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 PDFInfo
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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
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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
- 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
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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
- B23K11/16—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
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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
- B23K11/24—Electric supply or control circuits therefor
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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
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- B23K11/24—Electric supply or control circuits therefor
- B23K11/241—Electric supplies
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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
- 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 degrees C
- B23K35/3053—Fe as the principal constituent
- B23K35/3073—Fe as the principal constituent with Mn as next major constituent
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- C—CHEMISTRY; METALLURGY
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- 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
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- 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- 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
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C—ALLOYS
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C—ALLOYS
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- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- 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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- 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
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- 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
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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
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/006—Vehicles
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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
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/18—Sheet panels
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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
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/34—Coated articles, e.g. plated or painted; Surface treated articles
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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
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
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- 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
- 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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Abstract
Description
後述する本発明の成分組成からなる鋼板(高強度鋼板)を用いて溶接する場合、ナゲットを生成した後、溶接部をAc1点直上付近の温度に昇温することによって、溶接後にナゲット端部のマルテンサイトをフェライトおよびマルテンサイトからなる二相組織とすることが必要である。この温度が低すぎると上記の二相組織を得られず、ナゲット端部の応力集中緩和およびナゲット端部の靭性向上という2つの効果を得られない。その結果、き裂がナゲット内へ侵入するために、CTSが低下する。一方、この温度がAc3点に比べてより高ければナゲット端部はマルテンサイト単相の組織となり、その結果、脆弱な組織となる恐れがあると考えられる。このような理由から、本発明では溶接部を適切な温度で焼戻す組織制御を実現する技術の提供を課題としている。すなわち、本発明と特許文献1では、技術思想が異なる。
なお、本発明では、特許文献2のような上記の焼戻しマルテンサイトからなる領域は発生しない。本発明では、ナゲット端部をAc1点直上の温度に昇温することによって、ナゲット端部がフェライトを有する上記の二相組織となる。これにより、上述するナゲット端部の応力集中緩和およびナゲット端部の靭性向上の効果を得られる技術である。すなわち、本発明と特許文献2では、技術思想が異なる。
[1] 少なくとも1枚の高強度鋼板を含む2枚以上の鋼板が抵抗スポット溶接された抵抗スポット溶接部を有する抵抗スポット溶接継手であって、
前記高強度鋼板が、質量%で、
C:0.05~0.6%、
Si:0.1~2.0%、
Mn:1.5~4.0%、
P:0.10%以下、
S:0.005%以下、および
N:0.001~0.010%
を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、
前記鋼板の重ね面と交わるナゲットの境界上の2点を第1端部および第2端部とし、前記第1端部および前記第2端部を結ぶ線分Xの長さをD(mm)とし、前記第1端部および前記第2端部から、前記ナゲットの中心へ向けた前記線分X上の位置を点aおよび点bとし、
前記第1端部から前記点aまでおよび前記第2端部から前記点bまでの各距離L(mm)が、前記線分Xの長さD(mm)に対して式(1)の関係を満たす前記ナゲット内の領域をナゲット先端領域とするとき、
少なくとも1つの前記重ね面における、前記ナゲット先端領域の組織が、前記ナゲット先端領域全体に対する面積率で1%以上のフェライトを有し、
かつ、前記ナゲット先端領域における最軟化部の硬さHvが、前記ナゲットの中心部の硬さHvmに対して式(2)の関係を満たし、
前記重ね面に平行な直線Zと前記ナゲットの境界との交点を点q、前記直線Z上で熱影響部内の位置を点rとし、
前記直線Zと前記重ね面との板厚方向の距離M(mm)が式(3)の関係を満たし、かつ、前記点qから前記点rまでの距離T(mm)が式(4)の関係を満たす前記熱影響部内の領域をHAZ軟化領域とするとき、
