WO2020241500A1 - スポット溶接継手、及びスポット溶接継手の製造方法 - Google Patents
スポット溶接継手、及びスポット溶接継手の製造方法 Download PDFInfo
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- WO2020241500A1 WO2020241500A1 PCT/JP2020/020309 JP2020020309W WO2020241500A1 WO 2020241500 A1 WO2020241500 A1 WO 2020241500A1 JP 2020020309 W JP2020020309 W JP 2020020309W WO 2020241500 A1 WO2020241500 A1 WO 2020241500A1
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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
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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
- B23K11/36—Auxiliary equipment
-
- 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
Definitions
- the present disclosure relates to a spot welded joint and a method for manufacturing a spot welded joint.
- the present application claims priority based on Japanese Patent Application No. 2019-09703 filed in Japan on May 24, 2019, the contents of which are incorporated herein by reference.
- Patent Document 1 describes an energy absorbing member in which a hat material and a closing plate are joined to each other by spot welding.
- high-strength steel sheets with a tensile strength of 980 MPa or more are widely used as high-strength steel sheets for automobiles.
- high-strength steel sheets having a tensile strength of 1100 MPa or more have begun to be applied.
- a high-strength steel sheet having a tensile strength of 1100 MPa or more generally contains a hardened structure in order to obtain high strength.
- a nugget spot weld metal
- HAZ heat-affected zone
- the structural members (layer welded members) constituting the automobile body such as the A pillar, the B pillar, the roof rail, and the side sill need to have high strength.
- a structural member constituting an automobile body is manufactured by superimposing a plurality of steel plate members and joining flanges (overlapping portions) by resistance spot welding to form a tubular closed cross section.
- methods such as increasing the strength of the material (base material) and increasing the number of welding (spot) hit points are taken.
- In-plane tensile stress may be applied to a part of the flange of the member to be spot-welded by resistance in the event of an automobile collision.
- a region with low hardness such as a HAZ softened portion, the collision resistance performance of the member deteriorates.
- Such a HAZ softened portion has little influence on the evaluation results of the tensile shear test and the cross tensile test (JIS Z3137) used for the joint evaluation of resistance spot welding.
- strain may be locally concentrated on the HAZ softened portion and fracture may occur in the HAZ softened portion.
- Patent Document 2 describes a welded joint in which the characteristics of the spot welded portion are improved by heat-treating the spot welded portion at 100 to 400 ° C. to improve the strength of the L-shaped tensile joint.
- Patent Document 3 describes a method of post-energizing the spot welded portion to improve the strength of the cross tension joint.
- Patent Document 4 describes the ratio of TSS to material strength and CTS by heating the spot welded portion and the molten portion by high frequency induction heating immediately after welding with a coil wound around the spot weld electrode. A welding method for improving the joint strength evaluated from the product with the material strength is described.
- Patent Document 5 describes that the energy absorption capacity is enhanced by having a region called a soft zone having a strength of less than 1100 MPa in a part or all of the flange portion used for spot welding. Pillars are listed.
- Japanese Patent Application Laid-Open No. 2006-142905 Japanese Patent Application Laid-Open No. 2010-509451 Japanese Patent Application Laid-Open No. 2015-093282 Japanese Patent No. 5459750 Japanese Patent No. 5894081
- the present disclosure has been made in view of the above problems, and is a spot welded joint and a spot welded joint that can suppress breakage from a region sandwiched between spot weld metals even when an in-plane tensile stress is applied. It is an object to provide a manufacturing method of.
- the present inventors analyzed the strain distribution when an in-plane tensile stress was generated in a part in which a HAZ softened portion was formed. As a result, the strength of the parts is increased by quenching the vicinity of the surface (opposite the overlapping surface) between the welded metals of the high-strength steel plate that has been laminated-welded, and at the same time, tempering the vicinity of the overlapping surface. It has been found that it is possible to prevent strain from being locally concentrated on the HAZ softened portion and causing breakage in the HAZ softened portion even when an in-plane tensile stress is applied while suppressing the decrease to the minimum.
- the spot welded joint includes a first steel plate containing hard maltensite having an average Vickers hardness HVbase of 350 HV or more, and a second steel plate overlaid on the first steel plate.
- the first steel plate in the plate thickness direction, including the two spot-welded metals joining the first steel plate and the second steel plate, and including the two spot-welded metals.
- the first region formed in the range of 0.1 mm between the two spot-welded metals and from the surface on the second steel plate side in the plate thickness direction of the first steel plate.
- the metal structure of the first region contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 of the first region and the average Vickers hardness HVbase of the first steel plate are expressed by the following formula (1).
- the metal structure of the second region contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 of the second region and the average Vickers hardness HVbase of the first steel plate are as follows. (2) is satisfied.
- the difference between the maximum value and the minimum value of Vickers hardness in the first region may be 80 HV or less.
- the spot welded joint according to [1] or [2] even if the thickness of the first region in the plate thickness direction is 30 to 70% of the plate thickness of the first steel plate. Good.
- a first steel plate containing hard martensite having an average Vickers hardness HVbase of 350 HV or more and a second steel plate are superposed.
- Two spot-welded metals that join the first steel plate and the second steel plate that are overlapped are formed, and by laser irradiation, the first steel plate is placed between the two spot-welded metals and the second.
- quenching is performed in a range of 0.1 mm from the surface on the steel plate side of the above, and at the same time, quenching is performed in a range of 0.1 mm between the two spot weld metals and from the surface opposite to the surface on the second steel plate side. ..
- a spot welded joint capable of suppressing fracture from a region sandwiched between spot weld metals and a method for manufacturing a spot welded joint can be obtained even when an in-plane tensile stress is applied.
- the spot welded joint 1 includes a first steel plate 11, a second steel plate 12 stacked on the first steel plate, and a first steel plate 11 and a second steel plate. It is provided with two spot welded metals 2 for joining the twelve.
- the spot weld metal 2 is a nugget formed by resistance spot welding.
- Such a spot welded joint can be obtained by superimposing the first steel plate 11 and the second steel plate 12 and performing resistance spot welding.
