WO2020036198A1 - Resistance spot-welded member and manufacturing method therefor - Google Patents

Resistance spot-welded member and manufacturing method therefor Download PDF

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Publication number
WO2020036198A1
WO2020036198A1 PCT/JP2019/031949 JP2019031949W WO2020036198A1 WO 2020036198 A1 WO2020036198 A1 WO 2020036198A1 JP 2019031949 W JP2019031949 W JP 2019031949W WO 2020036198 A1 WO2020036198 A1 WO 2020036198A1
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Prior art keywords
nugget
energization
cooling
max
current value
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PCT/JP2019/031949
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French (fr)
Japanese (ja)
Inventor
直雄 川邉
松田　広志
池田　倫正
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Ｊｆｅスチール株式会社
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Priority to CN201980053050.7A priority Critical patent/CN112584959B/en
Priority to JP2019563908A priority patent/JP6908132B2/en
Priority to KR1020217003918A priority patent/KR102491219B1/en
Publication of WO2020036198A1 publication Critical patent/WO2020036198A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/10—Spot welding; Stitch welding
    • B23K11/11—Spot welding
    • 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
    • B23K11/24—Electric supply or control circuits therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K37/00—Auxiliary devices or processes, not specially adapted to a procedure covered by only one of the preceding main groups
    • B23K37/003—Cooling means
    • 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 invention relates to a resistance spot welding member and a method for manufacturing the same.
  • the welding method mainly used in the production process of an automobile is resistance spot welding, but the welded portion of this resistance spot welding is hard to easily undergo martensitic transformation due to rapid cooling of a molten portion. Become an organization. Further, tensile residual stress is generated in the weld due to heat shrinkage during the cooling process. Furthermore, hydrogen may be taken into the weld metal during welding from the plating layer on the steel sheet surface, oil or moisture on the steel sheet surface, or hydrogen may enter the weld from the use environment (for example, under an acidic environment). is there. Therefore, the welded part of resistance spot welding may be in a very disadvantageous state from the viewpoint of delayed fracture resistance.
  • Patent Document 1 discloses that the structure and hardness of the welded portion are controlled by increasing the pressing force and decreasing the current immediately after welding energization (main energization). A technique for preventing delayed destruction is disclosed.
  • the problem of delayed fracture due to the intrusion of hydrogen into the weld metal having high hydrogen embrittlement susceptibility during such welding is not limited to the case of resistance spot welding of a high-strength steel sheet for automobiles, and other problems may occur. It also exists in resistance spot welding of steel sheets.
  • the present inventor adjusts the hardness of the spot welded portion by improving the structure of the end of the nugget, which is the starting point of the delayed fracture, and is a plate set including a steel plate having a tensile strength of 980 MPa or more. Also, it was considered that a spot welded member excellent in delayed fracture resistance could be provided.
  • ⁇ C, Mn, and Si are listed as alloy elements in the steel sheet that affect the hardness of the nugget.
  • a method of controlling the hardness of the nugget end it is effective to provide a post-energization step for tempering after the main energization step for forming the nugget is completed in the resistance spot welding process. Means.
  • tempering by post-energization to the heat-affected zone outside the nugget hereinafter also referred to as welding heat-affected zone
  • the influence may be exerted, and the strength may be reduced due to excessive softening.
  • a high current equal to or more than the current value in the main energizing step is applied, and the amount of heat required for tempering is applied to the nugget end in a short time without excessively softening the heat-affected zone. It is necessary to control the hardness of the nugget end.
  • a method for manufacturing a resistance spot welded member wherein two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more are stacked, sandwiched by a pair of welding electrodes, and energized while applying pressure to produce a nugget. And a nugget by pressing and holding the steel plate with the welding electrode for a cooling time C t (ms) represented by the following formulas (5) and (6) after the main current supplying process. , And a post-energization step of energizing at a current value I p (kA) satisfying the following equation (7) after the cooling step.
  • the structure of the end of the nugget is improved while preventing the strength from being reduced due to the softening of the heat-affected zone, and the resistance is excellent in delayed fracture resistance.
  • a spot welding member can be obtained.
  • the resistance spot welding member of the present invention has two or more steel plates and a spot weld. Explanation will be given in the order of two or more steel plates and spot welds.
  • the steel plate of two or more includes a steel plate having a tensile strength of 980 MPa or more (sometimes referred to as a “high-strength steel plate”).
  • a steel sheet having a tensile strength of 980 MPa or more is used, delayed fracture of a spot weld tends to be a problem.
  • the resistance spot welding member of the present invention has an effect of adjusting the hardness of the spot welded portion, so that even when a high-strength steel plate is used, the delayed fracture resistance of the spot welded portion is improved. .
  • the component composition of the two or more steel sheets is not particularly limited, but in mass%, C: 0.6% or less, Si: 3.0% or less, Mn: 20.0% or less, P: 1.0% or less, A component composition containing 0.8% or less of S and 3.0% or less of Al, the balance being Fe and unavoidable impurities is preferable.
  • the thickness of the two or more steel plates is not particularly limited, but is preferably, for example, in the range of 0.5 mm or more and 2.0 mm or less.
  • a steel sheet having a thickness in this range can be suitably used as an automobile member.
  • # 2 or more steel sheets may be the same or different, may be the same type and same shape, or may be different types or different shapes.
  • the spot weld 12 is formed between two or more steel plates 15, and includes a central nugget 13 and a weld heat affected zone 14 outside the nugget.
  • the boundary between the nugget 13 and the heat affected zone 14 can be visually determined by performing corrosion using a picric acid aqueous solution on the section of the plate thickness of the spot weld.
  • the Vickers hardness H n (Hv) of the nugget end of the spot welded portion is equal to or less than H ob (Hv) represented by the following equation (3).
  • the coefficient concerning each element in X and Y is determined in consideration of the magnitude of the influence of each element.
  • H ob2 (800 ⁇ X max +300) / (0.7 + 20 ⁇ Y max ) (10)
  • Y max Y of the steel sheet in which the coefficient Y represented by the expression (2) is the largest.
  • the Vickers hardness H n of the nugget end is preferably set to (0.4 ⁇ H ob) or more. If the Vickers hardness H n of the nugget end (0.4 ⁇ H ob) smaller than, lowers the strength of the nugget itself, such as joint strength, it delayed joint performance other than fracture properties may deteriorate.
  • FIG. 4 also shows an enlarged view of the nugget end 16 in the spot welded portion 12.
  • the nugget end 16 has a thickness of 50 ⁇ m from the boundary between the nugget 13 and the weld heat-affected zone 14 toward the center of the nugget 13 in a section of the resistance spot welding member passing through the center of the nugget 13. Means the position.
  • the nugget end portions 16 are located at two positions 50 ⁇ m from the two intersections of the boundary line between the steel plates 15 and the nugget 13 toward the center of the nugget 13 in a thickness cross section passing through the center of the nugget 13.
  • the Vickers hardness H n of the nugget end there, define who the them smaller Vickers hardness as the Vickers hardness H n of the nugget end.
  • HV Vickers hardness
  • the certain range is an area of 40 to 60 ⁇ m from the boundary toward the center of the nugget. Therefore, the “nugget end” includes a region of 40 to 60 ⁇ m from the above boundary toward the center of the nugget.
  • the Vickers hardness H min (Hv) of the softest part of the heat affected zone of the spot welded part satisfies the following equation (4). 0.4 ⁇ H n ⁇ H min ⁇ 0.9 ⁇ H n (4)
  • the Vickers hardness H min (Hv) of the softest part of the weld heat affected zone is less than (0.4 ⁇ H n ) (Hv)
  • the strength is easily reduced due to excessive softening of the weld heat affected zone.
  • the Vickers hardness H min (Hv) of the softest part of the heat affected zone exceeds (0.9 ⁇ H n ) (Hv) (Hv)
  • locally high stress concentrates on the end of the nugget, causing a delay. Destruction is easy to occur.
  • the Vickers hardness H min of the softest part of the weld heat affected zone is (0.5 ⁇ H n ) or more. Is preferable, and (0.8 ⁇ H n ) or less is preferable.
  • FIG. 5A is a cross-sectional view in the thickness direction passing through the center of the nugget 13
  • FIG. 5B is a diagram showing the distance (mm) from the boundary and the Vickers hardness (near the boundary) between the nugget and the welding heat affected zone. HV).
  • the softest part of the welding heat affected zone 14 of the spot welded part 12 means the following position.
  • a region 3 mm outside the nugget 13 from the boundary between the nugget 13 and the welding heat affected zone 14 is spaced at intervals of 150 ⁇ m from the boundary. It means the portion (position) showing the softest hardness when measured. The hardness can be measured by a method described in Examples described later.
  • a steel plate arranged on the lower side (hereinafter, referred to as lower steel plate 1) and a steel plate arranged on the upper side (hereinafter, referred to as upper steel plate 2) are overlapped.
  • at least one of the lower steel plate 1 and the upper steel plate 2 is a steel plate having a tensile strength of 980 MPa or more.
  • a pair of welding electrodes that is, an electrode disposed on the lower side (hereinafter, referred to as lower electrode 4) and an electrode disposed on the upper side (hereinafter, referred to as upper electrode 5), are superimposed on a steel plate (the lower steel plate 1 and the lower steel plate 1).
  • the upper steel plate 2) is sandwiched and energized while applying pressure.
  • the configuration in which the lower electrode 4 and the upper electrode 5 pressurize and control the pressing force is not particularly limited, and conventionally known devices such as an air cylinder and a servomotor can be used.
  • the configuration for supplying a current during energization and controlling the current value is not particularly limited, and a conventionally known device can be used.
  • the present invention can be applied to both direct current and alternating current. In the case of AC, “current” means “effective current”.
  • the type of the tip of the lower electrode 4 or the upper electrode 5 is not particularly limited. For example, DR type (dome radius type), R type (radius type), D type (dome type) described in JIS C # 9304: 1999. ) And the like.
  • the tip diameter (diameter of the tip) of the electrode is, for example, 4 mm to 16 mm.
  • the main energization step performed as described above is a step of melting a steel sheet to form a nugget.
  • the energizing conditions and the pressing conditions for forming the nugget are not particularly limited, and conventionally used welding conditions can be adopted.
  • the current value is 1.0 kA or more and 15.0 kA or less
  • the pressing force is 2.0 kN or more and 7.0 kN or less.
  • the energization time is not particularly limited, and is, for example, 100 ms or more and 1000 ms or less.
  • the “nugget” is a portion that is melt-solidified at a spot weld in lap resistance welding.
  • a cooling step of cooling the nugget by holding the steel plate while pressing it with the welding electrode for a cooling time C t (ms) represented by the following equations (5) and (6) is performed. .
  • t is an average thickness (mm) of the steel plate to be joined.
  • the cooling step is a step necessary for obtaining a tempering effect by a post-energization step described later. If the cooling time C t (ms) does not satisfy the equations (5) and (6), the nugget end is heated by subsequent energization without being sufficiently cooled. In that case, the effect of tempering cannot be obtained, and the hardness of the nugget end cannot be reduced.
  • the cooling time C t (ms) depends on the thickness of the steel plates, and when joining steel plates having different thicknesses, the average value of the thickness of each steel plate is used.
  • the upper limit of the cooling time C t (ms) is not particularly defined, but it is preferable that C t ⁇ 800 ⁇ t.
  • C t is (800 ⁇ t) or more, the total time of the welding process itself becomes longer, and the productivity is reduced.
  • the post-energization step is an energization step with a current value I p (kA) satisfying the following equation (7). 0.8 ⁇ I min ⁇ I p ⁇ 1.5 ⁇ I max (7)
  • I max is a maximum current value (kA) in the main energizing step
  • I min is a minimum current value (kA) in the main energizing step.
