JP6108018B2

JP6108018B2 - Spot welding method

Info

Publication number: JP6108018B2
Application number: JP2016172227A
Authority: JP
Inventors: 千智吉永; 康信宮▲崎▼; 邦夫林
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2015-09-03
Filing date: 2016-09-02
Publication date: 2017-04-05
Anticipated expiration: 2036-09-02
Also published as: JP2017047476A

Description

本発明は、亜鉛系めっき鋼板を含む複数枚の鋼板のスポット溶接方法に関する。 The present invention relates to a spot welding method for a plurality of steel sheets including galvanized steel sheets.

近年、自動車分野では、低燃費化やＣＯ_２排出量の削減のため、車体を軽量化することや、衝突安全性の向上のため、車体部材を高強度化することが求められている。これらの要求を満たすためには、車体部材や各種部品などに高強度鋼板を使用することが有効である。 In recent years, in the automobile field, it has been required to reduce the weight of a vehicle body in order to reduce fuel consumption and reduce CO ₂ emissions, and to increase the strength of vehicle body members in order to improve collision safety. In order to satisfy these requirements, it is effective to use high-strength steel sheets for vehicle body members and various parts.

このような高強度鋼板よりなる車体の組立や部品の取付けなどの工程では、主として、抵抗スポット溶接の使用が広く普及している。スポット溶接して得られた溶接継手の品質指標の一つである継手強度には、せん断方向に引張荷重を負荷して測定する引張せん断強さ（ＴＳＳ）と、剥離方向に引張荷重を負荷して測定する十字引張強さ（ＣＴＳ）がある。 In the process of assembling a vehicle body made of such a high-strength steel plate and attaching parts, the use of resistance spot welding is mainly prevalent. The joint strength, one of the quality indicators of welded joints obtained by spot welding, includes tensile shear strength (TSS) measured by applying a tensile load in the shear direction and tensile load in the peeling direction. There is a cross tensile strength (CTS) to be measured.

高強度鋼板を含むスポット溶接継手におけるＣＴＳは、鋼板の引張強度が増加しても、増加しないか、又は、減少することがある。この理由は、変形能の低下により溶接部への応力集中が高まることや、溶接部に焼きが入ることにより溶接部の靱性が低下することとされている。このため、高強度鋼板にスポット溶接を行った場合に、ＣＴＳ等の継手強度を向上させる技術が望まれていた。 CTS in spot welded joints containing high-strength steel sheets may not increase or decrease even if the tensile strength of the steel sheet increases. The reason for this is that stress concentration on the welded portion is increased due to a decrease in deformability, and that the toughness of the welded portion is reduced due to quenching in the welded portion. For this reason, when spot welding is performed on a high-strength steel sheet, a technique for improving joint strength such as CTS has been desired.

このような要望のもと、高強度鋼板を含むスポット溶接継手における強度と靭性を確保するため、本溶接の前に通電する前通電工程、又は、本溶接の後に通電する後通電工程を実施する技術が知られている（例えば、特許文献１、参照）。 Under such demand, in order to ensure the strength and toughness of spot welded joints including high-strength steel sheets, a pre-energization step for energizing before main welding or a post-energization step for energizing after main welding is performed. A technique is known (for example, refer to Patent Document 1).

一方、車体の高防錆化の観点から、部材を耐食性に優れた鋼板で構成する必要がある。亜鉛系めっき鋼板は、耐食性が良好であることが幅広く知られており、上記の軽量化や高強度化の観点から、自動車用として用いられる亜鉛系めっき鋼板においては、めっき原板に高強度鋼板を用いた亜鉛系めっき高強度鋼板が使用されている。 On the other hand, from the viewpoint of increasing the rust prevention of the vehicle body, it is necessary to configure the member with a steel plate having excellent corrosion resistance. Zinc-based galvanized steel sheets are widely known to have good corrosion resistance. From the viewpoints of lightening and increasing the strength described above, zinc-based galvanized steel sheets used for automobiles are made of high-strength steel sheets. The used zinc-based plated high-strength steel sheet is used.