- the first steel plate 11 to be used for resistance spot welding is a steel plate having an average Vickers hardness (HVbase) of 350 HV or more in consideration of application to automobile frame parts such as B pillars. Further, the first steel sheet has a structure including a hardened structure such as hard martensite. On the other hand, the second steel plate 12 is not limited.
- the average Vickers hardness of the first steel plate 11 (sometimes simply referred to as hardness) means the average Vickers hardness of the first steel plate 11 to be welded before welding. When measured after spot welding, it means the average Vickers hardness measured at a position not affected by welding heat.
- the first steel plate 11 in all the cross sections of the first steel plate 11 including the two spot weld metals 2 and 2 in the plate thickness direction, the first steel plate 11 is the two spot weld metals. Between the first region 51 formed between 2 and 2 and within a range of 0.1 mm from the surface (that is, the overlapped surface) on the side of the second steel plate 12 and between the two spot weld metals 2 and 2. It has a second region 52 formed in a range of 0.1 mm from a surface opposite to the surface on the second steel plate 12 side (that is, the surface of the joint).
- the metal structure of the first region 51 contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 of the first region 51 and the first steel plate.
- the average Vickers hardness HVbase satisfies the following equation (1).
- the metal structure of the second region 52 contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 of the second region 52 and the first region 52.
- the average Vickers hardness HVbase of the steel sheet of the above satisfies the following equation (2).
- a metal structure satisfying the same conditions of the area% of tempered martensite and the average Vickers hardness as in the first region 51 may extend to the outside of the first region 51. Further, a metal structure satisfying the same conditions of the area% of hard martensite and the average Vickers hardness as in the second region 52 may extend to the outside of the second region 52. The reasons for limiting each configuration will be described below.
- a high-strength steel plate having an average Vickers hardness of 350 HV or more has a structure containing a hardened structure such as hard martensite (for example, 50 area% or more). Often have.
- a structure is obtained by a manufacturing method including a quenching step.
- hard martensite changes to a soft structure such as tempered martensite in HAZ formed around the weld metal due to the heat of welding. That is, a region (HAZ softened portion) having a hardness lower than that of the base material is formed.
- this HAZ softened portion may be the starting point of fracture.
- the region including the HAZ softened portion between the spot weld metals 2 is tempered so that the hardness around the HAZ softened portion is set to the same level as the hardness of the HAZ softened portion. It is possible to reduce it. However, in this case, although the breakage from the HAZ softened portion can be suppressed, the softened portion becomes large as a whole, so that there is a concern that the collision resistance performance (bending performance) of the part may deteriorate.
- the spot welded metal 2 according to the present embodiment is the spot weld metal 2 of the first steel plate 11 which has been lap welded in order to prevent cracking at the HAZ softened portion while minimizing a decrease in joint strength.
- the metal structure contains 50 area% or more of tempered martensite, and the average Vickers hardness HV1 in the first region and the average Vickers hardness HVbase of the first steel sheet are HVbase ⁇ 0.33 + 150 ⁇ HV1 ⁇ HVbase ⁇ 0. Satisfy 33 + 230]
- the hardness of the HAZ softened portion Vickers hardness caused by the heat effect of welding.
- the first region between the spot weld metals 2 and 2 including the HAZ softened portion is tempered to have a tempered martensite structure of 50 area% or more, and its Vickers hardness. Is controlled so as to satisfy the following equation (1).
- HV1 average Vickers hardness
- the difference between the maximum value and the minimum value of the Vickers hardness in the region is 80 HV or less.
- the concentration of strain can be further relaxed.
- the difference between the maximum value and the minimum value of Vickers hardness is 50 HV or less.
- the first region 51 is formed in the thickness cross section of the first steel plate 11 between the two spot weld metals 2 and 2 and in a range (thickness) of 0.1 mm from the surface on the second steel plate 12 side. Has been done. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1 mm, the HAZ softened portion, which is a locally reduced strength portion, may remain, and a sufficient effect may not be obtained.
- the region satisfying the hardness and structure of the first region 51 may extend to the outside of the first region 51.
- the region satisfying the hardness and structure of the first region 51 preferably extends from the surface on the second steel plate side to a range of 30% or more of the plate thickness of the first steel plate.
- the region satisfying the hardness and structure of the first region 51 extends from the surface on the second steel plate side to a range of more than 90% of the plate thickness of the first steel plate, the entire joint portion It is not preferable because there is a concern that the average hardness will decrease and the bending strength will decrease.
- ⁇ Second area> [The metal structure contains 50 area% or more of hard martensite, and the average Vickers hardness HV2 in the second region and the average Vickers hardness HVbase of the first steel sheet satisfy HVbase-30 ⁇ HV2 ⁇ HVbase +30].
- a local strength reducing portion such as a HAZ softened portion is formed between the two spot weld metals 2 and 2. If so, the strain is concentrated, but as a result of the examination by the present inventors, the strain is likely to be concentrated especially when the strength reduction portion exists near the surface of the first steel plate 11 (the surface opposite to the overlapped surface). I found out.
- the vicinity of the surface of the first steel plate 11 between the two spot weld metals 2 and 2 including the HAZ softened portion is hardened to reduce the hardness of the hardened region.
- the average Vickers hardness HV2 of the second region 52 and the average Vickers hardness HVbase of the first steel plate 11 satisfy the following formula (2).
- the average Vickers hardness (HV2) of the second region 52 is larger than the average Vickers hardness (HV1).
- the second region 52 is a range (thickness) of 0.1 mm (thickness) between the two spot weld metals 2 and 2 in the thickness cross section of the first steel plate 11 and from the surface opposite to the surface on the second steel plate side. It is formed in. If the thickness of the region having the above hardness in the plate thickness direction is less than 0.1 mm, the HAZ softened portion, which is a local strength reducing portion, may remain and a sufficient effect may not be obtained. A region satisfying the hardness and structure of the second region 52 may extend to the outside of the second region 52.
