WO2023063097A1 - 抵抗スポット溶接継手およびその抵抗スポット溶接方法 - Google Patents

抵抗スポット溶接継手およびその抵抗スポット溶接方法 Download PDF

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Publication number
WO2023063097A1
WO2023063097A1 PCT/JP2022/036459 JP2022036459W WO2023063097A1 WO 2023063097 A1 WO2023063097 A1 WO 2023063097A1 JP 2022036459 W JP2022036459 W JP 2022036459W WO 2023063097 A1 WO2023063097 A1 WO 2023063097A1
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Prior art keywords
nugget
resistance spot
less
energization
region
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PCT/JP2022/036459
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English (en)
French (fr)
Japanese (ja)
Inventor
玲子 遠藤
克利 ▲高▼島
広志 松田
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JFE Steel Corp
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JFE Steel Corp
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Priority to CN202280066403.9A priority Critical patent/CN118043157A/zh
Priority to MX2024004028A priority patent/MX2024004028A/es
Priority to EP22880793.9A priority patent/EP4371687A4/en
Priority to KR1020247010972A priority patent/KR102948516B1/ko
Priority to JP2022579782A priority patent/JP7508025B2/ja
Priority to US18/698,067 priority patent/US20250235949A1/en
Publication of WO2023063097A1 publication Critical patent/WO2023063097A1/ja
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • C21D9/505—Cooling thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • 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
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • B23K11/163—Welding of coated materials
    • B23K11/166—Welding of coated materials of galvanized or tinned materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • 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
    • B23K11/24—Electric supply or control circuits therefor
    • 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
    • B23K11/24—Electric supply or control circuits therefor
    • B23K11/241—Electric supplies
    • 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
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
    • B23K35/24—Selection of soldering or welding materials proper
    • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550°C
    • B23K35/3053—Fe as the principal constituent
    • B23K35/3073—Fe as the principal constituent with Mn as next major constituent
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/006—Vehicles
    • 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
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/18—Sheet panels
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    • 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
    • B23K2101/00—Articles made by soldering, welding or cutting
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    • 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
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/34—Coated articles ; Surface treated articles
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    • 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
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a resistance spot welded joint and its resistance spot welding method.
  • the joint strength of resistance spot welds joined by resistance spot welding is divided into the tensile strength in the shear direction (TSS) and the tensile strength in the peeling direction (CTS). tension strength).
  • TSS in resistance spot welds tends to increase with the tensile strength of the base metal
  • CTS is said to decrease when the tensile strength of the base metal is 780 MPa or more.
  • the failure morphology of the resistance spot weld (weld) changes from ductile plug failure in the base metal or heat affected zone (HAZ) around the resistance spot weld to brittleness in the nugget. transition to interfacial rupture or partial plug rupture.
  • the cause of the decrease in CTS is, for example, brittle fracture caused by hardening of the nugget edge after quenching. Therefore, in order to solve such problems, various post-energization methods have been studied in which current is again supplied after the main current has been supplied.
  • Patent Documents 1 to 5 can be cited as techniques for solving such problems.
  • a melt-solidified portion (nugget) and a heat-affected zone in a joint obtained by resistance-welding a resistance-welded steel plate having a specific chemical composition are tempered martensite or tempered bainite. revealing something.
  • Patent Document 2 discloses a welded joint obtained after performing a post-energization step at a temperature of Ac 1 point or less.
  • the spot weld of this weld joint has a region of tempered martensite formed between the center of the nugget and the softest zone of the lowest hardness in the heat affected zone.
  • Patent Document 3 discloses a resistance spot welded joint that defines the hardness of the outside of the nugget and the structure inside the nugget.
  • the resistance spot welds of this resistance spot welded joint are conditioned on the fact that the structure in the nugget is an equiaxed martensitic structure, and that a softened region with a lower degree than the base metal exists outside the nugget.
  • Patent Document 4 discloses a spot welding method that is divided into three steps: a first energization step for forming a nugget, a cooling step for non-energization, and a second energization step for softening the nugget.
  • the martensite formed in the first energization step is tempered into tempered martensite, thereby softening the nugget and tempering the inside of the nugget.
  • Patent Literature 5 discloses a resistance spot welding method that reduces the amount of hydrogen taken into the weld metal by combining the main current-carrying process and the post-current current process, and further suppresses the shrinkage of the nugget due to the occurrence of expulsion. ing.
  • Japanese Patent No. 5182855 Japanese Patent No. 5713147 Japanese Patent No. 5895430 Japanese Patent No. 6107939 Japanese Patent No. 6315161
  • Patent Document 1 only discloses the chemical composition of the resistance-welded steel sheet and the structure of the joint, and the welding conditions in the post-energization method in which the current is again applied after the main current, specifically the temperature range of the post-energization. nothing has been taken into account.
  • a steel plate high-strength steel plate
  • the welded portion is heated to a temperature in the vicinity of the Ac 1 point, so that the nugget edge after welding It is necessary to convert the martensite of the steel into a two-phase structure consisting of ferrite and martensite.
  • an object of the present invention is to provide a technique for realizing structure control for tempering a weld zone at an appropriate temperature. That is, the present invention and Patent Document 1 have different technical ideas.
  • Patent Literature 2 has a region made of tempered martensite between the nugget center and the softest portion with the lowest hardness in the heat affected zone.
  • the energization in the post-energization step is performed in one step, the temperature rises due to this energization, and the temperature cannot be kept constant. As a result, the HAZ cannot be sufficiently tempered.
  • Patent Document 2 When applying the technique of Patent Document 2 to a steel sheet having the chemical composition of the present invention, it is unclear whether it is possible to achieve both an improvement in CTS and an improvement in delayed fracture resistance.
  • a region composed of tempered martensite as described in Patent Document 2 does not occur.
  • the nugget edge by raising the temperature of the nugget edge to a temperature just above the Ac 1 point, the nugget edge becomes the above two-phase structure having ferrite. As a result, the above-described effect of alleviating the stress concentration at the nugget edge and improving the toughness of the nugget edge can be obtained. That is, the present invention and Patent Document 2 have different technical ideas.
  • Patent Document 3 is a technique in which deformation is concentrated in the softened region outside the nugget by making the structure in the nugget an equiaxed martensite structure and by allowing a softened region with a lower degree than the base material to exist outside the nugget. be.
  • the above-described two-phase structure having ferrite at the nugget edge is a technique that can obtain the effects of stress concentration relaxation at the nugget edge and improved toughness at the nugget edge. That is, in Patent Document 3, no consideration is given to structure control for tempering a welded portion in an appropriate temperature range as in the present invention.
  • Patent document 4 is a technique of tempering the martensite formed in the first energization process to make tempered martensite in the second energization process.
  • the present invention is a technique for forming the above-mentioned two-phase structure in which the nugget ends have ferrite, and the structure inside the nugget obtained by the present invention is a hardened structure without being tempered. That is, the present invention and Patent Document 4 have different technical ideas.