このような亜鉛系めっき高強度鋼板にスポット溶接を行うと、電極と接する鋼板の外側面から、ナゲットに向かって、割れが発生するという問題がある。図１に、亜鉛系めっき鋼板にスポット溶接を行った場合の溶接箇所の割れの概略を示す。亜鉛系めっき鋼板１にスポット溶接を行うと、電極と接する鋼板１の外側面からナゲット２に向かって進展する割れ３や、電極肩部から熱影響部４に進展する割れ５が発生することが知られている。 When spot welding is performed on such a zinc-based plated high-strength steel plate, there is a problem that cracks are generated from the outer surface of the steel plate in contact with the electrode toward the nugget. In FIG. 1, the outline of the crack of the welding location at the time of performing spot welding to a zinc-based plated steel plate is shown. When spot welding is performed on the galvanized steel sheet 1, a crack 3 that progresses from the outer surface of the steel sheet 1 in contact with the electrode toward the nugget 2 and a crack 5 that progresses from the electrode shoulder to the heat affected zone 4 may occur. Are known.

この割れは、電極の加圧力や鋼板の熱膨張、収縮による引張応力が溶接箇所に加わり、該溶接箇所の鋼板表面で溶融した亜鉛や、亜鉛と電極の銅との合金が、鋼板の結晶粒界に侵入して粒界強度を低下させて引き起こされる、いわゆる液体金属脆性に起因する割れであるといわれている。自動車車体では、溶接箇所の割れが著しいと強度が低下するという問題があり、鋼板の成分組成や組織を制御することにより、溶接箇所の割れを抑制する技術が知られている。 This crack is caused by the applied pressure of the electrode, the thermal expansion and contraction of the steel sheet, and the tensile stress due to the shrinkage applied to the welded part. It is said to be a crack caused by so-called liquid metal embrittlement caused by entering the boundary and lowering the grain boundary strength. In automobile bodies, there is a problem that the strength decreases when cracks in the welded portion are significant, and a technique for suppressing cracks in the welded portion by controlling the composition and structure of the steel sheet is known.

たとえば、特許文献２には、鋼板の成分組成を調整し、スポット溶接時に生成されるオーステナイト相を微細な結晶粒にして、他の相の結晶粒と複雑に入り組んだ金属組織を有するものとすることで、結晶粒界への溶融亜鉛の拡散浸入経路を複雑にして、溶融亜鉛を侵入し難くして、スポット溶接時の液体金属脆化割れを抑制する技術が開示されている。 For example, in Patent Document 2, the composition of the steel sheet is adjusted, the austenite phase generated at the time of spot welding is made into fine crystal grains, and it has a metal structure that is complicated with the crystal grains of other phases. Thus, a technique has been disclosed in which the diffusion and penetration path of molten zinc into the crystal grain boundary is complicated to make it difficult for the molten zinc to enter and suppress liquid metal embrittlement cracking during spot welding.

また、特許文献３には、鋼板の組織制御によって結晶粒界を複雑化するだけでは、溶接部の割れ発生を充分に抑制できないことがあるとして、鋼板の成分組成を調整し、熱間圧延鋼板の粒界酸化深さを５μｍ以下とし、合金化溶融亜鉛めっき処理前の冷間圧延鋼板にＦｅ系電気めっき処理を行うことによって、合金化溶融亜鉛めっき鋼板の粒界侵食深さを５μｍ以下にすることで、合金化溶融亜鉛めっき鋼板の溶接箇所における割れの発生を抑制する技術が開示されている。 Further, in Patent Document 3, the composition of the steel sheet is adjusted and the hot-rolled steel sheet is adjusted, assuming that the occurrence of cracks in the welded part may not be sufficiently suppressed only by making the grain boundaries complicated by controlling the structure of the steel sheet. The grain boundary oxidization depth of the alloyed hot-dip galvanized steel sheet is reduced to 5 μm or less by performing Fe-based electroplating on the cold-rolled steel sheet before the alloying hot-dip galvanizing process. Thus, a technique for suppressing the occurrence of cracks at the welded portion of the galvannealed steel sheet is disclosed.

また、特許文献４には、亜鉛などのめっきを施した鋼板から電縫鋼管を製造する際に、液体金属脆化抑制のために、ストリップ端部の突合せ部のめっきを除去する技術が開示されている。 Patent Document 4 discloses a technique for removing plating at a butt portion of a strip end portion in order to suppress embrittlement of liquid metal when an ERW steel pipe is manufactured from a steel plate plated with zinc or the like. ing.