  • the post-energization step is a step of reheating the nugget end and reducing the hardness of the nugget end by tempering. If the current value I p (kA) in the post-energization step is less than (0.8 ⁇ I min ), the amount of heat input is insufficient, and the hardness of the nugget end cannot be reduced. Further, when the current value I p (kA) in the post-energization step is equal to or more than (1.5 ⁇ I max ), the input heat becomes excessive and exceeds the temperature range in which the effect of tempering the nugget end can be obtained. The hardness of the end cannot be reduced.
  • the current value I p (kA) in the post-energization step is 0.95 ⁇ I min ⁇ I It is preferable to satisfy p ⁇ 1.2 ⁇ I max . Further, the lower limit of the current value I p (kA) in the post-energization step is more preferably (1.0 ⁇ I min ) or more. As described above, in the manufacturing method of the present invention, the post-energization step is performed after the step of determining the condition of the current value Ip based on the equation (7).
  • the energization time in the post-energization step is less than 20 ms, the nugget may not be sufficiently heated, and the tempering effect may not be obtained.
  • the energization time of the post-energization step exceeds 200 ms, the input heat becomes sufficient and the hardness of the end of the nugget decreases, but the strength of the heat-affected zone decreases due to excessive softening, and breakage from the heat-affected zone at low stress occurs. May occur.
  • the energization time in the post-energization step is preferably in the range of 20 to 200 ms. More preferably, it is 20 to 100 ms.
  • a repeated energization step may be performed.
  • the repetitive energizing step includes a cooling energizing step in which cooling is performed after the post-energizing step and energizing under the condition satisfying the following equation (8), and a re-energizing state re-energizing the cooling energizing state under the condition satisfying the following equation (9). Is performed n times.
  • I nc a current value (kA) in the n-th cooling energization
  • I nr a current value (kA) in the n-th re-energization
  • n is a natural number of 1 or more.
  • I max and I min are the same as in equation (7).
  • the nugget end By repeating cooling and heating in the repetitive energizing step, the nugget end can be maintained in an appropriate temperature range for a long time, and the effect of tempering can be further exhibited. Further, since the cooling energization is a process of cooling the nugget end, it may be performed without energization. If the current value I nc (kA) in the cooling energization exceeds I max , the effect of cooling the nugget end cannot be obtained, and the appropriate temperature range cannot be maintained by the subsequent re-energization.
  • the upper limit of the energization time of the cooling energization in the repetitive energization step is not particularly defined, but is preferably (800 ⁇ t) or less. If it exceeds (800 ⁇ t), the total time of the welding process itself may be long, and the productivity may be reduced.
  • the lower limit of the energization time of the cooling energization is not particularly specified, but is preferably 20 ms or more. If the time is less than 20 ms, the effect of cooling the nugget end cannot be obtained, and an appropriate temperature range may not be maintained by the subsequent energization.
  • Resistance spot welding was performed under each condition of the two-plate set and the three-plate set. Resistance spot welding was performed at room temperature, and the electrode was constantly cooled with water. Each of the lower electrode 4 and the upper electrode 5 had a tip diameter (tip diameter) of 6 mm and a radius of curvature of 40 mm, and was a chrome copper DR type electrode. The pressing force was controlled by driving the lower electrode 4 and the upper electrode 5 with a servomotor, and a single-phase alternating current having a frequency of 50 Hz was supplied during energization.
  • the lower steel sheet 1 has a plating treatment in which the tensile strength is 1470 MPa, X represented by the formula (1) is 0.24, and Y represented by the formula (2) is 0.020 (hot-dip galvanizing ( GI), the amount of adhesion is 50 g / m 2 per side), a 1.6 mm-thick steel plate and an upper steel plate 2 having a tensile strength of 1180 MPa, X represented by the formula (1) is 0.20, and the formula (2).
  • GA hot-dip galvanized
  • the tensile strength is a tensile strength determined by preparing a JIS No. 5 tensile test piece from each steel sheet in a direction parallel to the rolling direction and performing a tensile test in accordance with JIS Z 2241: 2011.
  • FIGS. 3 (a) and 3 (b) resistance spot welding is performed on the steel plate (length in the longitudinal direction: 150 mm, length in the short direction: 50 mm). Between the two sheets, a spacer 6 of 1.6 mm thickness and 50 mm square is sandwiched on both sides, and temporarily welded, and the center of the sheet set in which two steel sheets are overlapped is set under the conditions described above and in Table 1-1. Welding was performed to produce a welded joint.
  • FIGS. 3A to 3C are a plan view (FIG. 3A) and side views (FIGS. 3B and 3C) showing a test piece for resistance spot welding. Reference numeral 7 in FIGS.
  • 3A to 3C denotes a welding point
  • reference numeral 8 denotes a temporary welding point.
  • the lower steel plate 9, the middle steel plate 10, and the upper steel plate 11 are overlapped, and the spacers 6 are provided on both sides between the lower steel plate 9 and the middle steel plate 10. Welding was carried out. The dimensions of the steel plate and the spacer are the same as those of the two-plate set.
  • welded joints were manufactured under the same conditions and the same shape.
  • the obtained welded joint was cut at the center of the nugget, etched with picric acid to clarify the boundary of the melted portion, and then subjected to a load of 200 gf and a load holding time of 15 s at the end of the nugget and the softened portion. Vickers hardness was measured.
  • the hardness of the end of the nugget was a measured value of the hardness of the inside of the nugget 50 ⁇ m from the boundary of the fusion zone.
  • the hardness of the softest part was the measured value of the softest part in a case where a region having a length of 3 mm from the end of the nugget was measured at intervals of 150 ⁇ m from the end of the nugget to the outside of the nugget.
  • a cross tension test was performed on welded joints manufactured under the same conditions.
  • the cross tension test was performed on a test specimen having a shape based on JISZ # 3137.
  • the symbol "X" indicates that the joint strength was reduced by 10% or more compared to the joint strength when welding was performed only in the main energizing step without post-energization, and the symbol "X” indicates that the strength was not reduced by 10% or more.
  • ⁇ is described.

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Abstract

Provided are: a resistance spot-welded member in which the structure of a nugget end portion is improved, whereby the delayed fracture resistance of the member is enhanced; and a manufacturing method for the resistance spot-welded member. The resistance spot-welded member comprises two or more steel sheets and a spot-welded portion formed between the steel sheets, wherein, the tensile strength of at least one of the steel sheets is 980 MPa or greater, and given that the maximum of the coefficient X of the steel sheet is Xmax (where X=[C]+[Si]/40+[Mn]/200) and the minimum of the coefficient Y of the steel sheet is Ymin (where as Y=[P]+3×[S]), the Vickers hardness Hn (Hv) of a nugget end portion of the spot-welded portion is no greater than Hob (Hv), which is expressed as Hob=(800×Xmax+300)/(0.7+20×Ymin), and the Vickers hardness Hmin (Hv) of the softest part of the welding heat-affected portion in the spot-welded portion satisfies the inequality 0.4×Hn≤Hmin≤0.9×Hn.

Description

抵抗スポット溶接部材及びその製造方法Resistance spot welding member and method of manufacturing the same
　本発明は、抵抗スポット溶接部材及びその製造方法に関するものである。 The present invention relates to a resistance spot welding member and a method for manufacturing the same.
　自動車の燃費を向上させるための自動車車体の軽量化と衝突安全性の向上とを両立するために、使用する鋼板を高強度化してその板厚を低減する努力が続けられている。しかしながら、自動車用鋼板の引張強度９８０ＭＰａ級以上への高強度化に伴い、溶接部の耐遅れ破壊特性の低下に対する懸念が生じてきた。 努力 Efforts are being made to increase the strength of steel sheets used and reduce the thickness of the steel sheets in order to achieve both a reduction in the weight of the vehicle body and an improvement in collision safety to improve the fuel efficiency of the automobile. However, with the increase in the tensile strength of the steel sheet for automobiles to the 980 MPa class or higher, there has been a concern that the delayed fracture resistance of the welded portion is reduced.
　詳述すると、自動車の生産工程で主に用いられている溶接方法は抵抗スポット溶接であるが、この抵抗スポット溶接の溶接部は、溶融した部分が急冷されることによりマルテンサイト変態を起こしやすく硬い組織となる。また、該溶接部には冷却過程の熱収縮により引張残留応力が発生する。さらに、鋼板表面のめっき層、鋼板表面の油や水分などから、溶接中に溶接金属内に水素が取り込まれたり、使用環境（例えば酸性環境下）から溶接部へ水素が進入したりする場合がある。したがって、抵抗スポット溶接の溶接部は、耐遅れ破壊特性の観点からは非常に不利な状態となる場合がある。 In detail, the welding method mainly used in the production process of an automobile is resistance spot welding, but the welded portion of this resistance spot welding is hard to easily undergo martensitic transformation due to rapid cooling of a molten portion. Become an organization. Further, tensile residual stress is generated in the weld due to heat shrinkage during the cooling process. Furthermore, hydrogen may be taken into the weld metal during welding from the plating layer on the steel sheet surface, oil or moisture on the steel sheet surface, or hydrogen may enter the weld from the use environment (for example, under an acidic environment). is there. Therefore, the welded part of resistance spot welding may be in a very disadvantageous state from the viewpoint of delayed fracture resistance.
　従来は、鋼板強度がそれほど高くなかったために、溶接部への応力集中が比較的小さく、遅れ破壊が問題とされることはなかった。しかし、鋼板の引張強度が９８０ＭＰａ級以上の高強度鋼板においては、炭素等の焼入れ性元素を多く含むため、ナゲットおよびその近傍が非常に硬くなって、遅れ破壊が発生しやすい状態となる。 Conventionally, since the strength of the steel sheet was not so high, the stress concentration on the weld was relatively small, and delayed fracture did not pose a problem. However, in a high-strength steel sheet having a tensile strength of 980 MPa or more, since the steel sheet contains a large amount of hardenable elements such as carbon, the nugget and the vicinity thereof become very hard, so that delayed fracture is likely to occur.
　また、このようなナゲットが非常に硬い抵抗スポット溶接部は、ナゲット組織の粒界強度を低下させる要因となる元素であるＰ、Ｓに対する感受性が高い。このため、これらの元素を多く含む鋼板の抵抗スポット溶接部材ではナゲットの強度が低下し、遅れ破壊がより発生しやすい状態となる。この抵抗スポット溶接部材の遅れ破壊の問題は、高強度鋼板を含む板組であれば軟鋼との合わせ板組であっても生じる場合がある。また、該高強度鋼板においては、プレス成形が難しくなることから高強度鋼板を重ね合わせたときに鋼板間に隙間が生じやすい。このため、この隙間を一対の対向する電極で強制的に潰して溶接する抵抗スポット溶接では、ナゲットの端部にこの鋼板の隙間に起因する引張応力が追加され、さらに遅れ破壊が発生しやすい状態となることが考えられる。 抵抗 Further, such a resistance spot weld having an extremely hard nugget has a high sensitivity to P and S, which are elements that reduce the grain boundary strength of the nugget structure. For this reason, in a resistance spot welded member of a steel sheet containing many of these elements, the strength of the nugget is reduced, and delayed fracture is more likely to occur. The problem of delayed fracture of the resistance spot welded member may occur even if it is a plate set including a high strength steel sheet, even if it is a laminated plate set with mild steel. In addition, in the high-strength steel sheets, a gap is easily generated between the steel sheets when the high-strength steel sheets are overlapped because press forming is difficult. For this reason, in resistance spot welding in which this gap is forcibly crushed and welded by a pair of opposed electrodes, tensile stress due to the gap between the steel plates is added to the end of the nugget, and further delayed fracture is likely to occur. It is considered that
　このような溶接部の遅れ破壊を防止する方法として、特許文献１には、溶接通電（主通電）直後に加圧力を上昇させると共に電流を減少させることで溶接部の組織や硬さを制御し、遅れ破壊を防止する技術が開示されている。 As a method for preventing such a delayed fracture of the welded portion, Patent Document 1 discloses that the structure and hardness of the welded portion are controlled by increasing the pressing force and decreasing the current immediately after welding energization (main energization). A technique for preventing delayed destruction is disclosed.