前記高強度鋼板側における、前記HAZ軟化領域の硬さHvhが、前記ナゲットの中心部の硬さHvmに対して式(5)の関係を満たす、抵抗スポット溶接継手。
0<L≦0.15×D ・・・(1)
0.90×Hvm>Hv ・・・(2)
M=0.10×D ・・・(3)
0<T≦0.10×D ・・・(4)
0.90×Hvm>Hvh ・・・(5)
ただし、前記重ね面において前記鋼板間の隙間がある場合には、前記隙間の中間に位置し前記重ね面に平行な直線Yと交わる前記ナゲットの境界上の2点を前記第1端部および前記第2端部とする。
[2] 前記HAZ軟化領域における、粒径が100nm以上の炭化物の平均個数密度は、板断面5μm2当たり10個以上である、[1]に記載の抵抗スポット溶接継手。
[3] 前記HAZ軟化領域の組織が、前記HAZ軟化領域全体に対する面積率で50%以上の焼戻しマルテンサイトを有する、[1]または[2]に記載の抵抗スポット溶接継手。
[4] 前記高強度鋼板の前記成分組成が、さらに、質量%で、
Al:2.0%以下、
B:0.005%以下、
Ca:0.005%以下、
Cr:1.0%以下、
Cu:1.0%以下、
Ni:1.0%以下、
Mo:1.0%以下、
Ti:0.20%以下、
V:0.50%以下、
Nb:0.20%以下、および
O:0.03%以下
から選択される1種または2種以上を含有する、[1]~[3]のいずれか1つに記載の抵抗スポット溶接継手。
[5] [1]~[4]のいずれか1つに記載の抵抗スポット溶接継手の抵抗スポット溶接方法であって、
少なくとも1枚の前記高強度鋼板を含む2枚以上の鋼板を重ね合わせた板組を、1対の溶接電極で挟持し、加圧しながら通電して前記抵抗スポット溶接部を形成するに際し、
前記通電として、主通電工程と焼戻し後熱処理工程を有し、
前記主通電工程では、電流値I1(kA)で通電してナゲットを形成し、
前記焼戻し後熱処理工程では、
式(6)に示す冷却時間tc1(ms)の間、無通電状態を保持する第1冷却過程を行い、
次いで、式(7)に示す電流値I2(kA)で、式(8)に示す通電時間t2(ms)の間、前記抵抗スポット溶接部を通電する昇温過程を行い、
次いで、式(9)に示す冷却時間tc2(ms)の間、無通電状態を保持する第2冷却過程を行う、抵抗スポット溶接継手の抵抗スポット溶接方法。
800≦tc1 ・・・(6)
I1 < I2 ≦ 1.8×I1 ・・・(7)
100 < t2 ≦ 300 ・・・(8)
0 < tc2 < 300 ・・・(9)
[6] 前記第2冷却過程後、式(10)に示す電流値I3(kA)で、式(11)に示す通電時間t3(ms)の間、前記抵抗スポット溶接部を通電する第1保持過程を行う、[5]に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
0 < I3 < I2 ・・・(10)
0 < t3 < 2000 ・・・(11)
[7] 前記第1保持過程後、さらに後通電過程を有し、
該後通電過程では、
式(12)に示す冷却時間tc3(ms)の間、無通電状態を保持する第3冷却過程を行い、
その後、1つ前の通電を伴う過程における電流値に対して0.1倍以上1.3倍以下となる電流値I4で、0ms超え2000ms以下の通電時間t4の間、前記抵抗スポット溶接部を通電する第2保持過程を行う、[6]に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
tc3 < 300 ・・・(12)
[8] 前記後通電過程における前記第3冷却過程と前記第2保持過程を繰り返し行う、[7]に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
まず、図1~図3を参照して、本発明の抵抗スポット溶接継手を説明する。図1~図3には、一例として、本発明の抵抗スポット溶接継手における抵抗スポット溶接部およびその周辺の板厚方向断面図を示す。図1は重ね合わせる鋼板の枚数が2枚の場合であり、図2は重ね合わせる鋼板の枚数が2枚で、かつ鋼板間に板隙がある場合であり、図3は重ね合わせる鋼板の枚数が3枚の場合である。
本発明の抵抗スポット溶接継手11における、抵抗スポット溶接部について詳細に説明する。図1に示すように、抵抗スポット溶接継手11の抵抗スポット溶接部(以下、「溶接部」と称する)は、ナゲット3と熱影響部(HAZ)6を有する。本発明では、ナゲット端部およびナゲット端部近傍のHAZ内の領域における組織を次のように規定する。
0<L≦0.15×D ・・・(1)
0.90×Hvm>Hv ・・・(2)
ここで、式(1)におけるDは、上記の線分Xの長さを示す。式(2)における、Hvはナゲット先端領域31における最軟化部の硬さを示し、Hvmはナゲットの中心部の硬さを示す。