国際公開第２０１４／１９６４９９号International Publication No. 2014/196499 特開２００６−２６５６７１号公報JP 2006-265671 A 特開２００８−２３１４９３号公報JP 2008-231493 A 特開平０５−２７７５５２号公報Japanese Patent Laid-Open No. 05-277552

このようにスポット溶接箇所の割れの対策は検討されているものの、一部の亜鉛系めっき鋼板のスポット溶接継手において、又は、一部の非めっき鋼板と亜鉛系めっき鋼板のスポット溶接継手において、依然として、継手における所望の接合強度が得られないことがあった。
本発明は、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制し、継手強度を確保することが可能なスポット溶接方法を提供することを課題とする。 Although measures for cracking of spot welds have been studied in this way, in some spot welded joints of galvanized steel sheets, or in spot welded joints of some non-plated steel sheets and galvanized steel sheets, The desired joint strength in the joint may not be obtained.
It is an object of the present invention to provide a spot welding method capable of suppressing cracking at the corona bond directly outside and at the corona bond nugget and ensuring the joint strength.

（１）本発明の一態様に係るスポット溶接方法は、少なくとも溶接箇所が重ね合わされた複数枚の鋼板で構成される被溶接部材を溶接電極により加圧して通電する本溶接を行い、更に、前通電及び後通電の少なくとも一方の通電工程を行うスポット溶接方法であって、前記複数枚の鋼板の少なくとも一つが、引張強度が７８０ＭＰａ以上である高強度鋼板であり、前記複数枚の鋼板の少なくとも一つについて、少なくとも前記溶接箇所の重ね合わせ面が亜鉛系めっきで被覆され、前記複数枚の鋼板の総板厚ｔ（ｍｍ）が１．３５ｍｍ以上であり、前記溶接電極間の通電開始時から、溶接終了の際の当該溶接電極間の通電終了時まで、当該溶接電極による前記被溶接部材の加圧を保持したままとし、溶接終了の際、前記溶接電極間の通電終了時から当該溶接電極と前記被溶接部材とを非接触とするまでの通電後保持時間Ｈｔ（秒）を下記（４）式の範囲内とする。
０．０１５ｔ^２＋０．１１０≦Ｈｔ≦０．１６ｔ^２−０．４０ｔ＋０．５３・・・（４）
上記の構成からなるスポット溶接方法によれば、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制し、継手強度を確保することができる。 (1) In the spot welding method according to one aspect of the present invention, a main welding is performed in which a member to be welded composed of a plurality of steel plates on which at least welded portions are overlapped is pressurized with a welding electrode and energized. A spot welding method in which at least one energization step of energization and post-energization is performed, wherein at least one of the plurality of steel plates is a high strength steel plate having a tensile strength of 780 MPa or more, and at least one of the plurality of steel plates. For one, at least the overlapping surface of the welded portion is coated with zinc-based plating, the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more, from the start of energization between the welding electrodes, Until the end of energization between the welding electrodes at the end of welding, the pressurization of the member to be welded by the welding electrode is kept, and at the end of energization between the welding electrodes at the end of welding Luo the welding electrode and the energizing After the hold time to a non-contact and a workpiece to be welded Ht (seconds) in the range of the following equation (4).
0.015t ² + 0.110 ≦ Ht ≦ 0.16t ² −0.40t + 0.53 ( 4 )
According to the spot welding method having the above-described configuration, it is possible to suppress the cracking of the corona bond directly outside and the corona bond at the time of the nugget and ensure the joint strength.

（２）本発明の他の態様によれば、上記（１）のスポット溶接方法において、前記複数枚の鋼板の総板厚ｔ（ｍｍ）が２．４ｍｍ以上３．２ｍｍ以下である。
上記態様では、割れの抑制効果と継手強度の確保に加え、被溶接部材の軽量化、高強度化が可能となる。 ( 2 ) According to another aspect of the present invention, in the spot welding method of (1) , a total thickness t (mm) of the plurality of steel plates is 2.4 mm or more and 3.2 mm or less.
In the above aspect, in addition to ensuring the effect of suppressing cracking and securing the joint strength, the welded member can be reduced in weight and strength.