　また、特許文献２には、溶接通電（主通電）直後に加圧力を上昇させると共に無通電での冷却時間経過後に通電することで溶接部の組織や硬さを制御し、遅れ破壊を防止する技術が開示されている。 Further, in Patent Document 2, the structure and hardness of the welded portion are controlled by increasing the pressing force immediately after the welding current (main current) and energizing after the cooling time has elapsed without the current being applied, thereby preventing delayed fracture. Techniques are disclosed.
特開２０１５－９３２８２号公報JP-A-2005-93282 ＷＯ２０１４／１７１４９５号公報WO2014 / 171495
　しかし、これらの技術は、遅れ破壊を防止する効果を得るために長時間の後通電を要する。このため、投入熱量が過多となった場合には熱影響部が過度の軟化により強度低下し、低応力で熱影響部からの破断が発生する場合がある。また、これらの技術は、ナゲットの強度低下の要因となる元素であるＰ、Ｓの影響については何ら考慮されていない。 However, these technologies require long post-energization to obtain the effect of preventing delayed breakdown. For this reason, when the heat input is excessive, the heat-affected zone may be weakened due to excessive softening, and may be broken from the heat-affected zone with low stress. Further, these techniques do not consider the effects of P and S, which are elements that cause a reduction in the strength of the nugget.
　なお、このような溶接時に水素脆化感受性の高い溶接金属内に水素が侵入することにより遅れ破壊が発生するという問題は、自動車用の高強度鋼板を抵抗スポット溶接する場合に限らず、その他の鋼板の抵抗スポット溶接においても同様に存在する。 The problem of delayed fracture due to the intrusion of hydrogen into the weld metal having high hydrogen embrittlement susceptibility during such welding is not limited to the case of resistance spot welding of a high-strength steel sheet for automobiles, and other problems may occur. It also exists in resistance spot welding of steel sheets.
　本発明はかかる事情に鑑みてなされたものであって、その目的は、ナゲットの端部（ナゲット端部）の組織を改善することにより、耐遅れ破壊特性に優れた抵抗スポット溶接部材及びその製造方法を提供するものである。 The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the structure of the end portion of a nugget (nugget end portion) to thereby provide a resistance spot welded member having excellent delayed fracture resistance and its production. It provides a method.
　本発明者は、遅れ破壊の発生起点であるナゲット端部の組織を改善することにより、スポット溶接部の硬さを調整して、引張強度が９８０ＭＰａ以上の鋼板を含む板組の場合であっても、耐遅れ破壊特性に優れたスポット溶接部材を提供可能であると考えた。 The present inventor adjusts the hardness of the spot welded portion by improving the structure of the end of the nugget, which is the starting point of the delayed fracture, and is a plate set including a steel plate having a tensile strength of 980 MPa or more. Also, it was considered that a spot welded member excellent in delayed fracture resistance could be provided.
　そこで、遅れ破壊の要因であるナゲット端部の硬さおよび粒界強度低下の要因となる元素のＰ、Ｓについて調査し、以下のような知見を得た。 Then, the hardness of the nugget end part, which is the cause of delayed fracture, and the elements P and S, which are the causes of the decrease in grain boundary strength, were investigated, and the following findings were obtained.
　ナゲットの硬さに影響を与える鋼板中の合金元素としては、Ｃ、Ｍｎ、Ｓｉが挙げられる。鋼板中のこれらの元素の含有量が多いほど抵抗スポット溶接部材の遅れ破壊感受性が高くなる。しかし、これらの元素の含有量に応じてナゲット端部の硬さを低減することにより、上記鋼板を含む板組の抵抗スポット溶接部材の耐遅れ破壊特性を改善することが可能となる。 合金 C, Mn, and Si are listed as alloy elements in the steel sheet that affect the hardness of the nugget. The greater the content of these elements in the steel sheet, the higher the delayed fracture susceptibility of the resistance spot welded member. However, by reducing the hardness of the end of the nugget in accordance with the content of these elements, it becomes possible to improve the delayed fracture resistance of the resistance spot welded member of the sheet set including the steel sheet.
　しかしながら、鋼板がＰ、Ｓを多く含む場合、ナゲット端部のＰ、Ｓの濃度が高くなり、遅れ破壊がより発生しやすい状態となる。また、ナゲットは複数の被溶接鋼板が溶融し形成される組織である。よって、高強度鋼板がＰ、Ｓを多く含まない場合であっても、例えば板組に含まれる他の鋼板のＰ、Ｓの含有量が大きかった場合には、同様に遅れ破壊が発生しやすい状態となる。そのため鋼板のＣ、Ｍｎ、Ｓｉの含有量だけでなく、Ｐ、Ｓの含有量を考慮してナゲット端部の硬さを適切に制御することが必要となり、これにより耐遅れ破壊特性に優れた抵抗スポット溶接部材を作製可能となる。 However, when the steel sheet contains a large amount of P and S, the concentration of P and S at the end of the nugget becomes high, and a state in which delayed fracture is more likely to occur. The nugget is a structure formed by melting a plurality of steel plates to be welded. Therefore, even when the high-strength steel sheet does not contain much P and S, for example, when the P and S contents of the other steel sheets included in the sheet assembly are large, delayed fracture is also likely to occur. State. Therefore, it is necessary to appropriately control the hardness of the end of the nugget in consideration of not only the contents of C, Mn, and Si but also the contents of P and S of the steel sheet, thereby providing excellent delayed fracture resistance. A resistance spot welding member can be manufactured.
　また、ナゲット端部の硬さを制御する方法としては、抵抗スポット溶接プロセスにおいて、ナゲットを形成するための主通電工程が終了した後に、焼き戻しを目的とした後通電工程を付与することが有効な手段である。しかし、ナゲット端部を焼き戻すための後通電工程が長時間となった場合、ナゲットの外側の熱影響部（以下、溶接熱影響部と称する場合もある。）にまで後通電による焼き戻しの影響が及び、過度の軟化による強度低下を引き起こす場合がある。そこで、主通電工程後、主通電工程における電流値以上の高電流を通電し、焼き戻しに必要な熱量を短時間でナゲット端部に投入することで、熱影響部を過度に軟化させることなくナゲット端部の硬さを制御することが必要となる。 Further, as a method of controlling the hardness of the nugget end, it is effective to provide a post-energization step for tempering after the main energization step for forming the nugget is completed in the resistance spot welding process. Means. However, when the post-energization process for tempering the end of the nugget takes a long time, tempering by post-energization to the heat-affected zone outside the nugget (hereinafter also referred to as welding heat-affected zone) may be performed. The influence may be exerted, and the strength may be reduced due to excessive softening. Therefore, after the main energizing step, a high current equal to or more than the current value in the main energizing step is applied, and the amount of heat required for tempering is applied to the nugget end in a short time without excessively softening the heat-affected zone. It is necessary to control the hardness of the nugget end.
　本発明は以上のような知見に基づいてなされたものであり、要旨は以下のとおりである。 The present invention has been made based on the above findings, and the gist is as follows.
　［１］２以上の鋼板と、前記鋼板間に形成されたスポット溶接部と、を備える抵抗スポット溶接部材であって、前記２以上の鋼板の少なくとも１つの鋼板の引張強度が９８０ＭＰａ以上であり、前記２以上の鋼板において、下記（１）式で表される係数Ｘが最も大きくなる鋼板のＸをＸｍａｘとし、下記（２）式で表される係数Ｙが最も小さくなる鋼板のＹをＹｍｉｎとした場合に、前記スポット溶接部のナゲットの端部のビッカース硬さＨｎ（Ｈｖ）が下記（３）式で表されるＨｏｂ（Ｈｖ）以下であり、前記スポット溶接部の溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎ（Ｈｖ）が下記（４）式を満足する、抵抗スポット溶接部材。
Ｘ＝［Ｃ］＋［Ｓｉ］／４０＋［Ｍｎ］／２００　　（１）
Ｙ＝［Ｐ］＋３×［Ｓ］　　　（２）
Ｈｏｂ＝（８００× Ｘｍａｘ＋３００）／（０．７＋２０ × Ｙｍｉｎ）　　　（３）
０．４ × Ｈｎ≦Ｈｍｉｎ≦０．９ × Ｈｎ　　（４）
前記（１）式及び（２）式において［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ｐ］及び［Ｓ］は各元素の含有量（質量％）である。ただし、含まない場合は０とする。 [1] A resistance spot welding member including two or more steel sheets and a spot welded portion formed between the steel sheets, wherein at least one of the two or more steel sheets has a tensile strength of 980 MPa or more, wherein at least two steel plates, the X of the steel sheet which the coefficient X represented by the following formula (1) is maximized and X max, the Y coefficient Y is smallest steel sheet represented by the following formula (2) Y min , the Vickers hardness H n (Hv) of the end of the nugget of the spot weld is not more than H ob (Hv) expressed by the following equation (3), and the welding heat of the spot weld is A resistance spot welded member in which the Vickers hardness H min (Hv) of the softened part of the affected part satisfies the following expression (4)
X = [C] + [Si] / 40 + [Mn] / 200 (1)
Y = [P] + 3 × [S] (2)
H ob = (800 × X max +300) / (0.7 + 20 × Y min ) (3)
0.4 × H n ≦ H min ≦ 0.9 × H n (4)
In the formulas (1) and (2), [C], [Si], [Mn], [P] and [S] are the contents (% by mass) of the respective elements. However, it is set to 0 when not included.
　［２］抵抗スポット溶接部材の製造方法であって、引張強度が９８０ＭＰａ以上の鋼板を少なくとも１枚含む２枚以上の鋼板を重ね合わせて１対の溶接電極で挟持し加圧しながら通電してナゲットを形成する主通電工程と、前記主通電工程の後に下記の（５）式、（６）式で表される冷却時間Ｃｔ（ｍｓ）の間鋼板を前記溶接電極で加圧保持してナゲットを冷却する冷却工程と、前記冷却工程後に、下記（７）式を満足する電流値Ｉｐ（ｋＡ）で通電する後通電工程と、を有する、抵抗スポット溶接部材の製造方法。
Ｃｔ≧１６０ × ｔ２　（ｔ≦１．６）　　　（５）
Ｃｔ≧２５６ × ｔ　（ｔ＞１．６）　　　（６）
０．８ × Ｉｍｉｎ≦Ｉｐ＜１．５ × Ｉｍａｘ　　　　（７）
前記（５）式、（６）式、（７）式において、
ｔ：被接合鋼板の平均板厚（ｍｍ）
Ｉｍａｘ：主通電工程における最大電流値（ｋＡ）
Ｉｍｉｎ：主通電工程における最小電流値（ｋＡ）
である。 [2] A method for manufacturing a resistance spot welded member, wherein two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more are stacked, sandwiched by a pair of welding electrodes, and energized while applying pressure to produce a nugget. And a nugget by pressing and holding the steel plate with the welding electrode for a cooling time C t (ms) represented by the following formulas (5) and (6) after the main current supplying process. , And a post-energization step of energizing at a current value I p (kA) satisfying the following equation (7) after the cooling step.
C t ≧ 160 × t 2 (t ≦ 1.6) (5)
C t ≧ 256 × t (t> 1.6) (6)
0.8 × I min ≦ I p <1.5 × I max (7)
In the above equations (5), (6) and (7),
t: Average thickness of the steel plate to be joined (mm)
I max : maximum current value (kA) in the main energization process
I min : minimum current value (kA) in the main energization process
It is.