本発明の高強度鋼板側における、HAZ軟化領域61の硬さHvhが、ナゲット3の中心部の硬さHvmに対して式(5)の関係を満たす。
M=0.10×D ・・・(3)
0<T≦0.10×D ・・・(4)
0.90×Hvm>Hvh ・・・(5)
ここで、式(3)および式(4)におけるDは、前記線分Xの長さを示す。式(5)における、Hvmはナゲットの中心部の硬さを示し、HvhはHAZ軟化領域の硬さを示す。
ナゲット先端領域31のフェライトが、ナゲット先端領域31全体に対する面積率で1%未満の場合、溶接時の温度制御が適切になされていないことが分かる。上述のように、温度制御が適切になされない場合、溶接後に、ナゲット端部のマルテンサイトがフェライトおよびマルテンサイトからなる二相組織にならず、ナゲット端部の靱性を向上することができない。さらに、ナゲット端部近傍のHAZの組織もマルテンサイトとなる可能性が高くなり、その結果、HAZ軟化領域61の硬さHvhが上記の硬さを確保できない。
なお、この二相組織は、後述する焼戻し後熱処理工程での温度制御によって得られる。
ナゲット先端領域31における最軟化部の硬さHvが、ナゲット3の中心部の硬さHvmに対して式(2)の関係を満たす。
0.90×Hvm>Hv ・・・(2)
本発明の高強度鋼板側における、HAZ軟化領域61の硬さHvhが、ナゲット3の中心部の硬さHvmに対して式(5)の関係を満たす。
0.90×Hvm>Hvh ・・・(5)
なお、上述のとおり、本発明では高強度鋼板側の硬さを規定している。そのため、例えば図1に示す例において鋼板2側に高強度鋼板を用いた場合には、上記の「直線Z」は鋼板2側に引かれる線であり、上記の「直線Zと重ね面7との板厚方向」とは、鋼板2側の板厚方向を指す。すなわち、下側の鋼板1に高強度鋼板を用いた場合には、上記の「直線Z」は下側の鋼板1に引かれる線であり、上記の「直線Zと重ね面7との板厚方向」とは、鋼板1側の板厚方向を指す。
また、2枚の同種の高強度鋼板を重ね合わせた板組の場合には、上側の鋼板2を用いて各硬さを計測するものとし、2枚の異種の高強度鋼板を重ね合わせた板組の場合には、低強度鋼板側の硬さを計測するものとする。
HAZ軟化領域61における、粒径が100nm以上の炭化物の平均個数密度が、板断面5μm2当たり10個以上とすることが好ましい。
HAZ6の組織は、焼戻しマルテンサイト、マルテンサイトである。
ナゲット端部周辺のHAZ6の組織が焼戻しマルテンサイトになることで、ナゲット先端領域31近傍のHAZの靭性向上と応力集中緩和を実現することができる。このような理由から、ナゲット先端領域31の近傍となるHAZ軟化領域61の組織は、HAZ軟化領域61全体に対する面積率で50%以上の焼戻しマルテンサイトを有することが望ましい。このHAZ軟化領域61の焼戻しマルテンサイトは、面積率で60%以上とすることがより好ましい。
HAZ軟化領域61の焼戻しマルテンサイトの面積率の上限は規定しない。その理由は、HAZ軟化領域61の焼戻しマルテンサイトの面積率が100%であっても、靭性向上および応力集中緩和の効果は見込めるためである。すなわち、このHAZ軟化領域61の焼戻しマルテンサイトは、面積率で100%以下であることが望ましい。
本発明の抵抗スポット溶接継手における、高強度鋼板の母材の成分組成の限定理由について説明する。なお、以下の説明において、成分組成の「%」表示は、特に断らない限り「質量%」を指すものとする。
Cは鋼の強化に寄与する元素である。C含有量が0.05%未満では、鋼の強度が低くなり、引張強度780MPa以上の鋼板を製作することは極めて困難である。一方、C含有量が0.6%を超えると、鋼板の強度は高くなるものの、硬質なマルテンサイト量が過大となり、マイクロボイドが増加する。更にナゲットとその周辺のHAZが過度に硬化し、脆化も進むため、CTSを向上させることは困難である。そのため、C含有量は0.05~0.6%とする。C含有量は、好ましくは0.10%以上であり、好ましくは0.45%以下である。
Si含有量が0.1%以上であると、鋼の強化に有効に作用する。また、Siはフェライトフォーマー元素であることからナゲット端部のフェライトの生成に優位に働く。一方、Si含有量が2.0%を超えると、鋼は強化されるものの、靱性に悪影響を与えることがある。そのため、Si含有量は0.1~2.0%とする。Si含有量は、好ましくは0.2%以上であり、好ましくは1.8%以下である。
Mn含有量が1.5%未満であると、本発明のように長時間の冷却を与えずとも、高い継手強度を得ることができる。一方、Mn含有量が4.0%を超えると、溶接部の脆化あるいは脆化に伴う割れが顕著に現れるため、継手強度を向上させることは困難である。そのため、Mn含有量は1.5~4.0%とする。Mn含有量は、好ましくは2.0%以上であり、好ましくは3.5%以下である。
