaining an effect of post-energization to be described later (effect of structure improvement and segregation improvement) in an outer peripheral portion of the nugget 3 and the heat-affected zone 4 in the periphery of the nugget 3 . Accordingly, the productivity of the spot-welded joint is lowered.
  • the cooling time after main welding t S exceeding 300 (msec) is not realistic.
  • the cooling time after main welding t S is less than “7 ⁇ h+5” (msec)
  • the solidification of the molten zone becomes insufficient, resulting in that the width of the solidified region 5 becomes narrow.
  • the cooling time after main welding t S is less than “7 ⁇ h+5” (msec)
  • the prior austenite grain becomes too large, resulting in that the toughness of the heat-affected zone 4 is lowered, on the contrary, by the post-energization to be described later. Therefore, it is not possible to achieve the effect of post-energization to be described later (effect of structure improvement and segregation improvement), resulting in that it becomes difficult to sufficiently improve the joint strength.
  • a lower limit value of the cooling time after main welding t S is represented by a linear expression using the plate thickness h of the steel plate, as represented by the expression (3).
  • a spot-welded joint having a nugget diameter same as a nugget diameter of the welded joint obtained by first unconventional welding was obtained by overlapping two pieces of steel plates each having the above-described carbon equivalent and the above-described plate thickness, and performing spot welding through a method same as the above-described method except for the performance of the cooling after the main welding and the post-energization. Subsequently, the CTS (cross tensile strength) of each spot-welded joint was measured based on the method defined in JIS Z 3137. In the description hereinbelow, the spot-welded joint is referred to as a welded joint obtained by first conventional welding, according to need.
  • FIG. 5 is a diagram illustrating one example of a relationship between the cooling time after main welding t S and the plate thickness h of the steel plate.
  • a horizontal axis indicates h (mm)
  • a vertical axis indicates t S (msec).
  • the cooling time after main welding t S is set to not less than “7 ⁇ h+5” (msec) nor more than 300 (msec).
  • the cooling time after main welding t S it is more preferable to set the cooling time after main welding t S to not less than “7 ⁇ h+5” (msec) nor more than 250 (msec). Further, in order to facilitate the formation of the solidified region 5 , it is preferable that no energization is performed during the cooling time after main welding t S . However, it is also possible to energize the welding electrodes 2 A and 2 B with a current which is 0.5 times or less the main welding current I W for the cooling time after main welding t S for adjusting a formation speed and a temperature of the solidified region 5 .
  • the plate thickness h of the steel plate in the expression (3) a value same as the value of the plate thickness h of the steel plate in the above expression (2) is employed, for example. Further, it is preferable, in terms of working efficiency, that the pressurizing force F E applied when performing the main welding is retained as it is during the cooling time after main welding t S . However, it is also possible that the pressurizing force F E during the cooling time after main welding t S is different from the pressurizing force F E applied when performing the main welding, within a range satisfying the above expression (2).
  • the welding electrodes 2 A and 2 B are energized with a post-energization current I P (kA) satisfying the following expression (4) for a post-energization time t P (msec) satisfying the following expression (5) while retaining the pressurizing force F E (N) applied when performing the main welding, to thereby conduct post-energization.
  • the pressuring force F E during the post-energization time t P is set to the pressurizing force satisfying the above expression (2). It is preferable, in terms of working efficiency, that this pressurizing force F E is normally set to a pressurizing force same as the pressurizing force F E applied when performing the main welding (when the energization with the main welding current I W is performed), and when the molten zone is solidified from the melting boundary to form the shell-shaped solidified region 5 (during the cooling time after main welding t S ).
  • the pressurizing force F E during the post-energization time t P does not always have to be the same pressurizing force as that applied when performing these operations.
  • the post-energization current I P exerts a large influence on a structure and a segregation of the shell-shaped solidified region 5 , a structure and a segregation of the nugget 3 formed after the completion of solidification, and a structure and a segregation of the heat-affected zone 4 .
  • the post-energization current I P is set to “0.66 ⁇ I W ” (kA) or more and less than “I W ” (kA). Note that in order to obtain the effect of improving the structure and the segregation more securely, it is preferable to set the post-energization current I P to not less than “0.70 ⁇ I W ” (kA) nor more than “0.98 ⁇ I W ” (kA). Note that when an effective value is employed as the main welding current I W , it is preferable that the post-energization current I P also employs an effective value. Further, when a maximum value is employed as the main welding current I W , it is preferable that the post-energization current I P also employs a maximum value.