  • the welding method of Patent Document 5 does not include a cooling process other than a holding time during the energization process, it is not possible to achieve the above-described two-phase structure in which the nugget ends have ferrite as in the present invention.
  • the present invention is a technique for suppressing hydrogen entering steel by making the structure of the nugget end portion contain ferrite and the HAZ containing tempered martensite. That is, the present invention and Patent Document 5 have different technical ideas.
  • the present invention has been made in view of the above problems, and is a resistance spot welded joint in which a plurality of steel plates including at least one high-strength steel plate are resistance spot welded, and which has improved joint strength and delayed fracture resistance. It aims at providing the resistance spot welding method.
  • the present invention uses a plate assembly containing at least one high-strength steel plate, a cross tensile strength (CTS) reduction mechanism in resistance spot welding, and a method for improving cross tensile strength (CTS). have been diligently examined.
  • CTS cross tensile strength
  • the CTS decreases as the strength of the steel sheet increases.
  • the fracture morphology for low CTS ranges from ductile plug rupture in the base metal or heat affected zone (HAZ) around the resistance spot weld to interfacial or partial plug rupture in the nugget to brittle, Transition. This makes it difficult to ensure CTS with high-strength steel sheets.
  • the causes of interface fracture are (i) the formation of a hardened structure due to rapid cooling after nugget formation and embrittlement of the nugget edge, and (ii) stress concentration at the hardened nugget edge and HAZ, which causes the nugget edge to become brittle. cracks in the part.
  • the toughness of the structure at the nugget edge is increased, and the stress of the crack generated from the sheet separation is dispersed in the structure in the tempered HAZ. Let It is necessary to prevent cracks from penetrating into the interior of the nugget by avoiding stress concentration at the nugget edges due to its distribution.
  • the energization conditions after the main energization process are particularly focused. Specifically, from the results of welding under various energization conditions, temperature control was performed to realize the temperature history during energization at the nugget end portion shown in FIG. 5 and the temperature history during energization at the HAZ shown in FIG. As a result, the inventors have found that the effects described above can be obtained, and that a welded portion having the properties aimed at by the present invention can be formed.
  • the HAZ becomes a structure containing tempered martensite, which has the effect of suppressing hydrogen entering the steel and improves delayed fracture resistance. It was also found that stress concentration at the nugget edge can be relieved.
  • the high-strength steel plate is mass%, C: 0.05 to 0.6%, Si: 0.1 to 2.0%, Mn: 1.5-4.0%, P: 0.10% or less, S: 0.005% or less, and N: 0.001 to 0.010% and the balance has a component composition consisting of Fe and unavoidable impurities
  • the two points on the boundary of the nugget that intersect the overlapping surface of the steel plate are defined as the first end and the second end, and the length of the line segment X connecting the first end and the second end is D (mm) and the positions on the line segment X toward the center of the nugget from the first end and the second end are points a and b,
  • a resistance spot welding method for a resistance spot welded joint according to any one of [1] to [4], When forming the resistance spot welds by sandwiching a plate set in which two or more steel plates including at least one of the high-strength steel plates are superimposed with a pair of welding electrodes and applying current while applying pressure,
  • the energization includes a main energization step and a post-tempering heat treatment step, In the main energizing step, energizing at a current value I 1 (kA) to form a nugget, In the post-tempering heat treatment step, During the cooling time t c1 (ms) shown in Equation (6), a first cooling process is performed to maintain the non-energized state, Next, a temperature rising process is performed in which the resistance spot weld is energized at a current value I 2 (kA) shown in Equation (7) for an energization time t 2 (ms) shown in Equation (8),
  • a resistance spot welded joint and a method of resistance spot welding thereof that alleviates the stress concentration at the nugget edge and improves the toughness of the nugget edge.
  • the joint strength and delayed fracture resistance of the resistance spot-welded joint can be improved, and there is a significant industrial effect.
  • FIG. 1 is a cross-sectional view schematically showing the periphery of a resistance spot welded portion of a resistance spot welded joint according to one embodiment of the present invention.
  • FIG. 2 is a cross-sectional view schematically showing the periphery of a resistance spot welded portion of a resistance spot welded joint according to one embodiment of the present invention.
  • FIG. 3 is a cross-sectional view schematically showing the periphery of a resistance spot welded portion of a resistance spot welded joint according to one embodiment of the present invention.
  • FIG. 4 is a cross-sectional view illustrating an example of the resistance spot welding method of the present invention.
  • FIG. 1 is a cross-sectional view schematically showing the periphery of a resistance spot welded portion of a resistance spot welded joint according to one embodiment of the present invention.
  • FIG. 2 is a cross-sectional view schematically showing the periphery of a resistance spot welded portion of a resistance spot welded joint according to one embodiment of the present
  • FIG. 5 is a graph showing the relationship between the energization time and the temperature of the nugget end portion in the post-tempering heat treatment process of the present invention.
  • FIG. 6 is a graph showing the relationship between HAZ energization time and temperature in the post-tempering heat treatment step of the present invention.
  • FIG. 7 is a diagram illustrating an example of an energization pattern of the resistance spot welding method of the present invention.
  • FIGS. 1-3 show, as an example, cross-sectional views in the plate thickness direction of a resistance spot welded portion and its surroundings in a resistance spot welded joint of the present invention.
  • FIG. 1 shows the case where the number of steel plates to be overlapped is two
  • FIG. 2 shows the case where the number of steel plates to be overlapped is two and there is a gap between the steel plates
  • the present invention is a resistance spot welded joint in which a plurality of overlapping steel plates are resistance spot welded.
  • the steel sheets to be superimposed include at least one high-strength steel sheet to be described later.
  • the number of the plurality of steel plates is not particularly limited, and may be two or more. In addition, although the upper limit of the number of sheets of said several steel plates is not specifically defined, it is preferable to set it to four or less.
  • FIG. 1 shows a resistance spot-welded joint 11 in which two steel plates are superimposed and welded, and a high-strength steel plate is used for the steel plate 1 placed on the lower side and/or the steel plate 2 placed on the upper side.
  • a high-strength steel plate is used for the steel plate 2 on the upper side.
  • the high-strength steel sheet may have a plating layer as described later, the illustration of the plating layer on the surface of the steel sheet is omitted in FIG.
  • the steel sheet mating surfaces (lapped surfaces) 7 of the steel sheets 1 and 2 are formed with resistance spot welds, which will be described below.
  • a resistance spot weld (hereinafter referred to as “weld”) of resistance spot weld joint 11 has nugget 3 and heat affected zone (HAZ) 6 .
  • the structure of the nugget edge and the region in the HAZ near the nugget edge are defined as follows.
  • two points on the boundary of the nugget 3 that intersect with the overlapping surfaces 7 of the steel plates 1 and 2 are defined as a first end 8 and a second end 9 .