Pは不可避的不純物であるが、P含有量が0.10%を超えると、溶接部のナゲット端部に強偏析が現れるため継手強度を向上させることは困難である。そのため、P含有量は0.10%以下とする。P含有量は、好ましくは0.05%以下であり、より好ましくは0.02%以下である。なお、P含有量の下限は特に限定されない。ただし、過度の低減はコストの増加を招くので、P含有量は0.005%以上とすることが好ましい。
Sは、粒界に偏析して鋼を脆化させる元素である。さらに、Sは、硫化物と鋼板の局部変形能を低下させる。そのため、S含有量は0.005%以下とする。S含有量は、好ましくは0.004%以下とし、より好ましくは0.003%以下とする。なお、S含有量の下限は特に限定されない。ただし、過度の低減はコストの増加を招くので、S含有量は0.001%以上とすることが好ましい。
Nは、鋼の耐時効性を劣化させる元素である。Nは不可避的に含まれる元素である。そのため、N含有量は0.001~0.010%とする。N含有量は、好ましくは0.008%以下とする。
Alは、オーステナイト細粒化のため組織制御をすることができる元素であるが、多量に添加すると靭性が劣化する。このため、Alを含有する場合、Al含有量は2.0%以下とすることが好ましい。Al含有量は、より好ましくは1.5%以下とし、好ましくは1.2%以上とする。
Bは、焼入れ性を改善して鋼を強化することができる元素である。このため、Bを含有する場合、B含有量は0.0005%以上とすることが好ましい。B含有量は、より好ましくは0.0007%以上とする。しかし、Bを多量に添加しても、上記効果は飽和することから、B含有量は0.005%以下とする。B含有量は、より好ましくは0.0010%以下とする。
Caは、鋼の加工性向上に寄与することができる元素である。しかし、多量に添加すると靭性が劣化する。このため、Caを含有する場合、Ca含有量は0.005%以下とすることが好ましい。Ca含有量は、より好ましくは0.004%以下とし、好ましくは0.001%以上とする。
Crは、焼入れ性の向上により強度を向上させることができる元素である。しかし、Crは1.0%を超えて過剰に含有すると、HAZの靱性が劣化する恐れがある。このため、Crを含有する場合、Cr含有量は1.0%以下とすることが好ましい。より好ましくは0.8%以下とし、好ましくは0.01%以上とする。
Cu、Ni、Moは、鋼の強度向上に寄与することができる元素である。しかし、多量に添加すると靭性が劣化する。このため、これらの元素を含有する場合、それぞれ、Cu含有量は1.0%以下とし、Ni:1.0%以下とし、Mo:1.0%以下とすることが好ましい。Cu含有量は、より好ましくは0.8%以下とする。Cu含有量は、好ましくは0.005%以上とし、より好ましくは0.006%以上とする。Ni含有量は、より好ましくは0.8%以下とし、好ましくは0.01%以上とする。Mo含有量は、より好ましくは0.8%以下とする。Mo含有量は、好ましくは0.005%以上とし、より好ましくは0.006%以上とする。
Tiは、焼入れ性を改善して鋼を強化することができる元素である。しかし、多量に添加すると炭化物を形成し、その析出硬化によって靭性が著しく劣化する。このため、Tiを含有する場合、Ti含有量は0.20%以下とすることが好ましい。Ti含有量は、より好ましくは0.15%以下とする。Ti含有量は、好ましくは0.003%以上とし、より好ましくは0.004%以上とする。
Vは、析出硬化により組織制御をして鋼を強化することができる元素である。しかし、多量に添加するとHAZ靱性の劣化につながる。このため、Vを含有する場合、V含有量は0.50%以下とすることが好ましい。V含有量は、より好ましくは0.30%以下とする。V含有量は、好ましくは0.005%以上とし、より好ましくは0.006%以上とする。
Nbは、微細な炭窒化物を形成することで抵抗スポット溶接後のCTSおよび耐遅れ破壊特性を向上させる。その効果を得るためにはNbを0.005%以上含有させる。一方、多量にNbを添加すると、伸びが著しく低下するだけでなく、靭性を著しく損ねることから、Nb含有量は0.20%以下とする。このため、Nbを含有する場合、Nb含有量は0.20%以下とすることが好ましい。Nb含有量は、より好ましくは0.18%以下であり、さらに好ましくは0.15%以下であり、さらに一層好ましくは0.10%以下である。Nb含有量は、好ましくは0.005%以上であり、より好ましくは0.006%以上であり、さらに好ましくは0.007%以上である。
O(酸素)は非金属介在物を生成することにより、鋼の清浄度、靭性を劣化させる元素である。そのため、Oを含有する場合、O含有量は0.03%以下とすることが好ましい。O含有量は、0.02%以下とすることがより好ましい。また、O含有量は0.005%以上とすることが好ましい。