  • the welding electrodes 2 A and 2 B are energized with the post-energization current I P for a time satisfying the above expression (5) (post-energization time t P (msec)). Accordingly, the structure and the segregation in the solidified region 5 and the heat-affected zone 4 are improved, to thereby increase the reliability of the welded joint.
  • Patent Literature 5 discloses that “when the time exceeds 200 msec, the effect of improving the joint strength and reducing the fluctuation of joint strength becomes small, and further, the productivity is lowered”. Specifically, Patent Literature 5 discloses that the post-energization time t P should be set to 200 (msec) or less.
  • Patent Literature 5 describes a structure inside a nugget. However, no description is made regarding an improvement plan of CTS when a plug fracture occurs. Accordingly, the present inventors conducted systematic experiments regarding post-energization which further increases the CTS when the plug fracture occurs.
  • a spot-welded joint having a nugget diameter same as a nugget diameter of the welded joint obtained by first unconventional welding was obtained by overlapping two pieces of steel plates each having the above-described carbon equivalent and the above-described plate thickness, and performing spot welding through a method same as the above-described method except for the performance of the cooling after the main welding and the post-energization. Subsequently, the CTS (cross tensile strength) of each spot-welded joint was measured based on the method defined in JIS Z 3137. As described in the section of (cooling time after main welding: t S ), in the description hereinbelow, the spot-welded joint is referred to as the welded joint obtained by first conventional welding, according to need.
  • FIG. 6 is a diagram illustrating a first example of a relationship between the post-energization time t P and a square of a value obtained by dividing the post-energization current I P by the main welding current I W ((I P /I W ) 2 ).
  • a plot based on the post-energization time t P , the post-energization current I P , and the main welding current I W when the CTS in the welded joint obtained by first unconventional welding was improved but an amount of improvement was less than 20% or when it was not improved, when compared to the CTS in the welded joint obtained by first conventional welding, is indicated by ⁇ .
  • a horizontal axis indicates (I P /I W ) 2
  • a vertical axis indicates t P (ms).
  • the plug fracture in the spot-welded joint occurs in the heat-affected zone 4 . Therefore, it was estimated that a difference in plug fracture strengths is generated by a difference in resistance forces with respect to a propagation of crack in the heat-affected zone 4 , namely, a difference in toughness of the heat-affected zone 4 . Accordingly, a concentration distribution of P and S exerting a large influence on the toughness of the heat-affected zone 4 was measured through FE-EPMA. As a result of this, in FIG.
  • the post-energization current I P is required to be a current of a value at which the melting of the solidified region 5 does not occur. Specifically, it is required to satisfy the condition of I W >I P .
  • the I P /I W is an index of determining a heat input amount when performing the post-energization (a size of the nugget 3 ). Accordingly, the I P /I W is expressed as ⁇ ( ⁇ 1).
  • the heat generated in the post-energization is in proportion to a square of the post-energization current I P . Therefore, in FIG. 6 , the horizontal axis takes (I P /I W ) 2 . Further, a part of the heat generated in the post-energization is escaped to all over the welding electrodes 2 A and 2 B and steel plates 1 A and 1 B. A quantity of the heat to be escaped is set to ⁇ . Accordingly, a heat quantity Q which acts on the increase in temperature of the nugget 3 and the heat-affected zone 4 during the post-energization, can be represented by the following expression (6).
  • the automatic tempering caused by the post-energization is apt to occur.
  • a heat quantity exceeding a heat quantity A being a predetermined quantity is required.
  • FIG. 7 is a diagram illustrating, in a conceptual manner, one example of a relationship between the post-energization time t P and a degree of embrittlement of the outer peripheral portion of the nugget 3 and the heat-affected zone 4 .
  • FIG. 7 illustrates, in a conceptual manner, a sequence of events in which the segregation of P and S is reduced and the toughness is improved.
  • a vertical axis indicates a degree of embrittlement caused by the segregation or insufficient automatic tempering. As the value on the vertical axis is lowered, the segregation is reduced and the automatic tempering is sufficiently performed, resulting in that the toughness is improved.