  • D (mm) be the length of a line segment X connecting the first end portion 8 and the second end portion 9 .
  • the positions on the line segment X toward the center of the nugget 3 are points a and b, and the point a from the first end 8 and the point a from the second end 9
  • each distance to b be L (mm).
  • the region in the nugget 3 (the hatched region in FIG. 1) in which the distance L satisfies the relationship of the formula (1) with the length D of the line segment X is the “nugget tip region 31”. defined as
  • the structure of the nugget tip region 31 in at least one overlapping surface 7 has ferrite in an area ratio of 1% or more with respect to the entire nugget tip region 31, and the hardness Hv of the most softened portion in the nugget tip region 31 is satisfies the relationship of formula (2) with respect to the hardness Hvm of the central portion of .
  • D in Formula (1) indicates the length of the line segment X described above.
  • Hv indicates the hardness of the most softened portion in the nugget tip region 31
  • Hvm indicates the hardness of the central portion of the nugget.
  • the present invention also controls the organization of HAZ6.
  • the structure of the HAZ 6 of the present invention is formed on both ends of the nugget 3 on the side of the high-strength steel sheet, which will be described later.
  • the intersection of a straight line Z parallel to the overlapping surface 7 and the boundary of the nugget 3 is a point q
  • the position on the straight line Z within the HAZ 6 is a point r.
  • the distance M (mm) in the plate thickness direction between the straight line Z and the overlapping surface 7 satisfies the relationship of formula (3)
  • the distance T (mm) from the point q to the point r satisfies the relation of formula (4).
  • a region within HAZ 6 is defined as HAZ softening region 61 .
  • the above "straight line Z" is a line drawn on the side of the high-strength steel sheet of the present invention.
  • the hardness Hvh of the HAZ softened region 61 on the high-strength steel sheet side of the present invention satisfies the relationship of formula (5) with respect to the hardness Hvm of the central portion of the nugget 3 .
  • M 0.10 ⁇ D (3) 0 ⁇ T ⁇ 0.10 ⁇ D (4) 0.90 ⁇ Hvm>Hvh (5)
  • D in equations (3) and (4) indicates the length of the line segment X.
  • Hvm indicates the hardness of the nugget center
  • Hvh indicates the hardness of the HAZ softened region.
  • the nugget tip region 31 (not shown in FIG. 3) exists for each of the stacking surfaces 71 and 72.
  • FIG. 2 when there is a gap between the steel plates 1 and 2 on the overlapping surface 7 , the boundary of the nugget 3 located in the middle of the gap and intersecting with the straight line Y parallel to the overlapping surface 7 The two points are defined as a first end 8 and a second end 9 .
  • the nugget edge is heated to an appropriate temperature (a temperature just above the Ac 1 point) so that the nugget edge becomes the above two-phase structure and the HAZ structure near the nugget edge becomes tempered martensite.
  • the area ratio of ferrite in the nugget tip region 31 is set to 1% or more.
  • the area ratio of ferrite is preferably 3% or more, more preferably 5% or more, even more preferably 7% or more, and even more preferably 20% or more.
  • the structure of the nugget edge is made into a two-phase structure consisting of ferrite and martensite by controlling the temperature during welding as described above.
  • a HAZ softened region 61 is generated near the nugget edge and is locally tempered.
  • the nugget tip region 31 has ferrite as the two-phase structure, it is less brittle than when the nugget tip region 31 becomes full martensite, so cracks are less likely to propagate inside the nugget. As a result, the toughness of the nugget ends can be improved.
  • the upper limit of the area ratio of ferrite in the nugget tip region 31 is not particularly defined. From the viewpoint of controlling the temperature rise of the nugget edge to a temperature just above the Ac 1 point, the area ratio of ferrite in the nugget tip region 31 is preferably 80% or less, more preferably 60% or less, and even more preferably 50% or less, and more preferably 35% or less.
  • the structure (residual structure) other than ferrite in the nugget tip region 31 is martensite.
  • the area ratio of martensite in the nugget tip region 31 to the entire nugget tip region 31 is preferably 97% or less for the reason that the temperature is controlled to be just above the Ac 1 point.
  • the area ratio of martensite is more preferably 95% or less, still more preferably 80% or less, still more preferably 70% or less, and even more preferably 40% or less.
  • the area ratio of martensite is preferably 20% or more, more preferably 30% or more.
  • the nugget tip region 31 has a two-phase structure consisting of ferrite and martensite.
  • the above effects can be obtained.
  • the nugget tip region 31 has a two-phase structure, it is possible to prevent cracks from entering the nugget. As a result, stress concentration in the nugget tip region 31 can be avoided, and the nugget tip region 31 has toughness. As a result, even if a crack occurs from the sheet separation under CTS load, the crack does not propagate to the interior of the nugget 3 .
  • This two-phase structure is obtained by temperature control in the heat treatment step after tempering, which will be described later.
  • the texture of the nugget 3 and the nugget tip region 31 can be measured by the method described in Examples described later.
  • the structure of the nugget 3 other than the nugget tip region 31 is martensite.
  • the "hardness Hv of the most softened portion of the nugget tip region 31" refers to the lowest Vickers hardness measured in accordance with JISZ2244 (2020) in the nugget tip region 31.
  • the cross-sectional structure of the nugget is used as the test piece, and the boundary of the elliptical fusion zone (nugget) and the line formed by the overlapping surfaces of the steel sheets intersect 2
  • the hardness was measured at intervals of 0.2 (mm) from these two points toward the inside of the fusion zone.
  • the minimum value of these measured values in the nugget tip region 31 was taken as the "hardness Hv of the most softened portion of the nugget tip region 31".
  • the "hardness Hvm at the center of the nugget 3" refers to the Vickers hardness value measured in accordance with JISZ2244 (2020) at the center of the nugget 3. Specifically, according to the measurement method described in the examples described later, the cross-sectional structure of the nugget is used as the test piece, and the boundary of the elliptical fusion zone (nugget) and the line formed by the overlapping surfaces of the steel sheets intersect 2 The hardness was measured on the line segment X connecting the points (first end, second end) and at the position of the middle point between the two points. The measured value was defined as "hardness Hvm at the center of the nugget".
  • the hardness Hv of the most softened portion in the nugget tip region 31 and the hardness Hvm of the central portion of the nugget 3 do not satisfy the relationship of formula (2), the structure of the nugget tip region 31 becomes a martensite single phase, and the above A two-phase structure cannot be obtained. As a result, improvement in the toughness of the nugget tip region 31 and relaxation of stress concentration in the nugget tip region 31 cannot be obtained.
  • the hardness Hv of the most softened portion in the nugget tip region 31 is 0.85 times or less the hardness Hvm of the central portion of the nugget 3 .
  • the lower limit of the hardness Hv of the most softened portion in the nugget tip region 31 is not particularly defined.