　［３］前記後通電工程の後に、冷却し下記（８）式を満たす条件で通電する冷却通電と、前記冷却通電後に下記（９）式を満たす条件で再後通電する再後通電とをｎ回行う繰返通電工程を有する、［２］に記載の抵抗スポット溶接部材の製造方法。
０≦Ｉｎｃ≦Ｉｍａｘ　　　　（８）
０．８ × Ｉｍｉｎ≦Ｉｎｒ＜１．５ × Ｉｍａｘ　　　　（９）
前記（８）式、（９）式において、
Ｉｎｃ：ｎ回目の冷却通電における電流値（ｋＡ）
Ｉｎｒ：ｎ回目の再後通電における電流値（ｋＡ）
Ｉｍａｘ：主通電工程における最大電流値（ｋＡ）
Ｉｍｉｎ：主通電工程における最小電流値（ｋＡ）
ｎ：１以上の自然数
である。 [3] After the post-energization step, cooling energization for cooling and energizing under the condition satisfying the following equation (8) and re-energization for re-energizing under the condition satisfying the following equation (9) after the cooling energization are performed by n The method for producing a resistance spot welding member according to [2], further comprising a repetitive energizing step performed once.
0 ≦ I nc ≦ I max (8)
0.8 × I min ≦ I nr <1.5 × I max (9)
In the equations (8) and (9),
I nc : Current value (kA) at n-th cooling energization
Inr : current value (kA) in the nth re-energization
I max : maximum current value (kA) in the main energization process
I min : minimum current value (kA) in the main energization process
n is a natural number of 1 or more.
　本発明によれば、高強度鋼板の抵抗スポット溶接を行う場合であっても、熱影響部の軟化による強度低下を防止しつつナゲット端部の組織を改善し、耐遅れ破壊特性に優れた抵抗スポット溶接部材を得ることができる。 According to the present invention, even when resistance spot welding of a high-strength steel sheet is performed, the structure of the end of the nugget is improved while preventing the strength from being reduced due to the softening of the heat-affected zone, and the resistance is excellent in delayed fracture resistance. A spot welding member can be obtained.
図１は、スポット溶接部を説明するための模式図である。FIG. 1 is a schematic diagram for explaining a spot weld. 図２は、抵抗スポット溶接の例を模式的に示す断面図である。FIG. 2 is a cross-sectional view schematically illustrating an example of resistance spot welding. 図３（ａ）～図３（ｃ）は抵抗スポット溶接の試験片を示す図であり、図３（ａ）は平面図であり、図３（ｂ）および図３（ｃ）は側面図である。3 (a) to 3 (c) are views showing test pieces for resistance spot welding, FIG. 3 (a) is a plan view, and FIGS. 3 (b) and 3 (c) are side views. is there. 図４は、本発明のナゲット端部を説明する断面図である。FIG. 4 is a cross-sectional view illustrating a nugget end according to the present invention. 図５（ａ）は本発明の溶接熱影響部の最軟化部を説明する断面図であり、図５（ｂ）はナゲット端部および溶接熱影響部におけるビッカース硬さを示すグラフである。FIG. 5A is a cross-sectional view for explaining the softest part of the heat affected zone of the present invention, and FIG. 5B is a graph showing Vickers hardness at the nugget end and the heat affected zone.
　以下、本発明の実施形態について説明する。なお、本発明は以下の実施形態に限定されない。 Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments.
　本発明の抵抗スポット溶接部材は、２以上の鋼板と、スポット溶接部とを有する。２以上の鋼板、スポット溶接部の順で説明する。 抵抗 The resistance spot welding member of the present invention has two or more steel plates and a spot weld. Explanation will be given in the order of two or more steel plates and spot welds.
　鋼板
　２以上の鋼板は、引張強度が９８０ＭＰａ以上の鋼板（「高強度鋼板」という場合がある）を含む。引張強度が９８０ＭＰａ以上の鋼板を用いると、スポット溶接部の遅れ破壊が問題になりやすい。しかし、後述する通り、本発明の抵抗スポット溶接部材は、スポット溶接部の硬さの調整が行われているため、高強度鋼板を用いても、スポット溶接部の耐遅れ破壊特性が良好になる。なお、引張強度が９８０ＭＰａ以上の鋼板の使用量（使用枚数）が多いほど、遅れ破壊時間の問題が生じやすくなるため、本発明の効果は、２以上の鋼板のうち半数以上の鋼板の引張強度が９８０ＭＰａ以上の場合により顕著に現れる。 Steel plate The steel plate of two or more includes a steel plate having a tensile strength of 980 MPa or more (sometimes referred to as a “high-strength steel plate”). When a steel sheet having a tensile strength of 980 MPa or more is used, delayed fracture of a spot weld tends to be a problem. However, as will be described later, the resistance spot welding member of the present invention has an effect of adjusting the hardness of the spot welded portion, so that even when a high-strength steel plate is used, the delayed fracture resistance of the spot welded portion is improved. . In addition, since the problem of delayed fracture time is more likely to occur as the used amount (the number of sheets used) of a steel sheet having a tensile strength of 980 MPa or more increases, the effect of the present invention is as follows. Is more conspicuous when 980 MPa or more.
　また、鋼板の枚数は２以上であれば上限は特に限定されないが、溶接のしやすさ等を考慮すると鋼板の枚数は４以下が好ましい。 上限 Also, the upper limit is not particularly limited as long as the number of steel sheets is 2 or more, but the number of steel sheets is preferably 4 or less in consideration of ease of welding and the like.
　上記２以上の鋼板の成分組成は特に限定されないが、質量％で、Ｃ：０．６％以下、Ｓｉ：３．０％以下、Ｍｎ：２０．０％以下、Ｐ：１．０％以下、Ｓ：０．８％以下、Ａｌ：３．０％以下を含有し、残部がＦｅ及び不可避的不純物である成分組成が好ましい。 The component composition of the two or more steel sheets is not particularly limited, but in mass%, C: 0.6% or less, Si: 3.0% or less, Mn: 20.0% or less, P: 1.0% or less, A component composition containing 0.8% or less of S and 3.0% or less of Al, the balance being Fe and unavoidable impurities is preferable.
　上記２以上の鋼板の全て又は一部が、めっき処理されて表面にめっき層を有するものであってもよい。めっきとしては、例えば、Ｚｎ系めっきやＡｌ系めっき系が挙げられる。Ｚｎ系めっきとしては、溶融亜鉛めっき（ＧＩ）、Ｚｎ－Ｎｉ系めっき、Ｚｎ－Ａｌ系めっきなどが挙げられる。また、Ａｌ系めっきとしては、Ａｌ－Ｓｉ系めっき（例えば、１０～２０質量％のＳｉを含むＡｌ－Ｓｉ系めっき）などが例示できる。溶融めっき層は、合金化された合金化溶融めっき層であってもよい。合金化溶融めっき層としては、例えば、合金化溶融亜鉛めっき（ＧＡ）層が挙げられる。 全 て All or some of the two or more steel plates may be plated and have a plating layer on the surface. Examples of the plating include Zn-based plating and Al-based plating. Examples of the Zn-based plating include hot-dip galvanizing (GI), Zn-Ni-based plating, and Zn-Al-based plating. Examples of the Al-based plating include Al-Si-based plating (eg, Al-Si-based plating containing 10 to 20% by mass of Si). The hot dip coating layer may be an alloyed hot dip coating layer. Examples of the alloyed hot-dip galvanized layer include an alloyed hot-dip galvanized (GA) layer.
　上記２以上の鋼板の板厚は特に限定されないが、例えば０．５ｍｍ以上２．０ｍｍ以下の範囲内であることが好ましい。板厚がこの範囲内である鋼板は、自動車用部材として好適に使用することができる。 板 The thickness of the two or more steel plates is not particularly limited, but is preferably, for example, in the range of 0.5 mm or more and 2.0 mm or less. A steel sheet having a thickness in this range can be suitably used as an automobile member.
　２以上の鋼板は、同じでも異なっていてもよく、同種および同形状の鋼板であってもよいし、異種や異形状の鋼板であってもよい。 # 2 or more steel sheets may be the same or different, may be the same type and same shape, or may be different types or different shapes.
　２以上の鋼板は、抵抗スポット溶接部材を構成する鋼板であるため、重ね合された状態になる。重ね合された鋼板間には下記のスポット溶接部が形成される。 # 2 or more steel sheets are steel sheets constituting a resistance spot welding member, and thus are in a superposed state. The following spot welds are formed between the superposed steel plates.
　スポット溶接部
　スポット溶接部１２は、図１に示すように、２以上の鋼板間１５に形成され、中央のナゲット１３、ナゲットの外側の溶接熱影響部１４から構成される。ナゲット１３と溶接熱影響部１４との境界は、スポット溶接部の板厚断面において、ピクリン酸水溶液を用いた腐食を実施することで目視で判定可能である。 As shown in FIG. 1, the spot weld 12 is formed between two or more steel plates 15, and includes a central nugget 13 and a weld heat affected zone 14 outside the nugget. The boundary between the nugget 13 and the heat affected zone 14 can be visually determined by performing corrosion using a picric acid aqueous solution on the section of the plate thickness of the spot weld.
　２以上の鋼板において、下記（１）式で表される係数Ｘが最も大きくなる鋼板のＸをＸｍａｘとし、下記（２）式で表される係数Ｙが最も小さくなる鋼板のＹをＹｍｉｎとした場合に、スポット溶接部のナゲット端部のビッカース硬さＨｎ（Ｈｖ）が下記（３）式で表されるＨｏｂ（Ｈｖ）以下である。
Ｘ＝［Ｃ］＋［Ｓｉ］／４０＋［Ｍｎ］／２００　　（１）
Ｙ＝［Ｐ］＋３×［Ｓ］　　　（２）
Ｈｏｂ＝（８００× Ｘｍａｘ＋３００）／（０．７＋２０ × Ｙｍｉｎ）　　　（３）
（１）式において［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ｐ］及び［Ｓ］は各元素の含有量（質量％）である。ただし、含まない場合は０とする。 In two or more steel plates, the X of the steel sheet which the coefficient X represented by the following formula (1) is maximized and X max, the Y coefficient Y is smallest steel sheet represented by the following formula (2) Y min , The Vickers hardness H n (Hv) of the nugget end of the spot welded portion is equal to or less than H ob (Hv) represented by the following equation (3).
X = [C] + [Si] / 40 + [Mn] / 200 (1)
Y = [P] + 3 × [S] (2)
H ob = (800 × X max +300) / (0.7 + 20 × Y min ) (3)
In the formula (1), [C], [Si], [Mn], [P] and [S] are the contents (% by mass) of the respective elements. However, if it is not included, it is set to 0.