- the region satisfying the hardness and structure of the second region 52 is formed in a range of 10% or more of the plate thickness of the first steel plate from the surface opposite to the surface on the second steel plate 12 side. It is preferable to have. However, if the region satisfying the hardness and structure of the second region 52 extends from the surface on the second steel plate 12 side to a range of more than 70% of the first steel plate 11, the second region side bends. It is not preferable because there is a concern that the breaking elongation may decrease when an external tensile bending load is applied.
- the first region 51 and the second region 52 are formed in all cross sections of the first steel plate containing the two spot weld metals in the plate thickness direction. .. That is, when the cross-sections in the thickness direction of the first steel sheet are observed so as to include two weld metals, the above-mentioned first region 51 and second region 52 are observed in all the cross-sections.
- the diameter of the superposed surface 3 of the spot weld metal 2 is D
- the direction perpendicular to the thickness direction cross section of the first steel plate 11 in the first region 51 and the second region 52 (FIG. 1).
- a metal structure satisfying the same conditions of the area% of tempered martensite and the average Vickers hardness as in the first region 51 may extend to the outside in the width direction of the first region 51.
- a metal structure satisfying the same conditions of the area% of hard martensite and the average Vickers hardness as in the second region 52 may extend to the outside in the width direction of the second region 52.
- the widths of the first region 51 and the second region 52 are 1.0 ⁇ D, the direction in which the in-plane tensile stress is applied is a constant angle with respect to the direction connecting the spot weld metals 2 and 2 ( Even when stress is applied in the oblique direction), strain concentration on the HAZ softened portion where strain can be concentrated is suppressed. As a result, breakage at the HAZ softened portion is further suppressed.
- the widths of the first region 51 and the second region 52 are less than 1.0 ⁇ D, the direction of the in-plane tensile stress is a constant angle with respect to the direction connecting the spot weld metals 2 and 2. (Stress is applied in the diagonal direction), there is a concern that a sufficient effect cannot be obtained.
- the average hardness of the first steel plate 11 is measured using a Vickers hardness tester having a load of 1.0 kgf.
- the hardness of the portion affected by welding heat is lower than the hardness before welding. Therefore, for the hardness of the first steel plate 11, the hardness at a position not affected by heat due to welding of the first steel plate 11 is measured, and the average value thereof is used.
- the hardness at a position 15 mm or more away from the spot weld metal 2 in a direction free of other weld metals may be measured.
- the thickness of the first steel plate 11 in the first region 51 and the second region 52 from the surface and their average Vickers hardness a Vickers hardness tester with a load of 100 gf was used to determine the thickness of the first steel plate. Polishing and Vickers hardness measurement are repeated with respect to the cross section in the plate thickness direction to obtain a Vickers hardness distribution in the range sandwiched between the spot weld metals 2 and 2. Based on this distribution, the thickness of the first region 51 and the second region 52 and their average Vickers hardness are calculated. Specifically, the distribution of Vickers hardness is measured by the following method. First, a sample is taken so that the cross section in the thickness direction (cross section AA shown in FIG.
- polishing and Vickers hardness are measured until the cross section does not contain the spot weld metal 2, and the Vickers hardness distribution of the first steel plate 11 between the spot weld metals 2 and 2 is obtained. Since the hardness is considered to be the same for the cross section on the opposite side, it is sufficient to measure the half cross section as described above.
- the first steel plate 11 to be welded contains hard martensite is determined by the position of 1/8 of the plate thickness and the position of 3/8 from the surface of the position not affected by the heat due to the welding of the first steel plate 11.
- Samples taken from each of the 5 positions of 5/8 and 7/8 are etched with a repera corrosive solution, and a 100 ⁇ m square field is observed with an optical microscope at a magnification of 1000 times to make a judgment. Just do it. Martensite that looks white to reddish brown in the observation field is martensite. Of the martensite, martensite containing carbide is hard martensite, and martensite not containing carbide is tempered martensite. Judge.
- the area ratio of the tempered martensite in the first region 51 and the area ratio of the hard martensite in the second region 52 are the target regions (first region) on the same surface as the above-mentioned Vickers hardness measurement surface. If it is a region, the region that satisfies the formula (1), if it is the second region, the region that satisfies the formula (2)), the samples collected from five locations are etched with a repera corrosive liquid, and then an optical microscope A 100 ⁇ m square field is observed at a magnification of 1000 times, and the area ratio of martensite is measured assuming that martensite appears to be white to reddish brown in the observation field.
- the martensite area ratios of the first region 51 and the second region 52 can be obtained. Then, using the same sample, etching treatment with picral, observing a 100 ⁇ m square field with an optical microscope at a magnification of 1000 times, and within the observation field, martensite containing charcoal was hard martensite, Assuming that martensite containing no charcoal is tempered martensite, the ratios of tempered martensite and hard martensite among the martensite are determined.
- spot welding in which a weld metal is formed by spot welding is targeted.
- Spot welding also called spot welding, is welding in which two stacked steel plates are connected by dots.
- the spot welding means include arc spot welding, resistance spot welding, and laser spot welding.
- welding performed linearly is called continuous welding.
- the means of continuous welding include arc welding, laser welding, seam welding and the like. Compared to continuous welding, spot welding has a smaller welding area, so the construction time is shorter and power is saved. That is, spot welding is excellent in productivity.
- the spot weld joint 1 is not limited to the resistance spot weld joint in which the spot weld metal 2 is a nugget for resistance spot welding. ..
- the spot weld metal 2 may be formed by laser spot welding, or the spot weld metal 2 may be formed by arc spot welding.
- the first steel plate 11 is a hat member
- the second steel plate 12 is a closing plate
- the two spot welded metals 2 are the flange portion of the hat member and the closing. It is preferably formed in the overlapped portion with the plate. With such a configuration, it is particularly effective in improving the strength and collision resistance of the structural member.
- the spot welded joint 1 has two spot welded metals 2 and the region between the two spot welded metals 2 and 2 satisfies the above relationship, the effect is effective. can get.
- a plurality of (two or more) spot weld metals are formed. Even when more than two spot weld metals are formed, if the region between the two spot weld metals of interest has the above relationship, the effect can be obtained for that region.