  • the hardness Hv of the most softened portion in the nugget tip region 31 is equal to the hardness Hvm of the central portion of the nugget 3. It is preferably 0.40 times or more, more preferably 0.50 times or more the hardness Hvm at the center of the nugget 3, and 0 times the hardness Hvm at the center of the nugget 3. 0.60 times or more is more preferable.
  • the hardness Hvh of the HAZ softened region 61 on the high-strength steel sheet side of the present invention satisfies the relationship of formula (5) with respect to the hardness Hvm of the central portion of the nugget 3 . 0.90 ⁇ Hvm>Hvh (5)
  • the structure of the nugget end becomes a two-phase structure with ferrite, and by appropriately controlling the structure of the nugget tip region 31, the structure of the HAZ 6 is also changed. controlled.
  • the “hardness Hvh of the softened HAZ region 61” is the average Vickers hardness measured in accordance with JISZ2244 (2020) in the softened HAZ region 61 . Specifically, according to the measurement method described in the later-described Examples, the inside of the HAZ softened region 61 is measured at intervals of 0.2 (mm) under the condition that an indenter load of 300 gf is maintained for 15 seconds. The average value of these measured values was taken as the "hardness Hvh of the softened HAZ region 61". As described above, the present invention specifies the hardness of the high-strength steel sheet. Therefore, for example, when a high-strength steel plate is used on the steel plate 2 side in the example shown in FIG.
  • plate thickness direction refers to the plate thickness direction on the steel plate 2 side. That is, when a high-strength steel plate is used for the lower steel plate 1, the above “straight line Z” is a line drawn on the lower steel plate 1, and the above “thickness between the straight line Z and the overlapping surface 7 "direction" refers to the plate thickness direction on the steel plate 1 side.
  • each hardness is measured using the upper steel plate 2, and a plate in which two different high-strength steel plates are superimposed In the case of a set, the hardness on the low-strength steel plate side shall be measured.
  • the hardness Hvh of the HAZ softened region 61 and the hardness Hvm of the central portion of the nugget 3 do not satisfy the relationship of formula (5), the periphery of the nugget edge is not sufficiently tempered and becomes a hardened structure, and the nugget edge The texture and stiffness of the tip region 31 cannot be controlled within the above ranges. As a result, it is not possible to improve the toughness of the nugget edge and alleviate the stress concentration.
  • the hardness Hvh of the HAZ softened region 61 is preferably 0.85 times or less the hardness Hvm of the central portion of the nugget 3, and 0.80 times or less the hardness Hvm of the central portion of the nugget 3. is more preferable.
  • the lower limit of the hardness Hvh of the HAZ softening region 61 is not specified. Even when the entire structure of the HAZ softened region 61 becomes tempered martensite, it still has a certain degree of hardness. It is preferably twice or more, more preferably 0.45 times or more the hardness Hvm at the center of the nugget 3, and 0.60 times or more the hardness Hvm at the center of the nugget 3. more preferably.
  • the welding portion in addition to having the above configuration, the welding portion also has the following configuration, thereby further improving the effects of the present invention.
  • the average number density of carbides having a grain size of 100 nm or more in the softened HAZ region 61 is 10 or more per 5 ⁇ m 2 of the plate cross section.
  • the grain size of the carbide is set to 100 nm or more is to confirm that coarse carbide is generated as a result of sufficient progress of tempering.
  • the particle size of the carbide is preferably 500 nm or less.
  • the average number density of carbides in the softened HAZ region 61 is less than 10 per 5 ⁇ m 2 of plate cross section, tempering is insufficient. As a result, the nugget edge and its periphery have low toughness, and stress concentration relaxation may not be achieved. Therefore, the average number density of the carbides is preferably 10 or more per 5 ⁇ m 2 of the plate cross section, more preferably 20 or more per 5 ⁇ m 2 of the plate cross section, and 40 or more per 5 ⁇ m 2 of the plate cross section. is more preferred.
  • the upper limit of the average number density of carbides in the softened HAZ region 61 is not specified. Even if the entire structure of the HAZ softened region 61 becomes tempered martensite, it does not become 100% carbide.
  • the average number density of the carbides is preferably 155 or less per 5 ⁇ m 2 of the plate cross section, more preferably 90 or less per 5 ⁇ m 2 of the plate cross section, and 80 or less per 5 ⁇ m 2 of the plate cross section. is more preferable, and 70 or less per 5 ⁇ m 2 of plate cross section is even more preferable.
  • the particle size of carbides and the average number density of carbides can be measured by the methods described in Examples described later.
  • the structure of HAZ6 is tempered martensite, martensite.
  • the structure of the HAZ softened region 61 near the nugget tip region 31 preferably has tempered martensite of 50% or more in terms of area ratio with respect to the entire HAZ softened region 61 .
  • the area ratio of the tempered martensite in the HAZ softened region 61 is 60% or more.
  • the upper limit of the area ratio of tempered martensite in the HAZ softening region 61 is not specified.
  • the area ratio of tempered martensite in the HAZ softening region 61 is 100%, the effect of improving toughness and alleviating stress concentration can be expected. That is, the area ratio of the tempered martensite in the HAZ softened region 61 is desirably 100% or less.
  • the area ratio of the residual structure (martensite) in the HAZ softened region 61 is desirably less than 50% with respect to the entire HAZ softened region 61 .
  • C 0.05-0.6% C is an element that contributes to strengthening of steel. If the C content is less than 0.05%, the strength of the steel becomes low, and it is extremely difficult to produce a steel sheet with a tensile strength of 780 MPa or more. On the other hand, when the C content exceeds 0.6%, although the strength of the steel sheet increases, the amount of hard martensite becomes excessive and microvoids increase. Furthermore, the nugget and its surrounding HAZ are excessively hardened and embrittled, making it difficult to improve the CTS. Therefore, the C content should be 0.05 to 0.6%. The C content is preferably 0.10% or more and preferably 0.45% or less.
  • Si 0.1-2.0%
  • Si content 0.1% or more, it effectively acts to strengthen the steel.
  • Si since Si is a ferrite former element, it works predominantly for the generation of ferrite at the edge of the nugget.
  • the Si content exceeds 2.0%, although the steel is strengthened, toughness may be adversely affected. Therefore, the Si content should be 0.1 to 2.0%.
  • the Si content is preferably 0.2% or more and preferably 1.8% or less.
  • Mn 1.5-4.0%
  • the Mn content is less than 1.5%, a high joint strength can be obtained without long-term cooling as in the present invention.
  • the Mn content exceeds 4.0%, embrittlement of the welded portion or cracking due to the embrittlement will remarkably appear, making it difficult to improve the joint strength. Therefore, the Mn content should be 1.5 to 4.0%.
  • the Mn content is preferably 2.0% or more and preferably 3.5% or less.
  • P 0.10% or less
  • P is an unavoidable impurity, but if the P content exceeds 0.10%, strong segregation appears at the nugget edge of the weld, making it difficult to improve joint strength. . Therefore, the P content is set to 0.10% or less.