　スポット溶接部の硬さは、Ｃ、Ｓｉ、Ｍｎの影響を受けるため、Ｃ、Ｓｉ、Ｍｎを考慮する必要がある。本発明では、Ｘ＝［Ｃ］＋［Ｓｉ］／４０＋［Ｍｎ］／２００（上記（１）式）により、Ｃ、Ｓｉ、Ｍｎの影響を考慮する。また、Ｐ、Ｓは、ナゲット組織の強度低下の要因となる元素である。このため、遅れ破壊を改善する本発明においてはＰやＳの影響を考慮する必要がある。そこで、本発明では、Ｙ＝［Ｐ］＋３×［Ｓ］（上記（２）式）によりＳやＰの影響を考慮する。なお、Ｘ、Ｙにおける各元素にかかる係数は、それぞれの元素が及ぼす影響の大きさを加味して決定している。
　スポット溶接部のナゲット端部のビッカース硬さＨｎ（Ｈｖ）は、下記（３）式で表されるＨｏｂ（Ｈｖ）以下である。ナゲット端部のビッカース硬さが式（３）で表されるＨｏｂ（Ｈｖ）を超える場合は、遅れ破壊が発生し易い。
　耐遅れ破壊特性を向上させる効果をより顕著に発揮させたい場合には、ナゲット端部のビッカース硬さＨｎ（Ｈｖ）を下記（１０）式で表されるＨｏｂ２（Ｈｖ）以下とするか、もしくは（０．９５×Ｈｏｂ）以下とすることがより好ましい。
Ｈｏｂ２＝（８００× Ｘｍａｘ＋３００）／（０．７＋２０ × Ｙｍａｘ）　　　（１０）
ここで、（１０）式において、Ｙｍａｘ：上記（２）式で表される係数Ｙが最も大きくなる鋼板のＹ、である。
　なお、ナゲット端部のビッカース硬さＨｎは（０．４×Ｈｏｂ）以上とすることが好ましい。ナゲット端部のビッカース硬さＨｎが（０．４×Ｈｏｂ）より小さい場合は、ナゲットの強度自体が低下し、継手強度など、耐遅れ破壊特性以外の継手性能が低下する恐れがある。 Since the hardness of the spot weld is affected by C, Si and Mn, it is necessary to consider C, Si and Mn. In the present invention, the influence of C, Si, and Mn is considered by using X = [C] + [Si] / 40 + [Mn] / 200 (formula (1)). P and S are elements that cause a reduction in the strength of the nugget structure. Therefore, in the present invention for improving delayed fracture, it is necessary to consider the influence of P and S. Therefore, in the present invention, the influence of S and P is considered by using Y = [P] + 3 × [S] (Equation (2)). In addition, the coefficient concerning each element in X and Y is determined in consideration of the magnitude of the influence of each element.
The Vickers hardness H n (Hv) of the nugget end of the spot weld is equal to or less than H ob (Hv) represented by the following equation (3). If the Vickers hardness at the end of the nugget exceeds H ob (Hv) represented by the formula (3), delayed fracture is likely to occur.
When the effect of improving the delayed fracture resistance is desired to be more remarkably exhibited, the Vickers hardness H n (Hv) at the end of the nugget should be equal to or less than H ob2 (Hv) represented by the following equation (10). Or (0.95 × H ob ) or less.
H ob2 = (800 × X max +300) / (0.7 + 20 × Y max ) (10)
Here, in the expression (10), Y max : Y of the steel sheet in which the coefficient Y represented by the expression (2) is the largest.
Incidentally, the Vickers hardness H n of the nugget end is preferably set to (0.4 × H ob) or more. If the Vickers hardness H n of the nugget end (0.4 × H ob) smaller than, lowers the strength of the nugget itself, such as joint strength, it delayed joint performance other than fracture properties may deteriorate.
　ここで、図４を用いてナゲット端部について説明する。図４には、スポット溶接部１２におけるナゲット端部１６を拡大した図も示す。図４に示すように、ナゲット端部１６とは、抵抗スポット溶接部材のナゲット１３の中央を通る板厚断面において、ナゲット１３と溶接熱影響部１４との境界からナゲット１３の中央に向けて５０μｍの位置を意味する。また、ナゲット端部１６は、ナゲット１３の中央を通る板厚断面において、鋼板１５間の境界線とナゲット１３との２つの交点からそれぞれナゲット１３の中央に向けて５０μｍの位置の２か所であり、そのうちビッカース硬さの値が小さい方をナゲット端部のビッカース硬さＨｎとして定義する。なお、ビッカース硬さの値が同じ場合には、その値をナゲット端部のビッカース硬さＨｎとして定義する。
　なお本発明では、上記境界からナゲット１３の中央に向けて５０μｍの位置を中心として一定の範囲内で上記ビッカース硬さＨｎ（ＨＶ）を有していれば同様に上述の効果が得られる。この一定の範囲内とは、上記境界からナゲット中央に向けて４０～６０μｍの領域である。そのため、「ナゲット端部」には上記境界からナゲット中央に向けて４０～６０μｍの領域が含まれるものとする。
　３枚以上の鋼板を溶接する場合には、複数のナゲット端部が形成されるが、引張強度が９８０ＭＰａ以上の鋼板を含む板組の少なくとも一つのナゲット端部で（３）式を満たせばよい。 Here, the nugget end will be described with reference to FIG. FIG. 4 also shows an enlarged view of the nugget end 16 in the spot welded portion 12. As shown in FIG. 4, the nugget end 16 has a thickness of 50 μm from the boundary between the nugget 13 and the weld heat-affected zone 14 toward the center of the nugget 13 in a section of the resistance spot welding member passing through the center of the nugget 13. Means the position. Further, the nugget end portions 16 are located at two positions 50 μm from the two intersections of the boundary line between the steel plates 15 and the nugget 13 toward the center of the nugget 13 in a thickness cross section passing through the center of the nugget 13. There, define who the them smaller Vickers hardness as the Vickers hardness H n of the nugget end. Incidentally, if the value of the Vickers hardness are the same, it defines the value as Vickers hardness H n of the nugget end.
In the present invention, a similar effect of the above so long as it has the Vickers hardness H n (HV) within a certain range around the position of 50μm toward the center of the nugget 13 from the boundary is obtained. The certain range is an area of 40 to 60 μm from the boundary toward the center of the nugget. Therefore, the “nugget end” includes a region of 40 to 60 μm from the above boundary toward the center of the nugget.
When three or more steel plates are welded, a plurality of nugget ends are formed. At least one nugget end of a plate set including a steel plate having a tensile strength of 980 MPa or more may satisfy Expression (3). .
　本発明では、スポット溶接部の溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎ（Ｈｖ）が下記（４）式を満足する。
０．４ × Ｈｎ≦Ｈｍｉｎ≦０．９ × Ｈｎ　　（４）
　溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎ（Ｈｖ）が（０．４×Ｈｎ）（Ｈｖ）未満の場合は、溶接熱影響部の過度の軟化により、強度低下を引き起こし易い。また、溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎ（Ｈｖ）が（０．９×Ｈｎ）（Ｈｖ）超えの場合は、ナゲット端部に局所的に高い応力が集中し、遅れ破壊が発生し易い。また、強度低下を引き起こすことなく、遅れ破壊を抑制する効果をより顕著に発揮させたい場合には、溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎは（０．５×Ｈｎ）以上が好ましく、（０．８×Ｈｎ）以下が好ましい。 In the present invention, the Vickers hardness H min (Hv) of the softest part of the heat affected zone of the spot welded part satisfies the following equation (4).
0.4 × H n ≦ H min ≦ 0.9 × H n (4)
When the Vickers hardness H min (Hv) of the softest part of the weld heat affected zone is less than (0.4 × H n ) (Hv), the strength is easily reduced due to excessive softening of the weld heat affected zone. When the Vickers hardness H min (Hv) of the softest part of the heat affected zone exceeds (0.9 × H n ) (Hv), locally high stress concentrates on the end of the nugget, causing a delay. Destruction is easy to occur. Further, when it is desired to more remarkably exhibit the effect of suppressing delayed fracture without causing a decrease in strength, the Vickers hardness H min of the softest part of the weld heat affected zone is (0.5 × H n ) or more. Is preferable, and (0.8 × H n ) or less is preferable.
　ここで、図５（ａ）および図５（ｂ）を用いて、スポット溶接部の溶接熱影響部の最軟化部について説明する。図５（ａ）はナゲット１３の中央を通る板厚方向の断面図であり、図５（ｂ）はナゲットと溶接熱影響部の境界周辺における、境界からの距離（ｍｍ）とビッカース硬さ（ＨＶ）を示すグラフである。
　図５（ａ）および図５（ｂ）に示すように、スポット溶接部１２の溶接熱影響部１４の最軟化部とは、次の位置を意味する。ナゲット１３の中央と上述したナゲット端部１６（図示せず）とを通る直線上において、ナゲット１３と溶接熱影響部１４との境界からナゲット１３の外側３ｍｍの領域について、上記境界から１５０μｍ間隔で測定した場合に最も軟らかい硬さを示した部分（位置）を意味する。なお、硬さは後述する実施例に記載の方法で測定することができる。 Here, with reference to FIGS. 5A and 5B, the softest part of the heat affected zone of the spot weld will be described. FIG. 5A is a cross-sectional view in the thickness direction passing through the center of the nugget 13, and FIG. 5B is a diagram showing the distance (mm) from the boundary and the Vickers hardness (near the boundary) between the nugget and the welding heat affected zone. HV).
As shown in FIGS. 5A and 5B, the softest part of the welding heat affected zone 14 of the spot welded part 12 means the following position. On a straight line passing through the center of the nugget 13 and the above-described nugget end 16 (not shown), a region 3 mm outside the nugget 13 from the boundary between the nugget 13 and the welding heat affected zone 14 is spaced at intervals of 150 μm from the boundary. It means the portion (position) showing the softest hardness when measured. The hardness can be measured by a method described in Examples described later.
　次いで、本発明の抵抗スポット溶接部材の製造方法について説明する。本発明の製造方法は、主通電工程と、冷却工程と、後通電工程と、を有する。図１、２を用いて、本発明の製造方法を説明する。 Next, a method for manufacturing the resistance spot welding member of the present invention will be described. The manufacturing method of the present invention includes a main energizing step, a cooling step, and a post-energizing step. The manufacturing method of the present invention will be described with reference to FIGS.
　主通電工程とは、引張強度が９８０ＭＰａ以上の鋼板を少なくとも１枚含む２枚以上の鋼板を重ね合わせて、１対の溶接電極で挟持し、加圧しながら通電してナゲットを形成する工程である。 The main energization step is a step in which two or more steel sheets including at least one steel sheet having a tensile strength of 980 MPa or more are overlapped, sandwiched by a pair of welding electrodes, and energized while applying pressure to form a nugget. .
　図２に示すように、下側に配置される鋼板（以下、下鋼板１という）と上側に配置される鋼板（以下、上鋼板２という）とを重ね合わせる。このとき、下鋼板１、上鋼板２の少なくとも１つを引張強度が９８０ＭＰａ以上の鋼板とする。次いで、１対の溶接電極、すなわち下側に配置される電極（以下、下電極４という）および上側に配置される電極（以下、上電極５という）で、重ね合わせた鋼板（下鋼板１と上鋼板２）を挟持して、加圧しながら通電する。下電極４と上電極５によって加圧し、且つその加圧力を制御する構成は特に限定されず、エアシリンダやサーボモータ等の従来から知られている機器が使用できる。また、通電の際に電流を供給し、且つ電流値を制御する構成も特に限定されず、従来から知られている機器が使用できる。また、直流、交流のいずれにも本発明を適用できる。なお、交流の場合は、「電流」は「実効電流」を意味する。また、下電極４や上電極５の先端の形式も特に限定されず、例えば、ＪＩＳ　Ｃ　９３０４：１９９９に記載されるＤＲ形（ドームラジアス形）、Ｒ形（ラジアス形）、Ｄ形（ドーム形）等が挙げられる。また、電極の先端径（先端の直径）は、例えば４ｍｍ～１６ｍｍである。 よ う As shown in FIG. 2, a steel plate arranged on the lower side (hereinafter, referred to as lower steel plate 1) and a steel plate arranged on the upper side (hereinafter, referred to as upper steel plate 2) are overlapped. At this time, at least one of the lower steel plate 1 and the upper steel plate 2 is a steel plate having a tensile strength of 980 MPa or more. Next, a pair of welding electrodes, that is, an electrode disposed on the lower side (hereinafter, referred to as lower electrode 4) and an electrode disposed on the upper side (hereinafter, referred to as upper electrode 5), are superimposed on a steel plate (the lower steel plate 1 and the lower steel plate 1). The upper steel plate 2) is sandwiched and energized while applying pressure. The configuration in which the lower electrode 4 and the upper electrode 5 pressurize and control the pressing force is not particularly limited, and conventionally known devices such as an air cylinder and a servomotor can be used. In addition, the configuration for supplying a current during energization and controlling the current value is not particularly limited, and a conventionally known device can be used. The present invention can be applied to both direct current and alternating current. In the case of AC, “current” means “effective current”. The type of the tip of the lower electrode 4 or the upper electrode 5 is not particularly limited. For example, DR type (dome radius type), R type (radius type), D type (dome type) described in JIS C # 9304: 1999. ) And the like. The tip diameter (diameter of the tip) of the electrode is, for example, 4 mm to 16 mm.