  • a temperature in the outer peripheral portion of the nugget 3 reaches a substantially steady temperature ( ⁇ melting point) due to the main welding performed for forming the welded portion, and thus is completely increased. On the contrary, a temperature of the heat-affected zone 4 is not sufficiently increased by the main welding.
  • the temperature of the heat-affected zone 4 is lower than the temperature of the outer peripheral portion of the nugget 3 which is just solidified and thus has a high temperature. For this reason, it takes a long time to perform heat treatment by retaining the heat-affected zone 4 at a high temperature with the use of the post-energization, when compared to a time required for performing heat treatment on the outer peripheral portion of the nugget 3 . This can be estimated to be a reason why the result in FIG. 6 can be obtained.
  • an upper limit value of the post-energization time t P is not particularly defined, the upper limit value is preferably 2000 (msec) or less, when the productivity of the spot-welded joint is taken into consideration.
  • the mutually overlapped steel plates 1 A and 1 B are pressurized and retained by the welding electrodes 2 A and 2 B for a retention time t H (msec) defined by the following expression (9), and then the pressurizing is released.
  • the pressurizing force F E (N) applied when the steel plates 1 A and 1 B are pressurized and retained by the welding electrodes 2 A and 2 B for the retention time t H within the range represented by the expression (9), is within a range defined by the above expression (2), for example.
  • the retention time t H exerts an influence on an occurrence of a defect or a crack in a structure of the nugget 3 and the heat-affected zone 4 and inside the nugget 3 .
  • the retention time t H exceeds 300 (msec)
  • the productivity of the spot-welded joint is lowered. Therefore, in the present embodiment, the retention time t H is set to 300 (msec) or less.
  • the retention time t H is desirably short, in order to stably achieve a desired effect by starting air cooling in an early stage.
  • the temperature of the nugget 3 is lowered also when the post-energization is performed. Accordingly, even if the retention time t H is shortened, a contraction defect or a crack is difficult to occur. Therefore, if it is possible to immediately separate the welding electrodes 2 A and 2 B from the steel plates 1 A and 1 B, the retention time t H may also be set to 0 (zero). When the retention time is not set to 0 (zero), the expression (9) becomes the following expression (9a).
  • the steel plate 1 A and the steel plate 1 B are overlapped so that their plate surfaces face each other, as illustrated in FIG. 1 .
  • the overlapped steel plate 1 A and steel plate 1 B are sandwiched by the welding electrode 2 A and the welding electrode 2 B from up and down directions, and the energization is performed by applying a required pressurizing force.
  • a high-strength steel plate generally has a large electrical resistance, so that heat generation is apt to occur when performing main welding. Further, when performing main welding, a gap between mutually adjacent two pieces of steel plates may exist. If an internal pressure of a molten metal exceeds an external pressure which acts on a corona bond when performing main welding, an expulsion occurs.
  • One of objects of performing the pre-energization is to suppress the occurrence of expulsion.
  • FIG. 8 is a diagram illustrating an example of second form of an energization pattern when performing energization on welding electrodes.
  • a current value is set to a value of pre-energization current I f (kA), and a state where the current value is the value of pre-energization current I f (kA) is retained for a pre-energization time t f (msec), to thereby perform pre-energization.
  • the current value is set to 0 (zero), and a state where the current value is 0 (zero) is retained for a cooling time after pre-energization t C (msec).
  • the main welding is performed under a state where the current value is set to the value of the main welding current I W (kA).
  • the current value is set to 0 (zero), and a state where the current value is 0 (zero) is retained for the cooling time after main welding (solidification time) t S (msec).
  • the current value is set to the value of the post-energization current I P (kA), and a state where the current value is the value of the post-energization current I P (kA) is retained for the post-energization time t P (msec), thereby performing the post-energization.
  • the current value is set to 0 (zero). Note that the retention time t H (msec) indicated in FIG. 8 corresponds to a time of retaining the pressurizing force F E (N) after the post-energization is finished, as described in the first example.
  • the current value is not set to the value of the pre-energization current I f (kA) immediately, and is gradually increased (up-sloped) from 0 (zero) until when it reaches the value of the pre-energization current I f (kA).
  • the energization with the pre-energization current I f is performed while pressurizing the overlapped plurality of pieces of steel plates by the welding electrodes 2 A and 2 B at the pressurizing force F E satisfying the above expression (2).