- the fracture at the HAZ softened portion is suppressed regardless of the direction and location in the plane of the high-strength steel plate. It is more preferable because it can be used.
- the spot welded joint 1 can be applied to an A pillar, a side sill, a B pillar, or the like.
- the flange portion of the hat member is joined to the closing plate by a spot weld metal. If the above relationship is satisfied between the spot-welded metals of the B-pillar, even if an in-plane tensile stress is applied to the flange portion at the time of an automobile collision, fracture at the portion that was the HAZ softened portion is suppressed. it can.
- the first steel plate 11 and / or the second steel plate 12 may be a plated steel plate.
- corrosion resistance is improved.
- the plated steel sheet include hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electrogalvanized steel sheets, and aluminum-plated steel sheets.
- the spot welded joint according to the present embodiment can be manufactured by a manufacturing method including the following steps. That is, the method for manufacturing a spot welded joint according to this embodiment is (I) The first steel plate containing hard martensite and the second steel plate having an average Vickers hardness HVbase of 350 HV or more are superposed. (II) A plurality of spot-welded metals for joining the first steel plate and the second steel plate that are overlapped with each other are formed. (III) The first steel plate is tempered between the two spot-welded metals and within a range of 0.1 mm from the surface on the second steel plate side, and at the same time, between the two spot-welded metals and above. It has a step of quenching in a range of 0.1 mm from the surface opposite to the surface on the second steel plate side.
- first steel plate 11 containing hard martensite and the second steel plate 12 having an average Vickers hardness HVbase of 350 HV or more known steel plates can be used. These steel plates are overlapped and spot welded to form a spot weld metal to form a welded joint.
- the spot welding conditions are not limited and may be normal conditions.
- a part of the first steel sheet is hardened and a part is tempered by laser irradiation to form a first region and a second region.
- the temperature of the quenching region is too high, the region that needs to be tempered will also become the quenching region due to heat conduction. Therefore, it is necessary to control the heat input according to the plate thickness.
- tempering it is necessary to heat the temperature of the target region to a temperature of less than Ac 1 ° C. As shown in FIG.
- the heated region becomes tempered martensite, and the hardness decreases as the temperature rises up to Ac1 ° C.
- the heating temperature exceeds Ac 1 ° C.
- the tissue is transformed into austenite. Since this austenite transforms into hard martensite again when cooled, it exhibits high hardness at the site heated to Ac 1 ° C. or higher.
- laser irradiation is performed from the surface side of the first steel sheet (the surface opposite to the second steel sheet), and the temperature near the surface of the first steel sheet is heated to exceed Ac 1 ° C. By heating the temperature near the opposite surface to Ac 1 ° C. or lower, a predetermined first region and second region can be formed.
- heating in order to give a hardness distribution in the plate thickness direction, it is necessary to heat only the polar surface layer and heat in the depth direction by heat conduction. Further, if the material is heated to the outside of the target area, the heat removal in the target area may be insufficient and the tempered structure may not be obtained. For example, in high-frequency induction heating, heat is input to a certain depth, so that a preferable hardness distribution cannot be obtained. Further, in gas heating or arc heating, it is difficult to heat only a specific region. Therefore, in the method for manufacturing a spot welded joint according to the present embodiment, heating is performed by irradiating a laser beam.
- the laser irradiation conditions are not particularly limited and may be determined depending on the thickness of the first steel plate, the thickness of the first region or the second region to be obtained, and the like. For example, the following conditions are exemplified. Examples of conditions ⁇ Oscillator type: Semiconductor laser ⁇ Output: 500-3000W -Beam shape: Rectangle with width direction: 4 to 10 mm and traveling direction: 0.5 to 3 mm on the irradiation surface-Laser moving speed: 50 to 500 cm / min
- quenching is performed between the two spot weld metals of the first steel sheet and within a range of 0.1 mm from the surface on the second steel plate side, and at the same time, between the two spot weld metals. Moreover, quenching can be performed in a range of 0.1 mm from the surface opposite to the surface on the second steel plate side.
- a steel sheet having a thickness of 2.0 mm was held in a furnace at 950 ° C. for 5 minutes, and then quenched by hot stamping with a water-cooled die. After quenching, the oxide scale on the surface of the steel sheet was removed by shot blasting. The Vickers hardness of the used steel sheet after quenching was as shown in Table 1. In addition, this steel sheet had a structure containing hard martensite.
- a tensile test piece having a distance between reference points of 50 mm and a parallel portion width of 25 mm as shown in FIG. 4 was collected from the steel plate.
- a 25 mm square tab plate was collected from the same steel plate.
- Electrode DR type electrode (tip ⁇ 6 mm R40) Pressurized pressure: 400 kgf Energizing time: 24 cyc
- t thickness (mm) of the tensile test piece
- the test piece (joints Nos. 1 to 5, 11 and 12) after spot welding is irradiated with a laser from one side of the tensile test piece, and the central part in the width direction of the parallel part coincides with the center of the beam in the longitudinal direction.
- the entire parallel portion was heat-treated.
- Joint No. Laser irradiation was not performed on 6 to 10.
- Joint No. The laser irradiation conditions for 1 to 5 were as follows. ⁇ Oscillator type: Semiconductor laser ⁇ Output: 1200W -Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on the irradiation surface-Laser moving speed: 250 cm / min
- the laser irradiation conditions for No. 11 were as follows.
- ⁇ Oscillator type Semiconductor laser ⁇ Output: 700W -Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on the irradiation surface-Laser moving speed: 130 cm / min
- the laser irradiation conditions for 12 were as follows.
- ⁇ Oscillator type Semiconductor laser ⁇ Output: 750W -Beam shape: Rectangle with width direction: 8 mm and traveling direction: 1 mm on the irradiation surface-Laser moving speed: 80 cm / min
- the measurement surface was a cross section in the thickness direction at the center of the width direction of the test piece.
- a tensile test was carried out on each test piece (in-plane tensile stress was applied) to investigate the fracture position.
- the tensile speed during the tensile test was 10 mm / min. The results are shown in Table 1.