　上記のように行われる主通電工程は、鋼板を溶融させてナゲットを形成する工程である。そのナゲットを形成するための通電条件、加圧条件は特に限定されず、従来から用いられている溶接条件を採用することができる。例えば、電流値は１．０ｋＡ以上１５．０ｋＡ以下であり、加圧力は２．０ｋＮ以上７．０ｋＮ以下である。また、通電時間も特に限定されず、例えば、１００ｍｓ以上１０００ｍｓ以下である。なお、「ナゲット」とは、重ね抵抗溶接においてスポット溶接部に生じる溶融凝固する部分である。 主 The main energization step performed as described above is a step of melting a steel sheet to form a nugget. The energizing conditions and the pressing conditions for forming the nugget are not particularly limited, and conventionally used welding conditions can be adopted. For example, the current value is 1.0 kA or more and 15.0 kA or less, and the pressing force is 2.0 kN or more and 7.0 kN or less. Further, the energization time is not particularly limited, and is, for example, 100 ms or more and 1000 ms or less. The “nugget” is a portion that is melt-solidified at a spot weld in lap resistance welding.
　上記主通電工程後に、下記の（５）式、（６）式で表される冷却時間Ｃｔ（ｍｓ）の間、鋼板を溶接電極で加圧しながら保持してナゲットを冷却する冷却工程を行う。
Ｃｔ≧１６０ × ｔ２　（ｔ≦１．６）　　　（５）
Ｃｔ≧２５６ × ｔ　（ｔ＞１．６）　　　（６）
（５）式、（６）式において、ｔ：被接合鋼板の平均板厚（ｍｍ）である。 After the main energization step, a cooling step of cooling the nugget by holding the steel plate while pressing it with the welding electrode for a cooling time C t (ms) represented by the following equations (5) and (6) is performed. .
C t ≧ 160 × t 2 (t ≦ 1.6) (5)
C t ≧ 256 × t (t> 1.6) (6)
In the expressions (5) and (6), t is an average thickness (mm) of the steel plate to be joined.
　冷却工程は、後述する後通電工程によって焼き戻しの効果を得るために必要な工程である。冷却時間Ｃｔ（ｍｓ）が（５）式、（６）式を満たさない場合は、ナゲット端部が十分に冷却されないままその後の後通電によって加熱されることになる。そうなると、焼き戻しの効果が得られず、ナゲット端部の硬さを低減できない。冷却時間Ｃｔ（ｍｓ）は鋼板の板厚に依存し、板厚の異なる鋼板を接合する場合には各鋼板の板厚の平均値とする。また、ナゲット端部をより十分に冷却し、その後の後通電における焼き戻しの効果をより顕著に発揮させたい場合は、ｔ≦１．６の場合はＣｔ≧２００×ｔ２、ｔ＞１．６の場合はＣｔ≧３２０×ｔとすることが好ましい。
　本発明において特に冷却時間Ｃｔ（ｍｓ）の上限は規定しないが、Ｃｔ＜８００×ｔとすることが好ましい。Ｃｔが（８００×ｔ）以上の場合は、溶接工程自体の総時間が長くなって生産性が低下する。
以上より、本発明の製造方法では、（５）式及び（６）式に基づいて冷却時間の条件を決定する過程を経た後に、冷却工程が行われる。なお、被接合鋼板の平均板厚は、溶接される全ての鋼板の板厚の平均を意味する。 The cooling step is a step necessary for obtaining a tempering effect by a post-energization step described later. If the cooling time C t (ms) does not satisfy the equations (5) and (6), the nugget end is heated by subsequent energization without being sufficiently cooled. In that case, the effect of tempering cannot be obtained, and the hardness of the nugget end cannot be reduced. The cooling time C t (ms) depends on the thickness of the steel plates, and when joining steel plates having different thicknesses, the average value of the thickness of each steel plate is used. In addition, when it is desired to cool the nugget end more sufficiently and to exert the effect of tempering in the subsequent energization more remarkably, when t ≦ 1.6, C t ≧ 200 × t 2 , and t> 1. .6, it is preferable that C t ≧ 320 × t.
In the present invention, the upper limit of the cooling time C t (ms) is not particularly defined, but it is preferable that C t <800 × t. When C t is (800 × t) or more, the total time of the welding process itself becomes longer, and the productivity is reduced.
As described above, in the manufacturing method of the present invention, the cooling step is performed after the process of determining the condition of the cooling time based on the equations (5) and (6). The average thickness of the steel plates to be joined means the average of the thicknesses of all the steel plates to be welded.
　上記冷却工程後に後通電工程を行う。後通電工程とは、下記（７）式を満足する電流値Ｉｐ（ｋＡ）で通電する工程である。
０．８ × Ｉｍｉｎ≦Ｉｐ＜１．５ × Ｉｍａｘ　　　　（７）
（７）式において、Ｉｍａｘ：主通電工程における最大電流値（ｋＡ）、Ｉｍｉｎ：主通電工程における最小電流値（ｋＡ）である。 After the cooling step, a post-energization step is performed. The post-energization step is an energization step with a current value I p (kA) satisfying the following equation (7).
0.8 × I min ≦ I p <1.5 × I max (7)
In the equation (7), I max is a maximum current value (kA) in the main energizing step, and I min is a minimum current value (kA) in the main energizing step.
　後通電工程はナゲット端部を再加熱し、焼き戻しによりナゲット端部の硬さを低減する工程である。後通電工程における電流値Ｉｐ（ｋＡ）が（０．８×Ｉｍｉｎ）未満の場合は投入熱量が足りず、ナゲット端部の硬さを低減できない。また、後通電工程における電流値Ｉｐ（ｋＡ）が（１．５×Ｉｍａｘ）以上の場合は投入熱量が過多となり、ナゲット端部の焼き戻しの効果が得られる温度域を超えるため、ナゲット端部の硬さを低減できない。また、ナゲット端部の硬さを低減し、耐遅れ破壊特性を向上する効果をより顕著に発生させたい場合は、後通電工程における電流値Ｉｐ（ｋＡ）が０．９５×Ｉｍｉｎ≦Ｉｐ＜１．２×Ｉｍａｘを満たすことが好ましい。また、後通電工程における電流値Ｉｐ（ｋＡ）の下限は、（１．０×Ｉｍｉｎ）以上とすることがより一層好ましい。以上より、本発明の製造方法では、（７）式に基づいて電流値Ｉｐの条件を決定する過程を経た後に、後通電工程が行われる。 The post-energization step is a step of reheating the nugget end and reducing the hardness of the nugget end by tempering. If the current value I p (kA) in the post-energization step is less than (0.8 × I min ), the amount of heat input is insufficient, and the hardness of the nugget end cannot be reduced. Further, when the current value I p (kA) in the post-energization step is equal to or more than (1.5 × I max ), the input heat becomes excessive and exceeds the temperature range in which the effect of tempering the nugget end can be obtained. The hardness of the end cannot be reduced. Further, when it is desired to reduce the hardness of the end of the nugget and to make the effect of improving the delayed fracture resistance more remarkable, the current value I p (kA) in the post-energization step is 0.95 × I min ≦ I It is preferable to satisfy p <1.2 × I max . Further, the lower limit of the current value I p (kA) in the post-energization step is more preferably (1.0 × I min ) or more. As described above, in the manufacturing method of the present invention, the post-energization step is performed after the step of determining the condition of the current value Ip based on the equation (7).
　後通電工程の通電時間が２０ｍｓ未満では、ナゲットが十分に加熱されず、焼き戻しの効果を得られない場合がある。後通電工程の通電時間が２００ｍｓを超えると、投入熱量は十分となりナゲット端部の硬さは低減するが、熱影響部が過度の軟化により強度低下し、低応力で熱影響部からの破断が発生する場合がある。また、後通電工程における電流値が高く、投入熱量が過多となった場合には、ナゲットが再び溶融して、焼き戻しの効果を得られず、散り発生などの問題が生じる場合がある。したがって、後通電工程の通電時間は２０～２００ｍｓの範囲内が好ましい。より好ましくは、２０～１００ｍｓである。 If the energization time in the post-energization step is less than 20 ms, the nugget may not be sufficiently heated, and the tempering effect may not be obtained. When the energization time of the post-energization step exceeds 200 ms, the input heat becomes sufficient and the hardness of the end of the nugget decreases, but the strength of the heat-affected zone decreases due to excessive softening, and breakage from the heat-affected zone at low stress occurs. May occur. Further, when the current value in the post-energization step is high and the input heat quantity is excessive, the nugget may be melted again, and the effect of tempering may not be obtained, and a problem such as generation of scattering may occur. Therefore, the energization time in the post-energization step is preferably in the range of 20 to 200 ms. More preferably, it is 20 to 100 ms.
　また、上記後通電工程後に、繰返通電工程を行ってもよい。繰返通電工程とは、後通電工程の後に、冷却し下記（８）式を満たす条件で通電する冷却通電と、前記冷却通電後に下記（９）式を満たす条件で再後通電する再後通電とを、ｎ回行う工程である。
０≦Ｉｎｃ≦Ｉｍａｘ　　　　（８）
０．８ × Ｉｍｉｎ≦Ｉｎｒ＜１．５ × Ｉｍａｘ　　　　（９）
（８）式、（９）式において、Ｉｎｃ：ｎ回目の冷却通電における電流値（ｋＡ）、Ｉｎｒ：ｎ回目の再後通電における電流値（ｋＡ）、ｎ：１以上の自然数である。Ｉｍａｘ、Ｉｍｉｎは（７）式と同様である。 Further, after the above-described post-energization step, a repeated energization step may be performed. The repetitive energizing step includes a cooling energizing step in which cooling is performed after the post-energizing step and energizing under the condition satisfying the following equation (8), and a re-energizing state re-energizing the cooling energizing state under the condition satisfying the following equation (9). Is performed n times.
0 ≦ I nc ≦ I max (8)
0.8 × I min ≦ I nr <1.5 × I max (9)
In the equations (8) and (9), I nc : a current value (kA) in the n-th cooling energization, I nr : a current value (kA) in the n-th re-energization, and n is a natural number of 1 or more. . I max and I min are the same as in equation (7).
　繰返通電工程を行うことで、ナゲット端部の焼き戻しによる硬さ低減の効果をより顕著に発揮させ、溶接部材の耐遅れ破壊特性をより一層向上させることができる。 こ と By performing the repeated energization step, the effect of reducing the hardness by tempering the nugget end can be more remarkably exhibited, and the delayed fracture resistance of the welded member can be further improved.