  • the P content is preferably 0.05% or less, more preferably 0.02% or less.
  • the lower limit of the P content is not particularly limited. However, excessive reduction causes an increase in cost, so the P content is preferably 0.005% or more.
  • S 0.005% or less
  • S is an element that segregates at grain boundaries and embrittles steel. Furthermore, S reduces the local deformability of sulfides and steel sheets. Therefore, the S content is made 0.005% or less.
  • the S content is preferably 0.004% or less, more preferably 0.003% or less.
  • the lower limit of the S content is not particularly limited. However, excessive reduction causes an increase in cost, so the S content is preferably 0.001% or more.
  • N 0.001 to 0.010%
  • N is an element that deteriorates the aging resistance of steel.
  • N is an element that is inevitably included. Therefore, the N content is set to 0.001 to 0.010%.
  • the N content is preferably 0.008% or less.
  • the high-strength steel sheet used in the present invention contains each of the above elements, and the balance is Fe and unavoidable impurities.
  • the above composition is the basic composition of the high-strength steel sheet.
  • one or more elements selected from Al, B, Ca, Cr, Cu, Ni, Mo, Ti, V, Nb, and O are added as necessary. can be added.
  • each component of Al, B, Ca, Cr, Cu, Ni, Mo, Ti, V, Nb, and O below can be contained as necessary, these components may be 0%. .
  • Al 2.0% or less
  • Al is an element capable of controlling the structure for refining austenite grains, but if added in a large amount, the toughness deteriorates. Therefore, when Al is contained, the Al content is preferably 2.0% or less.
  • the Al content is more preferably 1.5% or less, preferably 1.2% or more.
  • B 0.005% or less
  • B is an element that can improve hardenability and strengthen steel. Therefore, when B is contained, the B content is preferably 0.0005% or more.
  • the B content is more preferably 0.0007% or more. However, even if a large amount of B is added, the above effect is saturated, so the B content is made 0.005% or less.
  • the B content is more preferably 0.0010% or less.
  • Ca 0.005% or less Ca is an element that can contribute to improving the workability of steel. However, when added in a large amount, the toughness deteriorates. Therefore, when Ca is contained, the Ca content is preferably 0.005% or less. The Ca content is more preferably 0.004% or less, preferably 0.001% or more.
  • Cr 1.0% or less Cr is an element that can improve strength by improving hardenability. However, if the Cr content exceeds 1.0% and is excessive, the toughness of the HAZ may deteriorate. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. It is more preferably 0.8% or less, preferably 0.01% or more.
  • Cu 1.0% or less
  • Cu, Ni, and Mo are elements that can contribute to improving the strength of steel.
  • the toughness deteriorates. Therefore, when these elements are contained, it is preferable that the Cu content be 1.0% or less, the Ni content be 1.0% or less, and the Mo content be 1.0% or less.
  • the Cu content is more preferably 0.8% or less.
  • the Cu content is preferably 0.005% or more, more preferably 0.006% or more.
  • the Ni content is more preferably 0.8% or less, preferably 0.01% or more.
  • Mo content is more preferably 0.8% or less.
  • the Mo content is preferably 0.005% or more, more preferably 0.006% or more.
  • Ti 0.20% or less
  • Ti is an element that can improve hardenability and strengthen steel. However, when added in a large amount, it forms carbides, and its precipitation hardening significantly deteriorates toughness. Therefore, when Ti is contained, the Ti content is preferably 0.20% or less. The Ti content is more preferably 0.15% or less. The Ti content is preferably 0.003% or more, more preferably 0.004% or more.
  • V 0.50% or less
  • V is an element capable of strengthening the steel by controlling the structure through precipitation hardening.
  • addition of a large amount leads to deterioration of HAZ toughness. Therefore, when V is contained, the V content is preferably 0.50% or less.
  • the V content is more preferably 0.30% or less.
  • the V content is preferably 0.005% or more, more preferably 0.006% or more.
  • Nb 0.20% or less Nb improves CTS and delayed fracture resistance after resistance spot welding by forming fine carbonitrides. In order to obtain the effect, 0.005% or more of Nb is contained. On the other hand, if a large amount of Nb is added, not only does the elongation remarkably decrease, but also the toughness remarkably deteriorates, so the Nb content is made 0.20% or less. Therefore, when Nb is contained, the Nb content is preferably 0.20% or less. The Nb content is more preferably 0.18% or less, still more preferably 0.15% or less, and even more preferably 0.10% or less. The Nb content is preferably 0.005% or more, more preferably 0.006% or more, still more preferably 0.007% or more.
  • O oxygen
  • oxygen is an element that deteriorates the cleanliness and toughness of steel by forming nonmetallic inclusions. Therefore, when O is contained, the O content is preferably 0.03% or less. The O content is more preferably 0.02% or less. Also, the O content is preferably 0.005% or more.
  • a high-strength steel sheet having the above chemical composition can have a tensile strength of 780 MPa or more.
  • the tensile strength of the high-strength steel sheet is preferably 1180 MPa or more.
  • the CTS may decrease, and the delayed fracture properties may also deteriorate.
  • even a high-strength steel sheet having a tensile strength of 780 MPa or more has toughness by making the structure of the nugget end part the above-mentioned two-phase structure and making the structure of the HAZ a tempered martensite. become an organization.
  • the high-strength steel sheet of the present invention can obtain the above effects even if it is a steel sheet (galvanized steel sheet) having a galvanized layer on the steel sheet surface after being subjected to galvanizing treatment.
  • a zinc plating layer refers to a plating layer containing zinc as a main component.
  • the plating layer containing zinc as a main component includes known zinc plating layers, such as hot-dip galvanization layers, electrogalvanization layers, Zn--Al plating layers and Zn--Ni layers.
  • the high-strength steel sheet of the present invention may be an alloyed galvanized steel sheet having an alloyed galvanized layer on the surface of the base material by performing an alloying treatment after performing the above-described galvanizing treatment.
  • the steel sheets to be superimposed in the present invention may be a plurality of steel sheets of the same type, or a plurality of steel sheets of different types.
  • a steel sheet having a galvanized layer on its surface surface-treated steel sheet
  • a steel sheet having no galvanized layer on its surface cold-rolled steel sheet
  • the thickness of the steel sheet is preferably 0.4 mm to 2.2 mm, for example.
  • Resistance spot welding method Next, one embodiment of the resistance spot welding method for manufacturing the resistance spot welded joint of the present invention having the welded portion described above will be described.
  • the resistance spot welded joint of the present invention is a resistance that joins a plate set in which two or more steel plates including at least one of the high-strength steel plates are superimposed with a pair of welding electrodes and is energized while applying pressure. It can be manufactured by spot welding.