- Joint No. In 1 to 5 (example of the present invention), a range of 0.1 mm (first region) from the overlapped surface is tempered to satisfy the formula (1), and a range of 0.1 mm from the surface (first region) is satisfied. Region 2) was hardened to satisfy equation (2). As a result, no crack was observed in the HAZ softened portion.
- the joint No. In 6 to 10 (comparative example), laser irradiation was not performed, so that the joint No. Equations (1) and (2) were not satisfied in the range corresponding to the first region or the second region of 1 to 5. As a result, in the tensile test, cracks occurred in the HAZ softened portion. Joint No. Laser irradiation was performed in Nos.
- Comparative Example 11 Comparative Examples
- Comparative Example 12 also had insufficient heat input, and although the second region was quenched so as to satisfy the formula (2), the tempering was insufficient in the first region, and the formula (1) was expressed in the first region. It was not satisfied, and the difference between the maximum value and the minimum value of Vickers hardness in the first region was 80 or more. As a result, in the tensile test, cracks occurred in the HAZ softened portion.
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Abstract
Description
本願は、2019年05月24日に、日本に出願された特願2019-097703号に基づき優先権を主張し、その内容をここに援用する。
抵抗スポット溶接される上記部材のフランジの一部には、自動車の衝突時に面内引張応力が負荷されることがある。一般に、HAZ軟化部のような硬さが低い領域があると部材の耐衝突性能が低下する。このようなHAZ軟化部は、抵抗スポット溶接の継手評価に用いられる引張せん断試験、及び十字引張試験(JIS Z3137)の評価結果への影響は小さい。しかしながら、面内引張応力が負荷された場合には、HAZ軟化部に局所的にひずみが集中してHAZ軟化部に破断を生じる場合がある。そのため、母材を高強度化し、スポット打点を増加しても、前述のHAZ軟化部が生じると、母材の強度と部品の形状とから想定される耐衝突性能を得られない場合がある。
従って、高強度鋼板からなる鋼板部材を自動車車体の構造部材に適用する場合には、ナゲットの周辺領域が破断の起点となるのを抑制することが求められる。
[1]本開示の一態様に係るスポット溶接継手は、平均ビッカース硬さHVbaseが350HV以上の、硬質マルテンサイトを含む第1の鋼板と、前記第1の鋼板に重ねられた第2の鋼板と、前記第1の鋼板と前記第2の鋼板とを接合している2つのスポット溶接金属と、を含み、前記2つのスポット溶接金属を含む前記第1の鋼板の板厚方向の全ての断面において、前記第1の鋼板が、前記2つのスポット溶接金属の間、かつ前記第2の鋼板側の面から前記第1の鋼板の板厚方向に0.1mmの範囲に形成された第1の領域と、前記2つのスポット溶接金属の間、かつ前記第2の鋼板側の面とは反対の面から前記板厚方向に0.1mmの範囲に形成された第2の領域と、を有し、前記第1の領域の金属組織が焼戻しマルテンサイトを50面積%以上含み、前記第1の領域の平均ビッカース硬さHV1と前記第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(1)を満たし、前記第2の領域の金属組織が硬質マルテンサイトを50面積%以上含み、前記第2の領域の平均ビッカース硬さHV2と前記第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(2)を満たす。
HVbase×0.33+150≦HV1≦HVbase×0.33+230 式(1)
HVbase-30≦HV2≦HVbase+30 式(2)
[2][1]に記載のスポット溶接継手では、前記第1の領域におけるビッカース硬さの最大値と最小値との差が80HV以下であってもよい。
[3][1]または[2]に記載のスポット溶接継手では、前記第1の領域の、前記板厚方向の厚みが、前記第1の鋼板の板厚の30~70%であってもよい。
[4]本開示の別の態様に係るスポット溶接継手の製造方法は、平均ビッカース硬さHVbaseが350HV以上の、硬質マルテンサイトを含む第1の鋼板と、第2の鋼板と、を重ね合わせ、重ね合わされた前記第1の鋼板と前記第2の鋼板とを接合する2つのスポット溶接金属を形成し、レーザー照射によって、前記第1の鋼板の、前記2つのスポット溶接金属同士の間かつ第2の鋼板側の面から0.1mmの範囲に焼戻しを行うと同時に、前記2つのスポット溶接金属の間かつ前記第2の鋼板側の面とは反対の面から0.1mmの範囲に焼入れを行う。