本発明の高強度鋼板は、亜鉛めっき処理を施して、鋼板表面に亜鉛めっき層を有する鋼板(亜鉛めっき鋼板)であっても、上記の効果を得ることができる。亜鉛めっき層とは、亜鉛を主成分とするめっき層を指す。亜鉛を主成分とするめっき層には、公知の亜鉛めっき層を含むものとし、例えば、溶融亜鉛めっき層、電気亜鉛めっき層、Zn-Alめっき層およびZn-Ni層等が含まれる。また、本発明の高強度鋼板は、上記の亜鉛めっき処理を施した後に合金化処理を施して、母材表面に合金化亜鉛めっき層を有する合金化亜鉛めっき鋼板であってもよい。
次に、上記した溶接部を有する本発明の抵抗スポット溶接継手を製造するための抵抗スポット溶接方法の一実施形態について説明する。
主通電工程とは、鋼板1、2の重ね面7を溶融して必要サイズのナゲット3を形成する工程である(図4を参照)。主通電工程では、電流値I1(kA)で通電してナゲットを形成する。
焼戻し後熱処理工程とは、主通電工程で形成されたナゲットにおける、ナゲット端部の組織をフェライトを有する組織(上記の二相組織)とし、かつ、HAZを焼戻すための後熱処理の工程である。焼戻し後熱処理工程では、主通電工程後、ナゲット端部およびその周辺のHAZ領域に対して冷却過程(第1冷却過程、第2冷却過程)および昇温過程を施す。また必要に応じて、第1保持過程、あるいは第1保持過程かつ後通電過程を施す。ナゲット端部の靭性を向上させる効果およびナゲット端部の応力集中を緩和させる効果を得るためには、焼戻し後熱処理工程における各過程の溶接条件を次のように制御することが重要である。
まず、主通電工程後、ナゲット端部がマルテンサイト変態を生じる温度まで下げる冷却(第1冷却過程)を行う。この第1冷却過程では、後述の焼戻しの効果を十分に得るために、式(6)に示す冷却時間tc1(ms)の間、無通電状態を保持することで溶接部を冷却する。
800 ≦ tc1 ・・・(6)
第1冷却過程後、昇温過程を行う。昇温過程では、主通電工程後、第1冷却過程によってナゲット端部およびその周辺のHAZ内の領域がマルテンサイト変態を生じる温度まで冷却した後、マルテンサイトとなった組織を焼戻すために、適切な温度域に昇温する通電(後通電)を行う。この「適切な温度域」とは、ナゲット端部(具体的には、上述のナゲット先端領域31)がフェライトを有する二相組織となるための温度域を指す。
I1 < I2 ≦ 1.8 × I1 ・・・(7)
100 <t2 ≦ 300 ・・・(8)
昇温過程後、HAZを焼戻すための冷却(第2冷却過程)を行う。この第2冷却過程では、式(9)に示す冷却時間tc2(ms)の間、無通電状態を保持することで溶接部を冷却する。
0 < tc2 < 300 ・・・(9)
第1保持過程は、必要に応じて行う過程である。焼戻し後熱処理工程がさらに第1保持過程を有する場合、第2冷却過程後に第1保持過程を行う。第1保持過程では、式(10)に示す電流値I3(kA)で、式(11)に示す通電時間t3(ms)の間、溶接部を通電する。
0 < I3< I2 ・・・(10)
0 < t3 <2000 ・・・(11)
後通電過程は、必要に応じて行う過程である。焼戻し後熱処理工程がさらに後通電過程を有する場合、第1保持過程を行った後に後通電過程を行う。
tc3 < 300 ・・・(12)
上述のように、上記作用効果をより一層効果的に得るために後通電過程を行うことができる。この際、後通電過程を行うことによる温度の上昇を抑制することを目的として、第3冷却過程を行う。そのため、第3冷却過程の冷却時間tc3(ms)は、300ms未満とすることが好ましい。冷却時間tc3は、より好ましくは250ms以下とし、さらに好ましくは100ms以下とする。冷却時間tc3の下限は特に規定しないが、10ms以上が好ましく、20ms以上がより好ましく、40ms以上がさらに好ましい。
後通電過程の第2保持過程は、後通電温度の維持を目的とする。この後通電過程の第2保持過程の電流値I4(kA)が、1つ前の通電を伴う過程における電流値に対して0.1倍以上1.3倍以下の関係を満たさない場合、後通電温度が上昇し過ぎる。その結果、後通電過程を行うことによる焼戻し効果が得難くなる。第2保持過程の電流値I4は、1つ前の通電を伴う過程における電流値に対して0.90倍以上0.95倍以下の関係を満たすことが好ましい。
以上説明したように、本発明の抵抗スポット溶接方法は、焼戻し後熱処理工程の溶接条件を適切に制御することによって、溶接部におけるナゲット端部の組織がフェライトを有する二相組織となる。また、この制御によって、ナゲット端部がAc1点近傍の温度となり、ナゲット端部近傍のHAZにおいては局所的な焼戻しを与えられる。これらにより、得られる溶接継手は、ナゲット端部の応力集中の緩和ができ、更にナゲット端部の靱性を向上できる。
CTSの評価は、十字引張試験に基づき行った。作製した抵抗スポット溶接継手を用いて、JISZ3137に規定の方法で十字引張試験を行い、CTS(十字引張力)を測定した。測定値がJIS A級(3.4kN)以上であったものに対して記号「○」を付し、JIS A級未満であったものに対して記号「×」を付した。なお、本実施例では、記号「○」の場合を良好と評価し、記号「×」の場合を劣ると評価する。評価結果は表5-1および表5-2に示した。