  • a plate assembly For example, as shown in Fig. 4, two steel plates 1 and 2 are superimposed to form a plate assembly. Then, the pair of welding electrodes 4 and 5 arranged on the lower side and the upper side of the plate set sandwich the plate set, and while applying pressure, the welding conditions are controlled to a predetermined value, and current is applied. As a result, the above-described welded portion can be formed by joining the plates that form the overlapping surfaces 7 of the steel plates (see FIG. 1).
  • the main energizing process and the post-tempering heat treatment process are included as the process of energizing the steel sheets 1 and 2 that are sandwiched between the welding electrodes 4 and 5.
  • Each step of the present invention will be described in detail below.
  • the main energizing step is a step of melting the overlapping surfaces 7 of the steel plates 1 and 2 to form a nugget 3 of a required size (see FIG. 4).
  • a nugget is formed by energizing at a current value I 1 (kA).
  • the nugget diameter adopted for resistance spot welding (welding) of automotive steel plates is generally 3.0 ⁇ t to 6.0 ⁇ t (t (mm) is the plate thickness). In the present invention, this numerical range is defined as the "target nugget diameter".
  • the energizing conditions and pressurizing conditions for forming the nugget 3 are not particularly limited as long as the nugget 3 having the target nugget diameter is obtained.
  • the energizing conditions and pressurizing conditions in the main energizing process are as follows. is preferably controlled to
  • the current value I 1 (kA) in the main energizing step is preferably 3.0 kA to 8.0 kA. If the current value I1 is too small, the target nugget diameter cannot be stably obtained. On the other hand, if the current value I 1 is too large, the nugget diameter may become too large, or the degree of melting of the steel plate may increase, and the melted weld portion may come out from between the plates as spatter, resulting in a smaller nugget diameter. may become. For this reason, the current value I 1 is set to 3.0 kA to 8.0 kA. The current value I 1 is more preferably 4.5 kA or more, more preferably 6.0 kA or more. The current value I 1 is more preferably 7.5 kA or less, more preferably 7.3 kA or less. However, as long as the required nugget diameter is obtained, the current value I1 may be shorter or longer than the above numerical range.
  • the energization time t 1 (ms) of the main energization step is preferably 120 ms to 400 ms. This is the time for stably forming the nugget 3 having the target nugget diameter, similarly to the current value I1 . If the energization time t 1 is less than 120 ms, there is concern that nuggets are less likely to form. On the other hand, if the energization time t 1 exceeds 400 ms, there are concerns that the nugget diameter to be formed may become larger than the target nugget diameter, and workability may deteriorate. However, as long as the required nugget diameter is obtained, the energization time t1 may be shorter or longer than the above numerical range.
  • the pressurizing force is preferably 2.0 kN to 7.0 kN. If the applied pressure is too large, the energized diameter will expand, making it difficult to secure the nugget diameter. On the other hand, if the applied pressure is too small, the energization diameter becomes small, and expulsion tends to occur. For this reason, the pressure is set to 2.0 kN to 7.0 kN.
  • the applied pressure is more preferably 3.0 kN or more, and more preferably 6.5 kN or less.
  • the applied force may be limited by the equipment capabilities used. The pressure may be lower or higher than the above numerical range as long as the pressure is such that the required nugget diameter can be obtained.
  • the post-tempering heat treatment step is a post-heat treatment step for making the structure of the nugget end portion of the nugget formed in the main energization step a structure having ferrite (the above-mentioned two-phase structure) and tempering the HAZ. .
  • the post-tempering heat treatment step after the main energization step, the nugget edge and its peripheral HAZ region are subjected to a cooling process (first cooling process, second cooling process) and a temperature raising process. If necessary, a first holding process or a first holding process and a post-energization process are performed.
  • a cooling process first cooling process, second cooling process
  • a first holding process or a first holding process and a post-energization process are performed.
  • first cooling process First, after the main energization step, cooling (first cooling step) is performed to lower the nugget end portion to a temperature at which martensite transformation occurs. In this first cooling process, in order to sufficiently obtain the effect of tempering, which will be described later, the weld zone is cooled by maintaining the non-energized state for the cooling time t c1 (ms) shown in Equation (6). 800 ⁇ t c1 (6)
  • the cooling time t c1 (ms) of the first cooling process is less than 800 ms, the martensite transformation does not occur sufficiently and martensite does not appear, resulting in a structure in which austenite remains. As a result, austenite remains as it is even after the subsequent temperature rising process, and finally becomes a martensite structure. As a result, the nugget ends have an embrittled structure, and the CTS is not improved. Therefore, the cooling time t c1 (ms) should be 800 ms or longer.
  • the cooling time t c1 is preferably 850 ms or longer, more preferably 900 ms or longer.
  • the upper limit of the cooling time t c1 (ms) of the first cooling process is not particularly limited. Since the steel sheet targeted by the present invention is a steel sheet for automobiles, a long welding time results in a decrease in welding efficiency. Therefore, the cooling time t c1 (ms) is preferably 2200 ms or less, more preferably 2000 ms or less.
  • a temperature rising process After the first cooling process, a temperature rising process is performed. In the temperature rising process, after the main energization process, the first cooling process cools the nugget edge and the region in the HAZ around it to a temperature at which martensite transformation occurs, and then tempers the structure that has become martensite. Conduct energization (post-energization) to raise the temperature to an appropriate temperature range.
  • This "appropriate temperature range” refers to a temperature range in which the nugget end portion (specifically, the nugget tip region 31 described above) becomes a two-phase structure having ferrite.
  • the welding portion is energized at the current value I 2 (kA) shown in Equation (7) for the energization time t 2 (ms) shown in Equation (8).
  • the above-mentioned "appropriate temperature" that is, the nugget edge and its peripheral area is set to Ac 3 points or less. It is particularly important to raise the temperature rapidly in a short period of time to a temperature range above 1 point Ac (see FIGS. 5 and 6). As a result, the structure of the nugget edge can be made into a two-phase structure containing ferrite, and the HAZ in the vicinity of the nugget edge can be effectively tempered.
  • the current value I2 in this process is too low, the effect of the tempering is reduced.
  • the current value I2 in this process is too high, the HAZ in the vicinity of the nugget edge cannot be tempered because it exceeds the Ac3 point.
  • the HAZ near the nugget edge becomes a martensite single phase or a two-phase structure of martensite and ferrite by energization in the subsequent process. .
  • stress concentration relaxation and toughness improvement at the nugget end cannot be achieved.
  • the nugget edge becomes a softened structure by the subsequent energization.
  • the temperature it is important to appropriately control the temperature so that the nugget ends have the above-described two-phase structure.
  • the current value I 2 (kA) in the temperature rising process shall satisfy the relationship of I 1 ⁇ I 2 ⁇ 1.8 ⁇ I 1 . If the current value I 2 in the heating process is equal to or less than the current value I 1 (kA) in the main energizing process, the temperature becomes less than the Ac 1 point, and the nugget end portion cannot be effectively tempered.