（３）本発明の他の態様によれば、上記（１）又は（２）のスポット溶接方法において、前記溶接電極を前記被溶接部材に接触させる直前に、下記（ａ）〜（ｄ）の条件のうち、一つまたは二つ以上を満たす。
（ａ）前記溶接電極の軸芯と、前記溶接電極と接触する鋼板表面の垂線とが平行でない状態、
（ｂ）一方の前記溶接電極の先端部から該先端部に最も近い鋼板表面までの距離と、他方の前記溶接電極の先端部から該先端部に最も近い鋼板表面までの距離が異なる状態、
（ｃ）一方の前記溶接電極の軸芯の延長線上に他方の前記溶接電極の軸芯がない状態、及び、
（ｄ）前記溶接箇所の重ね合わせ面の間に隙間を有した状態。
上記態様に規定される外乱因子がある場合に、コロナボンド直外及びコロナボンドのナゲット際の割れが顕著になるが、上記（１）又は（２）のスポット溶接方法によれば、このような割れを抑制し、かつ継手強度を確保することができる。 ( 3 ) According to another aspect of the present invention, in the spot welding method of the above (1) or (2) , immediately before the welding electrode is brought into contact with the welded member, the following (a) to (d): Satisfy one or more of the conditions.
(A) a state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet in contact with the welding electrode are not parallel;
(B) a state in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip;
(C) a state in which there is no axial center of the other welding electrode on an extension line of the axial center of the one welding electrode; and
(D) A state in which there is a gap between the overlapping surfaces of the welded portions.
When there is a disturbance factor stipulated in the above aspect, cracks at the corona bond directly outside and corona bond nugget become prominent, but according to the spot welding method of (1) or (2) above, Cracks can be suppressed and joint strength can be ensured.

（４）本発明の他の態様によれば、上記（１）から（３）のいずれかのスポット溶接方法において、前記後通電を行わない場合、予め、前記被溶接部材に、前記本溶接における前記溶接電極間の通電終了後、直ちに、当該溶接電極と前記被溶接部材とを非接触とするスポット溶接を行い、鋼板の重ね合わせ面のコロナボンド直外の割れ及びコロナボンドのナゲット際の割れの発生の有無を確認し、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れの少なくとも一方の割れが発生したとき、前記通電後保持時間の範囲内として溶接する。
また、上記（１）から（３）のいずれかのスポット溶接方法において、前記後通電を行う場合、予め、前記被溶接部材に、前記後通電における前記溶接電極間の通電終了後、直ちに、当該溶接電極と前記被溶接部材とを非接触とするスポット溶接を行い、鋼板の重ね合わせ面のコロナボンド直外の割れ及びコロナボンドのナゲット際の割れの発生の有無を確認し、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れの少なくとも一方の割れが発生したとき、前記通電後保持時間の範囲内として溶接する。
上記態様では、割れが発生した溶接箇所に対して、上記（１）から（３）のいずれかのスポット溶接方法を適用するため、効率的である。
( 4 ) According to another aspect of the present invention, in the spot welding method according to any one of (1) to ( 3 ), when the post-energization is not performed, the member to be welded is subjected to the main welding in advance. Immediately after the energization between the welding electrodes is completed, spot welding is performed so that the welding electrode and the member to be welded are not in contact with each other, cracks immediately outside the corona bond on the overlapped surface of the steel sheet and cracks in the corona bond nugget When at least one of the cracks immediately outside the corona bond and the crack at the corona bond nugget occurs, welding is performed within the range of the holding time after the energization.
Further, in the spot welding method according to any one of (1) to ( 3 ), when the post-energization is performed, immediately after the end of energization between the welding electrodes in the post-energization, the welded member is immediately Spot welding is performed in which the welding electrode and the member to be welded are not in contact with each other, and the presence or absence of cracks immediately outside the corona bond on the overlapped surface of the steel sheet and the crack at the time of corona bond nugget is confirmed. When at least one of the cracks and the cracks in the corona bond nugget occurs, welding is performed within the range of the holding time after energization.
In the said aspect, since the spot welding method in any one of said (1) to ( 3 ) is applied with respect to the welding location which the crack generate | occur | produced, it is efficient.