図1に示すように、本実施形態に係るスポット溶接継手1は、第1の鋼板11と、第1の鋼板に重ねられた第2の鋼板12と、第1の鋼板11と第2の鋼板12とを接合する2つのスポット溶接金属2とを備える。図1、図2においては、スポット溶接金属2は抵抗スポット溶接によって形成されたナゲットである。このようなスポット溶接継手は、第1の鋼板11と第2の鋼板12とを重ね合わせて抵抗スポット溶接を行うことによって得られる。
抵抗スポット溶接に供する第1の鋼板11は、Bピラー等の自動車骨格部品への適用を考慮し、平均ビッカース硬さ(HVbase)が350HV以上である鋼板である。また、第1の鋼板は、硬質マルテンサイトのような焼入れ組織を含む組織からなる。一方、第2の鋼板12については、限定されない。
第1の鋼板11の平均ビッカース硬さ(単純に硬度という場合がある)は、溶接に供される第1の鋼板11の溶接前の平均ビッカース硬さを意味する。スポット溶接後に測定する場合には、溶接熱影響を受けていない位置で測定された平均ビッカース硬さを意味する。
また、本実施形態に係るスポット溶接継手1では、第1の領域51の金属組織が、焼戻しマルテンサイトを50面積%以上含み、第1の領域51の平均ビッカース硬さHV1と第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(1)を満たす。
HVbase×0.33+150≦HV1≦HVbase×0.33+230 式(1)
さらに、本実施形態に係るスポット溶接継手1では、前記第2の領域52の金属組織が、硬質マルテンサイトを50面積%以上含み、前記第2の領域52の平均ビッカース硬さHV2と前記第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(2)を満たす。
HVbase-30≦HV2≦HVbase+30 式(2)
第1の領域51の外側にまで第1の領域51と同様の焼き戻しマルテンサイトの面積%及び平均ビッカース硬さの条件を満たす金属組織が広がっていてもよい。また、第2の領域52の外側にまで第2の領域52と同様の硬質マルテンサイトの面積%及び平均ビッカース硬さの条件を満たす金属組織が広がっていてもよい。
以下、各構成の限定理由について説明する。
焼入れ組織を含む鋼板に溶接を行った場合、溶接の熱により溶接金属の周囲に形成されるHAZにおいて、硬質マルテンサイトが焼戻しマルテンサイト等の軟質な組織に変化する。すなわち、母材より硬さが低い領域(HAZ軟化部)が形成される。溶接部を有する板の面内に引張応力が生じた際、このHAZ軟化部が破断の起点となる場合がある。
局所的な強度低下部が生じないようにする場合、例えばスポット溶接金属2の間のHAZ軟化部を含む領域を焼戻して、HAZ軟化部の周辺の硬度を、HAZ軟化部の硬度と同程度まで低下させることが考えられる。しかしながら、この場合、HAZ軟化部からの破断は抑制できるものの、部品全体としては軟化部が大きくなることから、部品の耐衝突性能(曲げ性能)が低下することが懸念される。
本実施形態に係るスポット溶接継手1は、継手強度の低下を最低限に抑えつつHAZ軟化部での割れを防止するため、重ね合わせ溶接を行った第1の鋼板11の、スポット溶接金属2,2同士の間の、重ね合わせ面3付近を焼き戻すとともに、スポット溶接金属2,2同士の間の第1の鋼板の表面(重ね合わせ面3とは反対側)付近を焼入れることによって、所望の金属組織と平均ビッカース硬さとを備えた第1の領域と第2の領域とを形成する。
[金属組織が焼戻しマルテンサイトを50面積%以上含み、第1の領域の平均ビッカース硬さHV1と第1の鋼板の平均ビッカース硬さHVbaseとが、HVbase×0.33+150≦HV1≦HVbase×0.33+230を満たす]
本発明者らの検討の結果、平均ビッカース硬さHVbaseが350HV以上の硬質マルテンサイトを含む第1の鋼板11にスポットを行った際、溶接の熱影響によって生じるHAZ軟化部の硬度(ビッカース硬さ)は、(溶接前の第1の鋼板11の硬度×0.33+150)~(溶接前の第1の鋼板11の硬度×0.33+230)程度になる。そのため、本実施形態に係るスポット溶接継手では、HAZ軟化部を含むスポット溶接金属2,2間の第1の領域を焼戻して焼戻しマルテンサイトが50面積%以上の組織とし、また、そのビッカース硬さが、下記式(1)を満たすように制御する。
第1の領域51の平均ビッカース硬さ(HV1)が式(1)を満たすことで、HAZ軟化部とその周囲との硬度差が80HV以下になる。この場合、HAZ軟化部への歪の集中を緩和できる。
HVbase×0.33+150≦HV1≦HVbase×0.33+230 式(1)
第1の領域51は、第1の鋼板11の板厚断面の、2つのスポット溶接金属2,2の間、かつ第2の鋼板12側の面から0.1mmの範囲(厚さ)に形成されている。上記の硬度を有する領域の板厚方向の厚さが0.1mm未満であると、局所的な強度低下部であるHAZ軟化部が残存し、十分な効果が得られない可能性がある。上記の第1の領域51の硬度及び組織の条件を満たす領域が第1の領域51の外側にまで広がっていてもよい。その場合、上記の第1の領域51の硬度と組織を満たす領域は、第2の鋼板側の面から第1の鋼板の板厚の30%以上の範囲に広がっていることが好ましい。但し、上記の第1の領域51の硬度と組織を満たす領域が、第2の鋼板側の面から第1の鋼板の板厚の90%超の範囲にまで広がっていると、継手部全体の平均硬度が低下し、曲げ耐力が低下することが懸念されるので好ましくない。
[金属組織が硬質マルテンサイトを50面積%以上含み、第2の領域の平均ビッカース硬さHV2と第1の鋼板の平均ビッカース硬さHVbaseとが、HVbase-30≦HV2≦HVbase+30を満たす]
第1の本実施形態に係るスポット溶接継手1に対し面内引張応力が付与された場合、2つのスポット溶接金属2,2間にHAZ軟化部のような局所的な強度低下部が形成されていると歪が集中するが、本発明者らの検討の結果、特に第1の鋼板11の表面(重ね合わせ面とは反対の面)付近に強度低下部が存在すると、歪が集中しやすいことが分かった。
そのため、本実施形態に係るスポット溶接継手1では、HAZ軟化部を含む2つのスポット溶接金属2,2間の第1の鋼板11の表面付近に対し、焼入れを行い、焼入れした領域の硬さを、溶接熱影響を受けていない第1の鋼板11の平均ビッカース硬さと同等にする。