  • the current value I 2 in the temperature rising process is preferably (1.01 ⁇ I 1 ) (kA) or more, more preferably (1.05 ⁇ I 1 ) (kA) or more, further preferably (1 .10 ⁇ I 1 ) (kA) or more.
  • the current value I 2 in the temperature rising process is preferably (1.7 ⁇ I 1 ) (kA) or less, more preferably (1.6 ⁇ I 1 ) (kA) or less, further preferably (1.5 ⁇ I 1 ) (kA) or less.
  • the energization time t 2 (ms) in the temperature raising process is set to 100 ⁇ t 2 ⁇ 300.
  • the energization time t2 is preferably 120 ms or longer, more preferably 140 ms or longer.
  • the energization time t2 is preferably 280 ms or less, more preferably 240 ms or less.
  • the cooling time t c2 (ms) of the second cooling process is more than 0 ms and less than 300 ms.
  • the cooling time t c2 is preferably 20 ms or longer.
  • the cooling time t c2 is preferably less than 200 ms, more preferably 150 ms or less.
  • the post-tempering heat treatment step may include the following steps, if necessary, after the second cooling step.
  • the first holding process is an optional process.
  • the post-tempering heat treatment step further includes a first holding step, the first holding step is performed after the second cooling step.
  • the welding portion is energized at the current value I 3 (kA) given by the formula (10) for the energization time t 3 (ms) given by the formula (11). 0 ⁇ I3 ⁇ I2 (10) 0 ⁇ t3 ⁇ 2000 (11)
  • the current value I 3 (kA) in the first holding process should be less than the current value I 2 (kA) in the temperature rising process. is preferred.
  • the current value I3 in the first holding process By setting the current value I3 in the first holding process to a current value lower than the current value I2 in the temperature rising process, it is possible to keep the nugget edge and its surroundings at a temperature of Ac 3 point or less. If the current value I 3 in the first holding process is equal to or higher than the current value I 2 (kA) in the temperature rising process, there is a possibility that the nugget end and its surroundings will again reach a temperature of Ac 3 or higher, It may not be possible to temper the HAZ near the nugget edge.
  • the current value I 3 in the first holding process is greater than 0 kA and less than the current value I 2 kA in the temperature rising process.
  • the current value I 3 is more preferably (0.95 ⁇ I 2 )(kA) or less, more preferably (0.2 ⁇ I 2 )(kA) or more.
  • the energization time t 3 (ms) in the first holding process is preferably more than 0 ms and less than 2000 ms.
  • the above-mentioned temperature raising process is a process of raising the temperature, so a high current value is required, but the first holding process is a process for tempering the HAZ by maintaining the temperature raised in the temperature raising process. is. Therefore, the energization time t3 in the first holding process may be long. However, from the viewpoint of enforcement efficiency, the energization time t3 is set to less than 2000 ms.
  • the energization time t3 of the first holding process is set to over 0 ms.
  • the energization time t 3 is more preferably 1800 ms or less, more preferably 1600 ms or less.
  • the energization time t3 is more preferably 150 ms or longer, more preferably 200 ms or longer.
  • the post-energization process is a process that is performed as necessary.
  • the post-tempering heat treatment step further includes a post-energization step, the post-energization step is performed after the first holding step.
  • a third cooling process is provided in which the non-energization state is maintained for the cooling time t c3 (ms) shown in Equation (12).
  • a second holding process is performed in which the resistance spot weld is energized at a current value I 4 that is 0.1 times or more and 1.3 times or less for an energization time t 4 of more than 0 ms and less than or equal to 2000 ms. tc3 ⁇ 300 (12)
  • the above-mentioned "process involving the previous energization” refers to the process having the energization performed immediately before based on the current energization process. That is, in the case where, for example, the temperature raising process and the first holding process are performed before the first post-energization process, the current value in the first holding process is referred to as the "current value in the process accompanied by the previous energization". In addition, for example, when the first holding process is not performed after the temperature raising process, the "current value in the process involving the previous energization" refers to the current value in the temperature raising process.
  • the third cooling process and the second holding process in the post-energization process may be performed only once, or may be repeatedly performed multiple times.
  • FIG. 7 shows an example of the energization pattern of the present invention.
  • the first cooling process, the temperature rising process, the second cooling process, the first holding process and two post-energizing processes are performed in this order. good too.
  • the cooling time t c3 (ms) of the third cooling process is preferably less than 300 ms.
  • the cooling time t c3 is more preferably 250 ms or less, more preferably 100 ms or less.
  • the lower limit of the cooling time t c3 is not particularly defined, it is preferably 10 ms or longer, more preferably 20 ms or longer, and even more preferably 40 ms or longer.
  • the purpose of the second holding step of the post-energization process is to maintain the post-energization temperature. If the current value I 4 (kA) in the second holding process of the subsequent energization process does not satisfy the relationship of 0.1 times or more and 1.3 times or less than the current value in the process accompanied by the previous energization, Post-heating temperature rises too much. As a result, it becomes difficult to obtain a tempering effect by performing the post-energization process.
  • the current value I 4 in the second holding process preferably satisfies a relationship of 0.90 to 0.95 times the current value in the previous process accompanied by energization.
  • the energization time t 4 (ms) of the second holding process in the post-energization process is more than 0 ms and does not satisfy 2000 ms or less, it is difficult to obtain the effect of tempering.
  • the energization time t 4 in the second holding process is preferably 300 ms or longer and preferably 500 ms or shorter.
  • the number of repetitions of the third cooling process and the second holding process in the post-energization process is preferably two or more.
  • the number of repetitions is preferably 5 or less, more preferably 4 or less.
  • the welded joint having the welded portion of the present invention interfacial rupture can be suppressed by obtaining a ductile fracture surface, and plug rupture or partial plug rupture in which most of the plug remains can be achieved.
  • the joint strength (CTS) of the obtained welded joint can be improved.
  • the delayed fracture resistance of the welded joint can be further improved. Therefore, even when a steel plate having the chemical composition of the steel plate described above is included as the high-strength steel plate in the set, the joint strength (CTS) and the delayed fracture resistance can be further improved.
  • Steel plates (steel plate A to steel plate J) with a tensile strength of 780 MPa to 1470 MPa and a thickness of 0.8 to 1.2 mm shown in Tables 1 and 2 were used as test pieces.
  • the size of the test piece was 150 mm long side and 50 mm short side.
  • Table 1 shows the chemical compositions of steel sheets A to J.
  • "-" in Table 1 indicates that the element is not intentionally added, and includes not only the case of not containing the element (0%) but also the case of unavoidably containing the element.
  • the "GA steel sheet” shown in Table 2 represents the galvannealed steel sheet described above.
  • a plurality of steel plates (in the example shown in FIG. 4, the lower steel plate 1 and the upper steel plate 2) are superimposed on each other, and the servo mounted on the C gun Resistance spot welding was performed using a resistance welding machine with a DC power source and a motor pressurization type.