  • the overlapped plurality of pieces of steel plates are pressurized to prevent a generation of gap between the adjacent two pieces of steel plates 1 A and 1 B.
  • a range of the pressurizing force F E in the pre-energization is set to a range same as the range of the pressurizing force F E applied in the main welding and the post-energization, thereby increasing the working efficiency.
  • the pre-energization current I f is set to equal to or more than the main welding current I W , there is a possibility that the expulsion occurs when performing the pre-energization.
  • the pre-energization current I f is set to less than 0.4 times the main welding current I W , a quantity of heat to be supplied to the steel plates 1 A and 1 B becomes insufficient. Consequently, there is a possibility that the steel plates 1 A and 1 B cannot be softened, and it is not possible to sufficiently reduce the gap between the steel plates 1 A and 1 B by the above-described pressurizing, resulting in that the expulsion occurs when performing the main welding.
  • the pre-energization current I f is set to 0.4 times or more the main welding current I W and less than the main welding current I W .
  • the pre-energization current I f when an effective value is employed as the main welding current I W , it is preferable that the pre-energization current I f also employs an effective value. Further, when a maximum value is employed as the main welding current I W , it is preferable that the pre-energization current I f also employs a maximum value.
  • the pre-energization time t f is less than 20 (msec)
  • a quantity of heat to be supplied to the steel plates 1 A and 1 B becomes insufficient. Consequently, there is a possibility that the steel plates 1 A and 1 B cannot be softened, and it is not possible to sufficiently reduce the gap between the steel plates 1 A and 1 B by the above-described pressurizing, resulting in that the expulsion occurs when performing the main welding.
  • the upper limit value of the pre-energization time t f is preferably 300 (msec) or less, when the productivity of the spot-welded joint is taken into consideration.
  • the cooling time after pre-energization t C can be set to a time exceeding 0 (zero). Note that if there is no occurrence of expulsion when performing the pre-energization, it is possible to set the cooling time after pre-energization t C to 0 (zero). Further, if the cooling time after pre-energization t C becomes “200+7 ⁇ h” (msec) or more, the steel plates 1 A and 1 B are cooled too much, resulting in that conformability of the steel plates 1 A and 1 B may be lost when performing the main welding. The larger the plate thickness h of the steel plate is, the slower the cooling rate of the steel plates 1 A and 1 B becomes.
  • an upper limit value of the cooling time after pre-energization t C is represented by a linear expression using the plate thickness h of the steel plate, as represented by the expression (12).
  • FIG. 9 is a diagram illustrating one example of a relationship between the cooling time after pre-energization t C and the plate thickness h of the steel plate.
  • a plot based on the cooling time after pre-energization t C and the plate thickness h of the steel plate when the expulsion did not occur in the aforementioned examination is indicated by ⁇ .
  • a plot based on the cooling time after pre-energization t C and the plate thickness h of the steel plate when the expulsion occurred in the aforementioned examination is indicated by ⁇ .
  • a horizontal axis indicates h (mm)
  • a vertical axis indicates t C (msec).
  • the cooling time after pre-energization t C is set to not less than 0 (zero) nor more than 200+7 ⁇ h” (msec).
  • the plate thickness h of the steel plate in the expression (12) a value same as the value of the plate thickness h of the steel plate in the above expression (2) is employed, for example. Further, it is preferable, in terms of working efficiency, that the pressurizing force F E applied when performing the pre-energization is retained as it is during the cooling time after pre-energization t C . However, it is also possible that the pressurizing force F E during the cooling time after pre-energization t C is different from the pressurizing force F E applied when performing the pre-energization, within a range satisfying the above expression (2).
  • energization with the main welding current I W is performed between the welding electrodes 2 A and 2 B while retaining the pressurizing force F E applied when performing the pre-energization as it is, to thereby conduct the main welding.
  • the main welding current I W and the main welding time are not particularly limited. Note that it is preferable, in terms of working efficiency, that the pressurizing force F E applied when performing the pre-energization is retained as it is during the main welding time. However, it is also possible that the pressurizing force F E during the main welding time is different from the pressurizing force F E applied when performing the pre-energization, within a range satisfying the above expression (2).
  • a method of determining the cooling time after main welding t S is a method same as that of the first example.