具体的には、第2の領域52の平均ビッカース硬さHV2と第1の鋼板11の平均ビッカース硬さHVbaseとが、下記式(2)を満たすようにする。
HVbase-30≦HV2≦HVbase+30 式(2)
第2の領域52の平均ビッカース硬さと、第1の鋼板11の平均ビッカース硬さとの差が30超であると、面内引張応力負荷時の歪の集中を十分に抑制することができない。
第1の領域51が焼戻しマルテンサイトを50面積%以上含む焼き戻し組織であって、第2の領域が硬質マルテンサイトを50面積%以上含む焼き入れ組織であることから、第1の領域51の平均ビッカース硬さ(HV1)より第2の領域52の平均ビッカース硬さ(HV2)は大きい。
第2の領域52は、第1の鋼板11の板厚断面の、2つのスポット溶接金属2,2の間、かつ第2の鋼板側の面とは反対の面から0.1mmの範囲(厚さ)に形成されている。上記の硬度を有する領域の板厚方向の厚さが0.1mm未満であると、局所的な強度低下部であるHAZ軟化部が残存し、十分な効果が得られない可能性がある。上記の第2の領域52の硬度と組織の条件を満たす領域が第2の領域52の外側にまで広がっていてもよい。その場合、上記の第2の領域52の硬度と組織を満たす領域は、第2の鋼板12側の面とは反対の面から第1の鋼板の板厚の10%以上の範囲に形成されていることが好ましい。但し上記の第2の領域52の硬度と組織を満たす領域が、第2の鋼板12側の面から第1の鋼板11の70%超の範囲にまでひろがっていると、第2の領域側が曲げ外になる引張曲げ荷重が負荷された際に、破断伸びが低下することが懸念されるので好ましくない。
言い換えれば、スポット溶接金属2の重ね合わせ面3における径をDとしたとき、第1の領域51及び第2の領域52の第1の鋼板11の板厚方向断面に垂直な方向(図1の紙面に垂直な方向、図2の紙面上の上下方向)の幅は、1.0×Dである。第1の領域51の幅方向外側にまで第1の領域51と同様の焼き戻しマルテンサイトの面積%及び平均ビッカース硬さの条件を満たす金属組織が広がっていてもよい。第2の領域52の幅方向外側にまで第2の領域52と同様の硬質マルテンサイトの面積%及び平均ビッカース硬さの条件を満たす金属組織が広がっていてもよい。
自動車の衝突が生じた場合、面内引張応力の方向は、スポット溶接金属2,2間を結ぶ方向(スポット溶接金属2,2の中心同士を結ぶ方向)に対して必ずしも平行ではない。すなわち、一定の角度を有する(斜め方向に応力がかかる)場合がある。第1の領域51及び第2の領域52の幅が1.0×Dあれば、面内引張応力の付加方向がスポット溶接金属2,2間を結ぶ方向に対し一定の角度となった場合(斜め方向に応力がかかった場合)であっても、歪が集中しうるHAZ軟化部のへの歪集中が抑制される。その結果、HAZ軟化部での破断がさらに抑制される。
一方、第1の領域51、第2の領域52の幅が1.0×D未満であると、面内引張応力の方向が、スポット溶接金属2,2間を結ぶ方向に対して一定の角度を有する(斜め方向に応力がかかる)場合には、十分な効果が得られないことが懸念される。
硬質マルテンサイト組織を含む鋼板では、溶接熱影響を受けた部分の硬度は、溶接前の硬度より低くなる。このため、第1の鋼板11の硬度は、第1の鋼板11の溶接による熱影響を受けていない位置の硬度を測定し、その平均値を用いる。溶接による熱影響を受けていない位置として、例えば、スポット溶接金属2から、他の溶接金属のない方向へ15mm以上離れた位置の硬度を測定すればよい。
具体的には、ビッカース硬さ計を用いて、荷重を1.0kgfとして、溶接による熱影響を受けていない10ヶ所の、第1の鋼板11の表面から板厚の1/8の位置、3/8の位置、5/8の位置、7/8の位置の硬度を測定し、その平均値を用いる。
ビッカース硬さの分布は、具体的には、以下の方法で測定する。
まず、2つのスポット溶接金属の中心を通る第1の鋼板11の板厚方向断面(図2に示すA-A断面)が測定面となるようにサンプルを採取する。
この測定面に対し、第1の鋼板11の板厚方向には、第1の鋼板11の表面及び重ね合わせ面からそれぞれ0.1mmの位置、及びその間を5等分した位置に対してビッカース硬さの測定を行う。この測定を、幅方向(スポット溶接金属2ともう一方のスポット溶接金属2とを結ぶ方向)に0.5mm間隔で繰り返して行う。
その後、サンプルを0.5mm研磨し、現出した断面(図2に示すB-B断面)に対し、上記と同様のビッカース硬さ測定を行う。
さらに、断面にスポット溶接金属2が含まれなくなるまで研磨及びビッカース硬さの測定を行い、スポット溶接金属2,2間における第1の鋼板11のビッカース硬さ分布を得る。硬度は反対側の断面も同等であると考えられるため、上記の通り半分の断面について測定を行えばよい。
上記では、スポット溶接金属2が抵抗スポット溶接のナゲットである場合について説明したが、本実施形態に係るスポット溶接継手1はスポット溶接金属2が抵抗スポット溶接のナゲットである抵抗スポット溶接継手に限定されない。例えば、スポット溶接金属2がレーザースポット溶接によって形成されてもよく、スポット溶接金属2がアークスポット溶接によって形成されてもよい。
本実施形態に係るスポット溶接継手は、以下の工程を含む製造方法によって製造できる。すなわち、本実施形態に係るスポット溶接継手の製造方法は、
(I)平均ビッカース硬さHVbaseが350HV以上の、硬質マルテンサイトを含む第1の鋼板と、第2の鋼板と、を重ね合わせ、
(II)重ね合わされた前記第1の鋼板と前記第2の鋼板とを接合する複数のスポット溶接金属を形成し、
(III)前記第1の鋼板の、前記2つのスポット溶接金属同士の間かつ第2の鋼板側の面から0.1mmの範囲に焼戻しを行うと同時に、前記2つのスポット溶接金属の間かつ前記第2の鋼板側の面とは反対の面から0.1mmの範囲に焼入れを行う
工程を有する。
これらの鋼板を重ね合わせてスポット溶接を行い、スポット溶接金属を形成して溶接継手とする。スポット溶接条件は限定されず、通常の条件とすればよい。
焼入れを行う場合には、対象とする領域の温度をAc1℃超に高める必要がある。好ましくはAc1+30℃以上である。ただし、焼き入れ領域の温度を高くし過ぎると、熱伝導によって、焼戻しを行う必要のある領域もまた焼き入れ領域となってしまう。そのため、板厚に応じた入熱コントロールが必要である。