　繰返通電工程において、冷却と加熱を繰り返すことにより、ナゲット端部を適切な温度域に長時間維持し、焼き戻しの効果をより一層発揮することができる。また、冷却通電はナゲット端部を冷却する過程であるから、無通電で行ってもよい。また、冷却通電における電流値Ｉｎｃ（ｋＡ）がＩｍａｘ超の場合は、ナゲット端部を冷却する効果が得られず、その後の再後通電で適切な温度域を維持できない。また、冷却通電でナゲット端部を冷却し、その後の再後通電で適切な温度域に保持する効果をより顕著に発揮したい場合は、冷却通電における電流値Ｉｎｃ（ｋＡ）が０≦Ｉｎｃ≦０．５×Ｉｍａｘを満足することが好ましい。
　また、再後通電における電流値Ｉｎｒ（ｋＡ）が（０．８×Ｉｍｉｎ）未満の場合は、ナゲット端部を十分に再加熱できず、適切な温度域を保持できない。また、再後通電における電流値Ｉｎｒ（ｋＡ）が（１．５×Ｉｍａｘ）以上の場合は、投入熱量が過多となりナゲットが再び溶融して、焼き戻しの効果を得られない。また、再後通電でナゲット端部を適切な温度域に保持する効果をより顕著に発揮したい場合は、再後通電における電流値Ｉｎｒ（ｋＡ）が０．９５×Ｉｍｉｎ≦Ｉｎｒ＜１．２×Ｉｍａｘを満足することが好ましい。また、再後通電工程における通電時間は、２０ｍｓ～２００ｍｓの範囲内とすることが好ましい。より好ましくは、２０ｍｓ～１００ｍｓである。
　なお、本発明において、特に繰返通電工程における冷却通電の通電時間の上限は規定しないが、（８００×ｔ）以下とすることが好ましい。（８００×ｔ）を超える場合は、溶接工程自体の総時間が長くなって生産性が低下することがある。
一方、冷却通電の通電時間の下限も特に規定しないが、２０ｍｓ以上とすることが好ましい。２０ｍｓ未満の場合は、ナゲット端部を冷却する効果が得られず、その後の再後通電で適切な温度域を維持できない場合がある。 By repeating cooling and heating in the repetitive energizing step, the nugget end can be maintained in an appropriate temperature range for a long time, and the effect of tempering can be further exhibited. Further, since the cooling energization is a process of cooling the nugget end, it may be performed without energization. If the current value I nc (kA) in the cooling energization exceeds I max , the effect of cooling the nugget end cannot be obtained, and the appropriate temperature range cannot be maintained by the subsequent re-energization. In addition, when it is desired to more remarkably exhibit the effect of cooling the nugget end portion by cooling energization and maintaining an appropriate temperature range by re-energization, the current value I nc (kA) in cooling energization is 0 ≦ I nc. It is preferable to satisfy ≦ 0.5 × I max .
If the current value I nr (kA) in the re-energization is less than (0.8 × I min ), the nugget end cannot be sufficiently reheated and an appropriate temperature range cannot be maintained. When the current value I nr (kA) in the re-energization is equal to or more than (1.5 × I max ), the input heat amount becomes excessive, the nugget is melted again, and the effect of tempering cannot be obtained. When it is desired to more remarkably exert the effect of maintaining the nugget end portion in an appropriate temperature range by re-energization, the current value I nr (kA) in the re-energization is 0.95 × I min ≦ I nr <1. .2 × I max is preferably satisfied. In addition, the energization time in the re-energization step is preferably in the range of 20 ms to 200 ms. More preferably, it is 20 ms to 100 ms.
In the present invention, the upper limit of the energization time of the cooling energization in the repetitive energization step is not particularly defined, but is preferably (800 × t) or less. If it exceeds (800 × t), the total time of the welding process itself may be long, and the productivity may be reduced.
On the other hand, the lower limit of the energization time of the cooling energization is not particularly specified, but is preferably 20 ms or more. If the time is less than 20 ms, the effect of cooling the nugget end cannot be obtained, and an appropriate temperature range may not be maintained by the subsequent energization.
　繰返通電工程では、上記の冷却通電と再後通電とをｎ回繰り返す。複数回行う場合、各回の条件は、上記範囲内であれば同じでも異なってもよい。ただし、後通電工程を繰り返し行うことにより溶接工程自体が長時間化し、抵抗スポット溶接の施工能率の低下を招く。このため、後通電工程の繰り返し数ｎは３以下が好ましい。 In the repetitive energizing step, the cooling energization and the re-energization are repeated n times. When performing a plurality of times, the conditions of each time may be the same or different as long as they are within the above range. However, by repeatedly performing the post-energization process, the welding process itself is lengthened, and the efficiency of resistance spot welding is reduced. Therefore, the number of repetitions n of the post-energization step is preferably 3 or less.
　以上では、２枚の鋼板を溶接する場合について主に述べたが、３枚以上の鋼板を溶接する場合についても、同様に本発明の製造方法を適用可能である。 In the above, the case where two steel plates are welded has been mainly described, but the manufacturing method of the present invention can be similarly applied to the case where three or more steel plates are welded.
　なお、電極が常に水冷されている状態で抵抗スポット溶接を行う。 抵抗 Perform resistance spot welding while the electrode is always water-cooled.
　以下、実施例を用いて本発明を説明する。なお、本発明は以下の実施例に限定されない。
　２枚板組と３枚板組の各条件において、抵抗スポット溶接を行った。抵抗スポット溶接は常温で行い、電極を常に水冷した状態で行った。下電極４と上電極５は、いずれも先端の直径（先端径）６ｍｍ、曲率半径４０ｍｍとし、クロム銅製のＤＲ形電極とした。また、下電極４と上電極５をサーボモータで駆動することによって加圧力を制御し、通電の際には周波数５０Ｈｚの単相交流を供給した。２枚板組の場合には下鋼板１および上鋼板２とし、３枚板組の場合には下鋼板９、中鋼板１０、上鋼板１１とした。板組としては、下記の（Ａ）、（Ｂ）、（Ｃ）とした。
（Ａ）：下鋼板１と上鋼板２として、引張強度１４７０ＭＰａ、（１）式で表されるＸが０．２４、（２）式で表されるＹが０．０２０となるめっき処理有り（溶融亜鉛めっき（ＧＩ）、付着量は片面当たり５０ｇ／ｍ２））の板厚１．４ｍｍの鋼板を用いた。
（Ｂ）：下鋼板１として、引張強度１４７０ＭＰａ、（１）式で表されるＸが０．２４、（２）式で表されるＹが０．０２０となるめっき処理有り（溶融亜鉛めっき（ＧＩ）、付着量は片面当たり５０ｇ／ｍ２））の板厚１．６ｍｍの鋼板、上鋼板２として、引張強度１１８０ＭＰａ、（１）式で表されるＸが０．２０、（２）式で表されるＹが０．０２４となるめっき処理無しの板厚２．０ｍｍの鋼板を用いた。
（Ｃ）：下鋼板９および中鋼板１０として、引張強度１４７０ＭＰａ、（１）式で表されるＸが０．２２、（２）式で表されるＹが０．０２４となるめっき処理無しの板厚１．４ｍｍの鋼板、上鋼板１１として、引張強度２７０ＭＰａ、（１）式で表されるＸが０．０２７、（２）式で表されるＹが０．０４４となるめっき処理有り（溶融亜鉛めっき（ＧＡ）、付着量は片面当たり４５ｇ／ｍ２）の板厚０．７ｍｍの鋼板を用いた。
　（Ａ）の場合、Ｘｍａｘ＝０．２４、Ｙｍｉｎ＝０．０２０であり、（Ｂ）の場合、Ｘｍａｘ＝０．２４、Ｙｍｉｎ＝０．０２０、（Ｃ）の場合、Ｘｍａｘ＝０．２２、Ｙｍｉｎ＝０．０２４である。引張強度は、各鋼板から、圧延方向に対して平行方向にＪＩＳ５号引張試験片を作製し、ＪＩＳ　Ｚ　２２４１：２０１１の規定に準拠して引張試験を実施して求めた引張強度である。 Hereinafter, the present invention will be described with reference to examples. Note that the present invention is not limited to the following embodiments.
Resistance spot welding was performed under each condition of the two-plate set and the three-plate set. Resistance spot welding was performed at room temperature, and the electrode was constantly cooled with water. Each of the lower electrode 4 and the upper electrode 5 had a tip diameter (tip diameter) of 6 mm and a radius of curvature of 40 mm, and was a chrome copper DR type electrode. The pressing force was controlled by driving the lower electrode 4 and the upper electrode 5 with a servomotor, and a single-phase alternating current having a frequency of 50 Hz was supplied during energization. In the case of a two-plate set, the lower steel plate 1 and the upper steel plate 2 were used, and in the case of a three-plate set, a lower steel plate 9, a middle steel plate 10, and an upper steel plate 11 were used. The plate sets were (A), (B), and (C) below.
(A): As the lower steel plate 1 and the upper steel plate 2, there is a plating treatment in which the tensile strength is 1470 MPa, X represented by the formula (1) is 0.24, and Y represented by the formula (2) is 0.020 ( A hot-dip galvanized (GI) steel sheet having a thickness of 1.4 mm with a coating weight of 50 g / m 2 per side) was used.
(B): The lower steel sheet 1 has a plating treatment in which the tensile strength is 1470 MPa, X represented by the formula (1) is 0.24, and Y represented by the formula (2) is 0.020 (hot-dip galvanizing ( GI), the amount of adhesion is 50 g / m 2 per side), a 1.6 mm-thick steel plate and an upper steel plate 2 having a tensile strength of 1180 MPa, X represented by the formula (1) is 0.20, and the formula (2). A 2.0 mm-thick steel sheet without plating treatment, in which Y represented by 0.024, is 0.024.
(C): As the lower steel plate 9 and the middle steel plate 10, a tensile strength of 1470 MPa, X represented by the formula (1) is 0.22, and Y represented by the formula (2) is 0.024 without plating treatment. As a steel plate having a thickness of 1.4 mm and the upper steel plate 11, there is a plating treatment in which the tensile strength is 270 MPa, X represented by the formula (1) is 0.027, and Y represented by the formula (2) is 0.044 ( A hot-dip galvanized (GA) steel sheet having a thickness of 0.7 mm and a coating amount of 45 g / m 2 per side was used.
In the case of (A), Xmax = 0.24, Ymin = 0.020, in the case of (B), Xmax = 0.24, Ymin = 0.020, and in the case of (C), Xmax = 0.22 and Y min = 0.024. The tensile strength is a tensile strength determined by preparing a JIS No. 5 tensile test piece from each steel sheet in a direction parallel to the rolling direction and performing a tensile test in accordance with JIS Z 2241: 2011.
　抵抗スポット溶接は、２枚板組の場合は、図３（ａ）および図３（ｂ）に示すように、上記鋼板（長手方向の長さ：１５０ｍｍ、短手方向の長さ：５０ｍｍ）の２枚の間に、厚さ１．６ｍｍで５０ｍｍ四方のスペーサ６を両側に挟み込んで仮溶接し、２枚の鋼板を重ね合わせた板組の中心を上記および表１－１に記載する条件で溶接し、溶接継手を作製した。図３（ａ）～図３（ｃ）は、抵抗スポット溶接の試験片を示す平面図（図３（ａ））および側面図（図３（ｂ）、図３（ｃ））であり、図３（ａ）～図３（ｃ）中の符号７は溶接点、符号８は仮溶接点である。また、３枚板組の場合は、図３（ｃ）に示すように、下鋼板９、中鋼板１０、上鋼板１１を重ね合わせ、下鋼板９と中鋼板１０の間にスペーサ６を両側に挟み込んで溶接を行った。なお、鋼板やスペーサの寸法は上記２枚板組の場合と同様である。 In the case of a two-plate set, as shown in FIGS. 3 (a) and 3 (b), resistance spot welding is performed on the steel plate (length in the longitudinal direction: 150 mm, length in the short direction: 50 mm). Between the two sheets, a spacer 6 of 1.6 mm thickness and 50 mm square is sandwiched on both sides, and temporarily welded, and the center of the sheet set in which two steel sheets are overlapped is set under the conditions described above and in Table 1-1. Welding was performed to produce a welded joint. FIGS. 3A to 3C are a plan view (FIG. 3A) and side views (FIGS. 3B and 3C) showing a test piece for resistance spot welding. Reference numeral 7 in FIGS. 3A to 3C denotes a welding point, and reference numeral 8 denotes a temporary welding point. In the case of a three-plate assembly, as shown in FIG. 3C, the lower steel plate 9, the middle steel plate 10, and the upper steel plate 11 are overlapped, and the spacers 6 are provided on both sides between the lower steel plate 9 and the middle steel plate 10. Welding was carried out. The dimensions of the steel plate and the spacer are the same as those of the two-plate set.