  • the cooling time after main welding t S is more preferably set to not less than “7 ⁇ h+5” (msec) nor more than 250 (msec).
  • it is preferable that no energization is performed during the cooling time after main welding t S but, it is also possible to perform energization with a current which is 0.5 times or less the main welding current I W during the cooling time after main welding t S for adjusting the formation speed and the temperature of the solidified region 5 .
  • the pressurizing force F E applied when performing the pre-energization and the main welding is retained as it is during the cooling time after main welding t S .
  • the pressurizing force F E during the cooling time after main welding t S is different from the pressurizing force F E applied when performing the pre-energization and the main welding, within a range satisfying the above expression (2).
  • the expression (13) is the same as the above expression (4).
  • a method of determining the post-energization current I P is a method same as that of the first example.
  • the post-energization current I P is preferably set to not less than “0.70 ⁇ I W ” (kA) nor more than “0.98 ⁇ I W ” (kA), in order to obtain the effect of improving the structure and the segregation more securely.
  • the pressurizing force F E during the post-energization time t P is different from the pressurizing force F E applied when performing the pre-energization and the main welding, within a range satisfying the above expression (2).
  • a spot-welded joint having a nugget diameter same as a nugget diameter of the welded joint obtained by second unconventional welding was obtained by overlapping two pieces of steel plates each having the above-described carbon equivalent and the above-described plate thickness, and performing spot welding through a method same as the above-described method except for the performance of the cooling after the main welding and the post-energization. Subsequently, the CTS (cross tensile strength) of each spot-welded joint was measured based on the method defined in JIS Z 3137. In the description hereinbelow, the spot-welded joint is referred to as a welded joint obtained by second conventional welding, according to need.
  • FIG. 10 is a diagram illustrating a second example of a relationship between the post-energization time t P and a square of a value obtained by dividing the post-energization current I P by the main welding current I W ((I P /I W ) 2 ).
  • a plot based on the post-energization time t P , the post-energization current I P , and the main welding current I W when the CTS in the welded joint obtained by second unconventional welding was improved but an amount of improvement was less than 20% or when it was not improved, when compared to the CTS in the welded joint obtained by second conventional welding, is indicated by ⁇ .
  • a horizontal axis indicates (I P /I W ) 2
  • a vertical axis indicates t P (msec).
  • FIG. 10 is a diagram corresponding to FIG. 6 .
  • a boundary line between ⁇ and ⁇ was determined as a regression curve (specifically, coefficients A and ⁇ in the expression (8) were determined). From a result of the determination, the above expression (14) was obtained.
  • the expression (14) corresponds to the above expression (5).
  • the coefficient ⁇ is “0.44”.
  • the coefficient ⁇ is “0.4”. Therefore, a lower limit value of the post-energization time t P in the second example becomes smaller than that in the first example. It can be considered that this is because a total heat input amount with respect to the heat-affected zone 4 becomes large because of the performance of pre-energization.
  • the upper limit value of the post-energization time t P is not particularly defined, the upper limit value is preferably 2000 (msec) or less, when the productivity of the spot-welded joint is taken into consideration.
  • the lower limit value of the post-energization time t P can be set to be small.
  • the present example also employs the above expression (5), instead of the expression (14).
  • the mutually overlapped steel plates 1 A and 1 B are pressurized and retained by the welding electrodes 2 A and 2 B for a retention time t H (msec) defined by the above expression (9), and then the pressurizing is released.
  • a method of determining the retention time t H is a method same as that of the first example. Note that as described in the first example, there is a need to set the retention time t H by considering the fact that the actual retention time t H becomes longer than the set retention time t H . Further, as described in the first example, it is also possible to set the retention time t H to 0 (zero).
  • FIG. 11A is a diagram (photograph) illustrating one example of a structure of a heat-affected zone of the above-described welded joint obtained by unconventional welding (the above-described welded joint obtained by first unconventional welding).
  • FIG. 11B is a diagram (photograph) illustrating one example of a structure of a heat-affected zone of the above-described welded joint obtained by conventional welding (the above-described welded joint obtained by first conventional welding). As illustrated in FIG. 11A and FIG.
  • the iron-based carbide mentioned here is mainly cementite (Fe 3 C).
  • the iron-based carbide is not limited to cementite.
  • ⁇ carbide (Fe 2.4 C) or the like is contained in the iron-based carbide.