  • test pieces were stacked as shown in Table 2 to form a board assembly.
  • Table 2 the "stacking position of steel sheets” is counted as “first sheet” and “second sheet” in order from the lower steel sheet.
  • resistance spot welding was performed under the welding conditions shown in Tables 3-1 and 3-2, and a nugget 3 of the required size was formed between the plates to produce a resistance spot welded joint.
  • Some board sets were made by stacking three steel plates. "-" in Tables 3-1 and 3-2 indicates that the process was not carried out.
  • the pressurizing force during energization was constant, and was 3.5 kN here.
  • the welding electrode 4 on the lower side and the welding electrode 5 on the upper side with respect to the plate set each had a tip diameter of 6 mm and a tip curvature radius of 40 mm, and used chromium-copper DR type electrodes.
  • Welding was performed by controlling the applied force with the lower welding electrode 4 and the upper welding electrode 5 and using a DC power source.
  • the nugget diameter was formed so as to be 5.5 ⁇ t (mm) or less when the plate thickness was t (mm).
  • the delayed fracture resistance was evaluated by the following method.
  • the prepared resistance spot welded joint was allowed to stand in the atmosphere at room temperature (20°C) for 24 hours, then immersed in a 3% NaCl + 1.0% NH 4 SCN aqueous solution, and then exposed to the cathode at a current density of 0.07 mA/cm 2 .
  • Electrolytic charging was performed for 96 hours, and then the presence or absence of delayed fracture was investigated.
  • Tables 5-1 and 5-2 those in which delayed fracture did not occur after immersion were marked with a symbol " ⁇ ", and those in which delayed fracture occurred after immersion were marked with a symbol " ⁇ ".
  • the symbol " ⁇ " it was evaluated as having "excellent delayed fracture resistance”.
  • two points on the boundary of the nugget 3 that intersect with the overlapping surface 7 of the steel plate are defined as a first end 8 and a second end 9, and the first end 8 and the second end
  • the length of the line segment X connecting the part 9 was set to D (mm).
  • the positions on the line segment X toward the center of the nugget 3 are points a and b, and the point a from the first end 8 and the point a from the second end 9
  • a nugget tip region 31 is defined as a region where each distance L (mm) to b satisfies the above formula (1).
  • the length D of the line segment X is shown in Tables 4-1 and 4-2. The sample was prepared such that the nugget tip region 31 was the viewing surface.
  • the measurement positions for the "hardness at the center of the nugget" shown in Tables 4-1 and 4-2 were on the line segment X and the center of the first end and the second end. The value measured at this position was defined as the hardness Hvm at the center of the nugget.
  • the "hardness of the most softened portion of the nugget tip region" shown in Tables 4-1 and 4-2 was obtained as follows.
  • the measurement position is on the above line segment X in the nugget tip region, and measurements are taken at intervals of 0.2 (mm) from the first end and the second end toward the inside of the nugget on the line segment X, Those values were measured.
  • the smallest value among the obtained measured values was taken as the hardness Hv of the most softened portion of the nugget tip region.
  • the measurement position of "Hardness of HAZ softened region" shown in Tables 4-1 and 4-2 was within the HAZ softened region.
  • the point q is the intersection of the straight line Z parallel to the overlapping surface and the nugget boundary
  • the point r is the position on the straight line Z within the heat affected zone.
  • the area was designated as the HAZ softening area.
  • a sample was prepared so that the HAZ softened region was the viewing surface.
  • Hardness of HAZ softened region shown in Tables 4-1 and 4-2 was obtained as follows. With the first end 8 (nugget end) as the origin, the measurement positions are 0.2 (mm) intervals in the HAZ softening region from the nugget end toward the base material and from the nugget end toward the steel plate surface direction. and measured their values. The average value of the measured values obtained was taken as the hardness Hvh of the softened HAZ region. The hardness of the obtained nuggets and HAZ are shown in Tables 4-1 and 4-2, respectively.
  • the present invention specifies the hardness of the high-strength steel sheet. Therefore, for example, when a high-strength steel plate is used on the steel plate 2 side in the example shown in FIG. "plate thickness direction” refers to the plate thickness direction on the steel plate 2 side. That is, when a high-strength steel plate is used for the lower steel plate 1, the above “straight line Z” is a line drawn on the lower steel plate 1, and the above “thickness between the straight line Z and the overlapping surface 7 "direction” refers to the plate thickness direction on the steel plate 1 side.
  • the HAZ structure was also observed in the same manner as the nugget structure evaluation. Specifically, as shown in FIG. 1, the steel sheet structure in the HAZ softening region is observed. A sample was prepared so that the HAZ softened region was the viewing surface. Using this sample, the structure of the nugget tip region 31 was observed at a magnification of 1,000 to 100,000 times using a scanning electron microscope (SEM). For the steel sheet structure, the area ratio of each structure was measured by the point count method (according to ASTM E562-83 (1988)). The area ratio of each tissue obtained is shown in Tables 4-1 and 4-2. In Tables 4-1 and 4-2, "TM" for structure indicates tempered martensite, and "M" indicates martensite.
  • the average number density (pieces/5 ⁇ m 2 ) of cementite having a particle size of 100 nm or more is obtained by observing the observation surface with a TEM at a magnification of 10,000 times, and calculating the number density per 5 ⁇ m 2 of plate cross section at five arbitrarily selected locations. asked.
  • the average value of the obtained values was defined as the average number density per 5 ⁇ m 2 of the plate cross section of carbides having a particle size of 100 nm or more.
  • the average number density is shown in Tables 4-1 and 4-2. If the particle size of the carbide becomes large, there is a possibility that it is a precipitate other than the carbide generated by tempering, so the particle size of the carbide is set to 500 nm or less.
  • resistance spot welded joints in which a plurality of steel sheets including at least one high-strength plated steel sheet are resistance spot welded have excellent shear tensile strength. It was a good welded joint with excellent delayed fracture resistance. On the other hand, a good welded joint could not be obtained in the comparative example.
  • Reference Signs List 1 2, 10 steel plate 3 nugget 4, 5 welding electrode 6 heat affected zone 7 steel plate mating surface 8 first end 9 second end 11 resistance spot welded joint 31 nugget tip region 61 HAZ softening region

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PCT/JP2022/036459 2021-10-12 2022-09-29 抵抗スポット溶接継手およびその抵抗スポット溶接方法 Ceased WO2023063097A1 (ja)

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WO2025127893A1 (ko) * 2023-12-15 2025-06-19 주식회사 포스코 강재, 용접열영향부 및 그 제조방법
JP7823801B1 (ja) * 2024-09-26 2026-03-04 Ｊｆｅスチール株式会社 抵抗スポット溶接継手の製造方法
WO2026069947A1 (ja) * 2024-09-26 2026-04-02 Ｊｆｅスチール株式会社 抵抗スポット溶接継手の製造方法

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