一方、焼戻しを行う場合には、対象とする領域の温度をAc1℃未満の温度に加熱する必要がある。図3に示すように、加熱された領域は、焼戻しマルテンサイトとなり、Ac1℃までは温度上昇につれて硬度が減少する。一方、加熱温度がAc1℃を超えると、組織がオーステナイトに変態する。このオーステナイトは、冷却時に再度硬質マルテンサイトに変態するので、Ac1℃超に加熱された部位では、高い硬度を示す。
このことを利用して、第1の鋼板の表面(第2の鋼板とは反対の面)側からレーザー照射を行い、第1の鋼板の表面付近の温度をAc1℃超になるように加熱し、その反対の面付近の温度をAc1℃以下となるように加熱すれば、所定の第1の領域及び第2の領域を形成することができる。
上記のような加熱を行う場合、板厚方向へ硬さ分布を与えるためには、極表層へのみ入熱させ、深さ方向には熱伝導によって熱を与える必要がある。また、狙いの領域外まで加熱すると狙いの領域での抜熱が不十分となり焼き戻し組織が得られない場合がある。
例えば、高周波誘導加熱では、一定の深さまで入熱されてしまうので、好ましい硬さ分布が得られない。また、ガス加熱やアーク加熱では、特定の領域だけを狙って加熱することが困難である。
そのため、本実施形態に係るスポット溶接継手の製造方法では、レーザービームの照射によって加熱を行う。溶接金属間全体を加熱するため、溶接金属径以上のビーム幅を有するレーザービームを一定の速度で移動させながら加熱することが好ましい。
レーザーの照射条件は、特に限定されず、第1の鋼板の板厚、得たい第1の領域または第2の領域の厚さ等によって決定すればよいが、例えば以下の条件が例示される。
例示される条件
・発振器の種類:半導体レーザー
・出力:500~3000W
・ビーム形状:照射面において、幅方向:4~10mm、進行方向:0.5~3mmの矩形
・レーザー移動速度:50~500cm/min
電極:DR型電極(先端φ6mm R40)
加圧力:400kgf
通電時間:24cyc
抵抗スポット溶接により、引張試験片とタブ板との間には、ナゲット径が4×√t(t:引張試験片の板厚(mm))である溶接金属が二か所形成された。
継手No.1~5へのレーザー照射条件は以下の通りとした。
・発振器の種類:半導体レーザー
・出力:1200W
・ビーム形状:照射面において、幅方向:8mm、進行方向:1mmの矩形
・レーザー移動速度:250cm/min
また、継手No.11へのレーザー照射条件は以下の通りとした。
・発振器の種類:半導体レーザー
・出力:700W
・ビーム形状:照射面において、幅方向:8mm、進行方向:1mmの矩形
・レーザー移動速度:130cm/min
また、継手No.12へのレーザー照射条件は以下の通りとした。
・発振器の種類:半導体レーザー
・出力:750W
・ビーム形状:照射面において、幅方向:8mm、進行方向:1mmの矩形
・レーザー移動速度:80cm/min
また、各試験体に対して引張試験を実施し(面内引張応力を負荷し)、破断位置を調査した。引張試験時の引張速度は10mm/minとした。
結果を表1に示す。
一方、継手No.6~10(比較例)はレーザー照射を行わなかったことで、継手No.1~5の第1の領域または第2の領域に相当する範囲において、式(1)、式(2)を満たさなかった。その結果、引張試験において、HAZ軟化部での割れとなった。
継手No.11~12(比較例)はレーザー照射を行ったが、レーザー照射条件が好ましくなかった。その結果、比較例11は入熱不足により第一の領域でHAZ軟化部が明瞭に残存しており、HAZ軟化部での破断となった。比較例12もまた入熱不足であり、第2の領域は式(2)を満たすように焼き入れされたものの、第1の領域では焼き戻し不足となり、第1の領域において式(1)を満たさず、また前記第1の領域におけるビッカース硬さの最大値と最小値との差が80以上となった。その結果、引張試験において、HAZ軟化部での割れとなった。
2 スポット溶接金属
3 重ね合わせ面
11 第1の鋼板
12 第2の鋼板
51 第1の領域
52 第2の領域
Claims (4)
- 平均ビッカース硬さHVbaseが350HV以上の、硬質マルテンサイトを含む第1の鋼板と、
前記第1の鋼板に重ねられた第2の鋼板と、
前記第1の鋼板と前記第2の鋼板とを接合している2つのスポット溶接金属と、
を含み、
前記2つのスポット溶接金属を含む前記第1の鋼板の板厚方向の全ての断面において、
前記第1の鋼板が、
前記2つのスポット溶接金属の間、かつ前記第2の鋼板側の面から前記第1の鋼板の板厚方向に0.1mmの範囲に形成された第1の領域と、
前記2つのスポット溶接金属の間、かつ前記第2の鋼板側の面とは反対の面から前記板厚方向に0.1mmの範囲に形成された第2の領域と、を有し、
前記第1の領域の金属組織が焼戻しマルテンサイトを50面積%以上含み、前記第1の領域の平均ビッカース硬さHV1と前記第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(1)を満たし、
前記第2の領域の金属組織が硬質マルテンサイトを50面積%以上含み、前記第2の領域の平均ビッカース硬さHV2と前記第1の鋼板の平均ビッカース硬さHVbaseとが、下記式(2)を満たす、
ことを特徴とするスポット溶接継手。
HVbase×0.33+150≦HV1≦HVbase×0.33+230 式(1)
HVbase-30≦HV2≦HVbase+30 式(2) - 前記第1の領域におけるビッカース硬さの最大値と最小値との差が80HV以下であることを特徴とする請求項1に記載のスポット溶接継手。
- 前記第1の領域の、前記板厚方向の厚みが、前記第1の鋼板の板厚の30~70%である、
ことを特徴とする請求項1または2に記載のスポット溶接継手。 - 平均ビッカース硬さHVbaseが350HV以上の、硬質マルテンサイトを含む第1の鋼板と、第2の鋼板と、を重ね合わせ、
重ね合わされた前記第1の鋼板と前記第2の鋼板とを接合する2つのスポット溶接金属を形成し、
レーザー照射によって、前記第1の鋼板の、前記2つのスポット溶接金属同士の間かつ第2の鋼板側の面から0.1mmの範囲に焼戻しを行うと同時に、前記2つのスポット溶接金属の間かつ前記第2の鋼板側の面とは反対の面から0.1mmの範囲に焼入れを行う、
ことを特徴とするスポット溶接継手の製造方法。
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