　得られた溶接継手を常温（２０℃）で大気中に静置して、２４時間経過した後に、遅れ破壊の有無を調査した。結果を表１－２に示す。遅れ破壊については、２４時間静置後に遅れ破壊が発生しなかったものに記号「○」を、破壊が発生したものに記号「×」を記載した。遅れ破壊の判定については、溶接後にナゲットの剥離（接合界面でナゲットが二つに剥離する現象）が目視で観察されたものを、遅れ破壊が発生したものとした。 し て The obtained welded joint was allowed to stand in the atmosphere at room temperature (20 ° C.), and after 24 hours, the presence or absence of delayed fracture was examined. The results are shown in Table 1-2. Regarding the delayed fracture, the symbol “」 ”was written for the case where the delayed fracture did not occur after standing for 24 hours, and the symbol“ x ”was described for the case where the fracture occurred. Regarding the judgment of delayed fracture, the one in which peeling of the nugget (a phenomenon in which the nugget peels off at the joint interface) after welding was visually observed was regarded as that in which delayed fracture occurred.
　また、遅れ破壊試験とは別に同条件・同形状で溶接継手を作製した。得られた溶接継手に対して、ナゲット中央にて切断し、ピクリン酸にてエッチングして溶融部の境界線を明瞭にした後に、荷重２００ｇｆ、荷重保持時間１５ｓでナゲット端部および最軟化部のビッカース硬さを測定した。ナゲット端部の硬さは、溶融部の境界線から５０μｍナゲット内側部分の硬さ測定値とした。最軟化部の硬さは、ナゲット端部からナゲット外側に１５０μｍ間隔で、ナゲット端部から３ｍｍの長さの領域を測定した場合における、最軟化部分の測定値とした。 別 に In addition to the delayed fracture test, welded joints were manufactured under the same conditions and the same shape. The obtained welded joint was cut at the center of the nugget, etched with picric acid to clarify the boundary of the melted portion, and then subjected to a load of 200 gf and a load holding time of 15 s at the end of the nugget and the softened portion. Vickers hardness was measured. The hardness of the end of the nugget was a measured value of the hardness of the inside of the nugget 50 μm from the boundary of the fusion zone. The hardness of the softest part was the measured value of the softest part in a case where a region having a length of 3 mm from the end of the nugget was measured at intervals of 150 μm from the end of the nugget to the outside of the nugget.
　また、遅れ破壊試験とは別に、同条件で作製した溶接継手で十字引張試験を実施した。十字引張試験については、ＪＩＳＺ　３１３７に基づいた形状の試験体で実施した。後通電を行わず、主通電工程のみで溶接した場合の継手強度と比較して、１０％以上強度低下したものには記号「×」を、１０％以上強度低下しなかったものには記号「○」を記載した。 In addition to the delayed fracture test, a cross tension test was performed on welded joints manufactured under the same conditions. The cross tension test was performed on a test specimen having a shape based on JISZ # 3137. The symbol "X" indicates that the joint strength was reduced by 10% or more compared to the joint strength when welding was performed only in the main energizing step without post-energization, and the symbol "X" indicates that the strength was not reduced by 10% or more. ○ ”is described.
　各評価で得られた結果を表１－２に示す。表１－２に示す「判定」の欄には、「遅れ破壊」および「継手強度」の両方が「〇」であったものに記号「〇」を記載し、「遅れ破壊」および「継手強度」の少なくとも１つが「×」であったものに記号「×」を記載した。表１－２から明らかなように、発明例では全て継手強度が低下せず、耐遅れ破壊特性に優れた溶接継手が得られたのに対して、比較例では良好な溶接継手が得られなかった。 結果 The results obtained in each evaluation are shown in Table 1-2. In the column of “Judgment” shown in Table 1-2, when both “delay fracture” and “joint strength” are “〇”, the symbol “〇” is described, and “delay fracture” and “joint strength” are described. The symbol "x" was described when at least one of "" was "x". As is clear from Table 1-2, in all of the inventive examples, the joint strength did not decrease and a welded joint excellent in delayed fracture resistance was obtained, whereas in the comparative example, a good welded joint was not obtained. Was.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　１　　　　　下鋼板
　２　　　　　上鋼板
　３　　　　　ナゲット
　４　　　　　下電極
　５　　　　　上電極
　６　　　　　スペーサ
　７　　　　　溶接点
　８　　　　　仮溶接点
　９　　　　　下鋼板
　１０　　　　中鋼板
　１１　　　　上鋼板
　１２　　　　スポット溶接部
　１３　　　　ナゲット
　１４　　　　溶接熱影響部
　１５　　　　鋼板
　１６　　　　ナゲット端部 1 Lower steel plate 2 Upper steel plate 3 Nugget 4 Lower electrode 5 Upper electrode 6 Spacer 7 Welding point 8 Temporary welding point 9 Lower steel plate 10 Middle steel plate 11 Upper steel plate 12 Spot welded part 13 Nugget 14 Welding heat affected zone 15 Steel plate 16 Nugget end

Claims (3)

  1. 　２以上の鋼板と、前記鋼板間に形成されたスポット溶接部と、を備える抵抗スポット溶接部材であって、
    　前記２以上の鋼板の少なくとも１つの鋼板の引張強度が９８０ＭＰａ以上であり、
    　前記２以上の鋼板において、下記（１）式で表される係数Ｘが最も大きくなる鋼板のＸをＸｍａｘとし、下記（２）式で表される係数Ｙが最も小さくなる鋼板のＹをＹｍｉｎとした場合に、前記スポット溶接部のナゲットの端部のビッカース硬さＨｎ（Ｈｖ）が下記（３）式で表されるＨｏｂ（Ｈｖ）以下であり、
    　前記スポット溶接部の溶接熱影響部の最軟化部のビッカース硬さＨｍｉｎ（Ｈｖ）が下記（４）式を満足する、抵抗スポット溶接部材。
    Ｘ＝［Ｃ］＋［Ｓｉ］／４０＋［Ｍｎ］／２００　　（１）
    Ｙ＝［Ｐ］＋３×［Ｓ］　　　（２）
    Ｈｏｂ＝（８００× Ｘｍａｘ＋３００）／（０．７＋２０ × Ｙｍｉｎ）　　　（３）
    ０．４ × Ｈｎ≦Ｈｍｉｎ≦０．９ × Ｈｎ　　（４）
    前記（１）式及び（２）式において［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ｐ］及び［Ｓ］は各元素の含有量（質量％）である。ただし、含まない場合は０とする。     A resistance spot welded member comprising two or more steel plates and a spot weld formed between the steel plates,
    The tensile strength of at least one steel plate of the two or more steel plates is 980 MPa or more,
    Wherein at least two steel plates, the X of the steel sheet which the coefficient X represented by the following formula (1) is maximized and X max, the Y coefficient Y is smallest steel sheet represented by the following formula (2) Y min , the Vickers hardness H n (Hv) at the end of the nugget of the spot welded portion is equal to or less than H ob (Hv) represented by the following equation (3);
    A resistance spot welded member in which the Vickers hardness H min (Hv) of the softest part of the heat affected zone of the spot welded part satisfies the following expression (4).
    X = [C] + [Si] / 40 + [Mn] / 200 (1)
    Y = [P] + 3 × [S] (2)
    H ob = (800 × X max +300) / (0.7 + 20 × Y min ) (3)
    0.4 × H n ≦ H min ≦ 0.9 × H n (4)
    In the formulas (1) and (2), [C], [Si], [Mn], [P] and [S] are the contents (% by mass) of the respective elements. However, it is set to 0 when not included.
  2. 　抵抗スポット溶接部材の製造方法であって、
    　引張強度が９８０ＭＰａ以上の鋼板を少なくとも１枚含む２枚以上の鋼板を重ね合わせて１対の溶接電極で挟持し加圧しながら通電してナゲットを形成する主通電工程と、
    　前記主通電工程の後に下記の（５）式、（６）式で表される冷却時間Ｃｔ（ｍｓ）の間鋼板を前記溶接電極で加圧保持してナゲットを冷却する冷却工程と、
    　前記冷却工程後に、下記（７）式を満足する電流値Ｉｐ（ｋＡ）で通電する後通電工程と、を有する、抵抗スポット溶接部材の製造方法。
    Ｃｔ≧１６０ × ｔ２　（ｔ≦１．６）　　　（５）
    Ｃｔ≧２５６× ｔ　（ｔ＞１．６）　　　（６）
    ０．８ × Ｉｍｉｎ≦Ｉｐ＜１．５ × Ｉｍａｘ　　　　（７）
    前記（５）式、（６）式、（７）式において、
    ｔ：被接合鋼板の平均板厚（ｍｍ）
    Ｉｍａｘ：主通電工程における最大電流値（ｋＡ）
    Ｉｍｉｎ：主通電工程における最小電流値（ｋＡ）
    である。     A method for manufacturing a resistance spot welding member,
    A main energization step in which two or more steel sheets including at least one steel sheet having a tensile strength of 980 MPa or more are stacked, sandwiched by a pair of welding electrodes, and energized while applying pressure to form a nugget;
    A cooling step of cooling the nugget by holding the steel plate under pressure with the welding electrode for a cooling time C t (ms) represented by the following equations (5) and (6) after the main energization step;
    A method for manufacturing a resistance spot welded member, comprising: after the cooling step, an energizing step of energizing at a current value I p (kA) satisfying the following expression (7).
    C t ≧ 160 × t 2 (t ≦ 1.6) (5)
    C t ≧ 256 × t (t> 1.6) (6)
    0.8 × I min ≦ I p <1.5 × I max (7)
    In the above equations (5), (6) and (7),
    t: Average thickness of the steel plate to be joined (mm)
    I max : maximum current value (kA) in the main energization process
    I min : minimum current value (kA) in the main energization process
    It is.
  3. 　前記後通電工程の後に、冷却し下記（８）式を満たす条件で通電する冷却通電と、前記冷却通電後に下記（９）式を満たす条件で再後通電する再後通電とをｎ回行う繰返通電工程を有する、請求項２に記載の抵抗スポット溶接部材の製造方法。
    ０≦Ｉｎｃ≦Ｉｍａｘ　　　　（８）
    ０．８ × Ｉｍｉｎ≦Ｉｎｒ＜１．５ × Ｉｍａｘ　　　　（９）
    前記（８）式、（９）式において、
    Ｉｎｃ：ｎ回目の冷却通電における電流値（ｋＡ）
    Ｉｎｒ：ｎ回目の再後通電における電流値（ｋＡ）
    Ｉｍａｘ：主通電工程における最大電流値（ｋＡ）
    Ｉｍｉｎ：主通電工程における最小電流値（ｋＡ）
    ｎ：１以上の自然数
    である。     After the post-energization step, cooling energization for cooling and energizing under the condition satisfying the following formula (8) and re-energizing for re-energizing after the cooling energizing under the condition satisfying the following formula (9) are repeated n times. The method for producing a resistance spot welded member according to claim 2, further comprising a step of returning electricity.
    0 ≦ I nc ≦ I max (8)
    0.8 × I min ≦ I nr <1.5 × I max (9)
    In the equations (8) and (9),
    I nc : Current value (kA) at n-th cooling energization
    Inr : current value (kA) in the nth re-energization
    I max : maximum current value (kA) in the main energization process
    I min : minimum current value (kA) in the main energization process
    n is a natural number of 1 or more.
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