  • another metal of Mn, Cr, or the like is contained in the iron-based carbide.
  • the present inventors examined the state of precipitation of the iron-based carbides in the heat-affected zone of each of a plurality of welded joints obtained by unconventional welding having the CTS which is improved by 20% or more when compared to the CTS in the welded joint obtained by conventional welding. As a result of this, it was confirmed that any welded joint obtained by unconventional welding having the CTS which is improved by 20% or more when compared to the CTS in the welded joint obtained by conventional welding, always satisfies the precipitation condition of iron-based carbides to be described below.
  • FIG. 12A is a diagram explaining one example of the precipitation condition of iron-based carbides.
  • FIG. 12B is a diagram illustrating a part of a region A in FIG. 12A in an enlarged manner.
  • FIG. 12A is a diagram schematically illustrating a cross section that passes through a center of a welding mark formed on surfaces of the steel plates 1 A and 1 B by the spot welding, and is cut along the plate thickness direction of the steel plates 1 A and 1 B.
  • a center of the welding mark for example, a target position (spot position) of (endmost regions of) the welding electrodes 2 A and 2 B can be employed.
  • a contour of an actually formed welding mark is approximated by circle, and a center of the circle is set to the center of the welding mark.
  • the precipitation condition of iron-based carbides described above is that ten or more of iron-based carbides in each of which a length of a longest portion is 0.1 ( ⁇ m) or more are precipitated (exist) in a square region 123 whose length of one side is 10 ⁇ m in which a plate thickness direction and a plate surface direction of the steel plates 1 and 2 are set to a vertical direction and a horizontal direction, respectively, being a region within the heat-affected zone 4 of such a cross section.
  • a position of a center of the square region 123 is a position 102 , at the cross section, separated by 100 ( ⁇ m) from a position 120 of an end portion of the nugget 3 in a direction perpendicular to a tangent 121 to a line indicating the end portion of the nugget 3 , at that position 120 .
  • the position 120 of the end portion of the nugget 3 is a position, out of positions on the line indicating the end portion of the nugget 3 , within a range whose center is set to a center in the plate thickness direction of the spot-welded joint and having a length of 1 ⁇ 4 times a total plate thickness t sum being a total value of plate thicknesses of the steel plates 1 A and 1 B before being subjected to the spot welding, along the plate thickness direction (within a range indicated by t sum /4 in FIG. 12A ).
  • a length including a part of gap between the steel plates 1 A and 1 B is represented as the total plate thickness t sum , for the convenience of representation.
  • the total value of the plate thicknesses of the steel plates 1 A and 1 B before being subjected to the spot welding which does not include the length of the part of the gap between the steel plates 1 A and 1 B, is set to the total plate thickness t sum , as described above.
  • the position of the center in the plate thickness direction of the spot-welded joint it is possible to employ, for example, a position of a center of a length in the plate thickness direction of a part passing through the center of the welding mark in the above-described cross section.
  • the length of the longest portion of the iron-based carbide it is possible to employ, for example, a maximum value of a distance between arbitrary two points on a line indicating an end portion of the iron-based carbide, in the above-described cross section. Further, it is also possible to employ a maximum value of a length of a straight line between two points on a line configuring the end portion of the iron-based carbide, being a length of a straight line passing through a position of a center of gravity of the iron-based carbide, in the above-described cross section, as the length of the longest portion of the iron-based carbide.
  • the reason why the square region 123 is determined as described above, is because such a region 123 is a region inside the heat-affected zone 4 , and is also a region in which a crack occurs in an initial stage when a plug fracture occurs in a cross tensile test.
  • At least one of the steel plates 1 A and 1 B is the above-described high-strength steel plate.
  • the precipitation condition of iron-based carbides described above can also be applied to a case where three pieces or more of steel plates including at least one piece of high-strength steel plate are spot-welded.
  • the above-described cross section is polished. Thereafter, an electron micrograph of a region including the square region 123 is photographed. From the electron micrograph, a length of a longest portion of each iron-based carbide is measured, and a number of iron-based carbides in each of which the length of the longest portion is 0.1 ( ⁇ m) or more is counted. From the number of iron-based carbides, it can be judged whether or not the precipitation condition of iron-based carbides described above is satisfied. Note that in the following description, the above-described square region 123 is referred to as iron-based carbide number counting region, according to need.