耐遅れ破壊特性は、次の方法で評価した。作製した抵抗スポット溶接継手を、常温(20℃)で大気中に24時間静置し、次いで3%NaCl+1.0%NH4SCN水溶液に浸漬し、次いで0.07mA/cm2の電流密度で陰極電解チャージを96hr実施し、その後、遅れ破壊の有無を調査した。表5-1および表5-2中、浸漬後に遅れ破壊が発生しなかったものには記号「○」を記載し、浸漬後に遅れ破壊が発生したものには記号「×」を記載した。ここでは、記号「〇」の場合に、「優れた耐遅れ破壊特性」を有すると評価した。
本実施例では、上述のCTSおよび耐遅れ破壊特性の評価を用いて、継手の評価を行った。表5-1および表5-2中、CTSおよび耐遅れ破壊特性の各評価がいずれも「〇」の場合に、継手評価を「〇(合格)」とした。一方、CTSおよび耐遅れ破壊特性の各評価のうちいずれか1つが「×」の場合、あるいはCTSおよび耐遅れ破壊特性の両方の評価が「×」の場合に、継手評価を「×(不合格)」とした。
ナゲット端部の組織の観察は、次のように行った。作製した抵抗スポット溶接継手を、円状に形成されたナゲットの中心を通る位置で切断して試験片とし、該試験片を超音波洗浄した。その後に、該試験片に樹脂埋めを行ったサンプルの板厚断面を研磨し、ナイタール溶液を用いてエッチングを行い、サンプルを準備した。
上記の組織評価と同様の方法でサンプルを作成した。ナゲットおよびHAZの硬さはヴィッカース硬度計により、JISZ2244に規定の方法で測定した。測定荷重は、300gfの圧子にて15秒負荷する条件で行った。
具体的には、図1に示すように、重ね面に平行な直線Zとナゲットの境界との交点を点q、直線Z上で熱影響部内の位置を点rとした。直線Zと重ね面との板厚方向の距離M(mm)が上記式(3)を満たし、かつ、点qから点rまでの距離T(mm)が上記式(4)を満たすHAZ内の領域をHAZ軟化領域とした。該HAZ軟化領域が観察面となるように、サンプルを準備した。
得られたナゲットおよびHAZの硬さを、表4-1および表4-2にそれぞれ示した。
また、2枚の同種の高強度鋼板を重ね合わせた板組の場合には、上側の鋼板2を用いて各硬さを測定した。2枚の異種の高強度鋼板を重ね合わせた板組の場合、すなわち板組c、d、e、o、pの場合には、低強度鋼板側の硬さを測定した。
ナゲットの組織評価と同様の方法で、HAZの組織の観察も行った。
具体的には、図1に示すようにHAZ軟化領域の鋼板組織を観察する。該HAZ軟化領域が観察面となるように、サンプルを準備した。該サンプルを用いて、走査電子顕微鏡(SEM)を用いて1000倍~100000倍の倍率でナゲット先端領域31の組織の観察を行った。鋼板組織はポイントカウント法(ASTM E562―83(1988)に準拠)により、各組織の面積率を測定した。得られた各組織の面積率を、表4-1および表4-2に示した。表4-1および表4-2中、組織に関する「TM」は焼戻しマルテンサイトを示し、「M」はマルテンサイトを示す。
図1に示すように、HAZ軟化領域の鋼板組織を観察した。これは、得られた抵抗スポット溶接部材の板厚断面を研磨し、次いで3%ナイタールで腐食した後、該HAZ軟化領域が観察面となるようにサンプルを準備した。TEM(透過型電子顕微鏡)を用いて10000倍の倍率でこのサンプルの観察面を観察した。さらに、Image-Proを用いて、下限を0.005μmとして、セメンタイトの円相当直径を算出することで、セメンタイトの粒径を求めた。
粒径が100nm以上のセメンタイトの平均個数密度(個/5μm2)は、TEMを用いて10000倍の倍率で観察面を観察し、任意に選択した5箇所の板断面5μm2当たりの個数密度を求めた。得られた値の平均値を、粒径が100nm以上の炭化物の板断面5μm2当たりの平均個数密度とした。該平均個数密度を、表4-1および表4-2に示した。
なお、炭化物の粒径が大きくなると焼戻しで発生する炭化物以外の析出物である可能性があるため、炭化物の粒径は500nm以下とした。
3 ナゲット
4、5 溶接電極
6 熱影響部
7 鋼板合わせ面
8 第1端部
9 第2端部
11 抵抗スポット溶接継手
31 ナゲット先端領域
61 HAZ軟化領域
Claims (8)
- 少なくとも1枚の高強度鋼板を含む2枚以上の鋼板が抵抗スポット溶接された抵抗スポット溶接部を有する抵抗スポット溶接継手であって、
前記高強度鋼板が、質量%で、
C:0.05~0.6%、
Si:0.1~2.0%、
Mn:1.5~4.0%、
P:0.10%以下、
S:0.005%以下、および
N:0.001~0.010%
を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、