  • Steel plates A, B, and C represented in Table 1 were prepared.
  • the steel plate A is obtained by applying Al plating to a surface of a hot-stamped steel plate having a plate thickness of 2.0 (mm) and a tensile strength in the class of 1470 MPa.
  • the steel plate B is obtained by applying Al plating to a surface of a hot-stamped steel plate having a plate thickness of 1.6 (mm) and a tensile strength in the class of 1470 MPa.
  • the steel plate C is obtained by applying Zn plating to a surface of a hot-stamped steel plate having a plate thickness of 1.4 (mm) and a tensile strength in the class of 1470 MPa.
  • steel plates D and E represented in Table 1 were prepared.
  • the steel plate D is obtained by applying Zn plating to a surface of a cold-rolled steel plate having a plate thickness of 1.2 (mm) and a tensile strength in the class of 1180 MPa.
  • the steel plate E is a cold-rolled steel plate having a plate thickness of 1.4 (mm) and a tensile strength in the class of 980 MPa.
  • the steel plates A to E are steel plates each containing the above-described composition of components within the ranges of upper limits and lower limits described above.
  • the strength ratio to joint obtained by conventional welding is obtained by multiplying a value as a result of dividing a value obtained by subtracting, from the CTS in the spot-welded joint formed under the welding conditions represented by the numbers 1-1 to 1-33, and 2-1 to 2-18 (the CTS in the welded-joint obtained by unconventional welding), the CTS in the spot-welded joint formed under conditions same as the welding conditions except for the performance of the cooling after the main welding and the post-energization (the CTS in the welded joint obtained by conventional welding), by the CTS in the spot-welded joint formed under the welding conditions (the CTS in the welded joint obtained by unconventional welding), by 100. Note that also in FIG. 5 , FIG. 6 , and FIG.
  • a type of plot is changed based on whether or not the strength ratio to joint obtained by conventional welding is improved by 20% or more.
  • the reason why the criterion is whether or not the strength ratio to joint obtained by conventional welding is improved by 20% or more, is because if the strength ratio to joint obtained by conventional welding is improved by 20% or more, it can be said that there is a significant difference between the CTS in the welded joint obtained by unconventional welding and the CTS in the welded joint obtained by conventional welding.
  • a target position of electrodes was set as a center of a welding mark. Further, the two pieces of steel plates were cut so as to pass through the center of the welding mark and to be along the plate thickness direction of the two pieces of steel plates, and the cross section was polished. The polished cross section was observed with the scanning electron microscope, to decide the above-described iron-based carbide number counting region. First, one of two positions separated by a length of 1 ⁇ 8 times a total plate thickness of the two pieces of steel plates before being subjected to the welding, in the plate thickness direction from a center of the spot-welded joint in the plate thickness direction, being a position at an end portion of a nugget of the polished cross section, was specified.
  • a position separated from this position by 100 ( ⁇ m) in a direction perpendicular to a tangent to a line indicating the end portion of the nugget (a line indicating a contour of the nugget) at the specified position was specified from a region within a heat-affected zone of the polished cross section. Further, a square region, being a region in which the above position is set as its center, whose length of one side is 10 ⁇ m in which a plate thickness direction and a plate surface direction of the two pieces of steel plates are set to a vertical direction and a horizontal direction, respectively, was set to the above-described iron-based carbide number counting region. Further, a maximum value of a distance between arbitrary two points on a line indicating an end portion of iron-based carbide, was set to a longest portion.
  • dome radius-type electrodes each made of copper and having a tip with a radius of curvature of 40 (mm).
  • the steel plates A, B, and C were subjected to welding at a pressurizing force of 5000 (N) by using electrodes each having a tip diameter of 8 (mm).
  • the steel plates D and E were subjected to welding at a pressurizing force of 3500 (N) by using electrodes each having a tip diameter of 6 (mm). Note that the pressurizing force was set to be unchanged during the performance of pressurizing.
  • the present invention can be utilized in an industry which uses spot welding as a manufacturing technique, for example.
US14/889,111 2013-06-05 2014-06-02 Spot-welded joint and spot welding method Abandoned US20160082543A1 (en)

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CA2912591A1 (en) 2014-12-11
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BR112015028782A2 (pt) 2017-07-25
TWI587954B (zh) 2017-06-21

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