前記鋼板の重ね面と交わるナゲットの境界上の2点を第1端部および第2端部とし、前記第1端部および前記第2端部を結ぶ線分Xの長さをD(mm)とし、前記第1端部および前記第2端部から、前記ナゲットの中心へ向けた前記線分X上の位置を点aおよび点bとし、
前記第1端部から前記点aまでおよび前記第2端部から前記点bまでの各距離L(mm)が、前記線分Xの長さD(mm)に対して式(1)の関係を満たす前記ナゲット内の領域をナゲット先端領域とするとき、
少なくとも1つの前記重ね面における、前記ナゲット先端領域の組織が、前記ナゲット先端領域全体に対する面積率で1%以上のフェライトを有し、
かつ、前記ナゲット先端領域における最軟化部の硬さHvが、前記ナゲットの中心部の硬さHvmに対して式(2)の関係を満たし、
前記重ね面に平行な直線Zと前記ナゲットの境界との交点を点q、前記直線Z上で熱影響部内の位置を点rとし、
前記直線Zと前記重ね面との板厚方向の距離M(mm)が式(3)の関係を満たし、かつ、前記点qから前記点rまでの距離T(mm)が式(4)の関係を満たす前記熱影響部内の領域をHAZ軟化領域とするとき、
前記高強度鋼板側における、前記HAZ軟化領域の硬さHvhが、前記ナゲットの中心部の硬さHvmに対して式(5)の関係を満たす、抵抗スポット溶接継手。
0<L≦0.15×D ・・・(1)
0.90×Hvm>Hv ・・・(2)
M=0.10×D ・・・(3)
0<T≦0.10×D ・・・(4)
0.90×Hvm>Hvh ・・・(5)
ただし、前記重ね面において前記鋼板間の隙間がある場合には、前記隙間の中間に位置し前記重ね面に平行な直線Yと交わる前記ナゲットの境界上の2点を前記第1端部および前記第2端部とする。 - 前記HAZ軟化領域における、粒径が100nm以上の炭化物の平均個数密度は、板断面5μm2当たり10個以上である、請求項1に記載の抵抗スポット溶接継手。
- 前記HAZ軟化領域の組織が、前記HAZ軟化領域全体に対する面積率で50%以上の焼戻しマルテンサイトを有する、請求項1または2に記載の抵抗スポット溶接継手。
- 前記高強度鋼板の前記成分組成が、さらに、質量%で、
Al:2.0%以下、
B:0.005%以下、
Ca:0.005%以下、
Cr:1.0%以下、
Cu:1.0%以下、
Ni:1.0%以下、
Mo:1.0%以下、
Ti:0.20%以下、
V:0.50%以下、
Nb:0.20%以下、および
O:0.03%以下
から選択される1種または2種以上を含有する、請求項1~3のいずれか1項に記載の抵抗スポット溶接継手。 - 請求項1~4のいずれか1項に記載の抵抗スポット溶接継手の抵抗スポット溶接方法であって、
少なくとも1枚の前記高強度鋼板を含む2枚以上の鋼板を重ね合わせた板組を、1対の溶接電極で挟持し、加圧しながら通電して前記抵抗スポット溶接部を形成するに際し、
前記通電として、主通電工程と焼戻し後熱処理工程を有し、
前記主通電工程では、電流値I1(kA)で通電してナゲットを形成し、
前記焼戻し後熱処理工程では、
式(6)に示す冷却時間tc1(ms)の間、無通電状態を保持する第1冷却過程を行い、
次いで、式(7)に示す電流値I2(kA)で、式(8)に示す通電時間t2(ms)の間、前記抵抗スポット溶接部を通電する昇温過程を行い、
次いで、式(9)に示す冷却時間tc2(ms)の間、無通電状態を保持する第2冷却過程を行う、抵抗スポット溶接継手の抵抗スポット溶接方法。
800≦tc1 ・・・(6)
I1 < I2 ≦ 1.8×I1 ・・・(7)
100 < t2 ≦ 300 ・・・(8)
0 < tc2 < 300 ・・・(9) - 前記第2冷却過程後、式(10)に示す電流値I3(kA)で、式(11)に示す通電時間t3(ms)の間、前記抵抗スポット溶接部を通電する第1保持過程を行う、請求項5に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
0 < I3 < I2 ・・・(10)
0 < t3 < 2000 ・・・(11) - 前記第1保持過程後、さらに後通電過程を有し、
該後通電過程では、
式(12)に示す冷却時間tc3(ms)の間、無通電状態を保持する第3冷却過程を行い、
その後、1つ前の通電を伴う過程における電流値に対して0.1倍以上1.3倍以下となる電流値I4で、0ms超え2000ms以下の通電時間t4の間、前記抵抗スポット溶接部を通電する第2保持過程を行う、請求項6に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
tc3 < 300 ・・・(12) - 前記後通電過程における前記第3冷却過程と前記第2保持過程を繰り返し行う、請求項7に記載の抵抗スポット溶接継手の抵抗スポット溶接方法。
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