本発明によれば、亜鉛系めっき鋼板を含む板組のスポット溶接において、外乱因子が存在する場合であっても、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制し、継手強度を確保することができる。 According to the present invention, in spot welding of a plate assembly including a galvanized steel sheet, even when a disturbance factor is present, cracks at the corona bond directly outside and at the corona bond nugget are suppressed, and the joint strength is ensured. can do.

亜鉛系めっき鋼板にスポット溶接を行った場合の溶接箇所の割れの概略を示す図である。It is a figure which shows the outline of the crack of the welding location at the time of performing spot welding to a zinc-based plated steel plate. 亜鉛系めっき鋼板にスポット溶接を行った溶接箇所の板厚方向の断面におけるコロナボンド直外又はコロナボンドのナゲット際の割れを示す概略図である。It is the schematic which shows the crack at the time of the corona bond right outside or corona bond nugget in the cross section of the plate | board thickness direction of the welding location which spot-welded the zinc-based plated steel plate. コロナボンド直外又はコロナボンドのナゲット際の割れと継手強度における、総板厚ｔに対する通電後保持時間Ｈｔの関係を示す図である。It is a figure which shows the relationship of the holding time Ht after electricity supply with respect to the total board thickness t in the crack in the nugget of corona bond right outside or a corona bond, and joint strength. 溶接電極の軸芯と、該溶接電極と接触する鋼板表面の垂線とが平行でない状態を説明するための概略図である。It is the schematic for demonstrating the state where the axial center of a welding electrode and the perpendicular of the steel plate surface which contacts this welding electrode are not parallel. 対向する溶接電極の各先端部から各鋼板表面までの各距離が異なる状態を説明するための概略図である。It is the schematic for demonstrating the state from which each distance from each front-end | tip part of the welding electrode which opposes to each steel plate surface differs. 一方の前記溶接電極の軸芯の延長線上に他方の前記溶接電極の軸芯がない状態を説明するための概略図である。It is the schematic for demonstrating the state which has no axial center of the other said welding electrode on the extension line of the axial center of one said welding electrode. 溶接箇所の重ね合わせ面の間に隙間を有した状態を説明するための概略図である。It is the schematic for demonstrating the state which had the clearance gap between the overlapping surfaces of a welding location. 後通電を行わない場合の溶接電極への通電パターンの例を示す図である。It is a figure which shows the example of the electricity supply pattern to the welding electrode in the case of not performing post-energization. 後通電を行う場合の溶接電極への通電パターンの例を示す図である。It is a figure which shows the example of the electricity supply pattern to the welding electrode in the case of performing post-energization. 溶接電極の軸芯と、該溶接電極と接触する鋼板表面の垂線とが平行でない状態でスポット溶接を行った溶接箇所の断面写真である。It is a cross-sectional photograph of the welding location which performed spot welding in the state where the axial center of a welding electrode and the perpendicular of the steel plate surface which contacts this welding electrode are not parallel. 対向する溶接電極の各先端部から各鋼板表面までの各距離が異なる状態でスポット溶接を行った溶接箇所の断面写真である。It is a cross-sectional photograph of the welding location which performed spot welding in the state from which each distance from each front-end | tip part of the opposing welding electrode to each steel plate surface differs. 一方の前記溶接電極の軸芯の延長線上に他方の前記溶接電極の軸芯がない状態でスポット溶接を行った溶接箇所の断面写真である。It is a cross-sectional photograph of the welding location which performed spot welding in the state where there is no axial center of the other said welding electrode on the extension line of the axial center of one said welding electrode. 溶接箇所の重ね合わせ面の間に隙間を有した状態でスポット溶接を行った溶接箇所の断面写真である。It is a cross-sectional photograph of the welding location which performed spot welding in the state which had the clearance gap between the overlapping surfaces of a welding location.

本発明の発明者らは、スポット溶接継手において、所望の継手強度が得られない理由について調査した。図２に、亜鉛系めっき鋼板にスポット溶接を行った後の溶接箇所における、ナゲットを含む板厚方向の断面を示す。図２に示すように、所望の引張強度が得られないスポット溶接継手には、鋼板の重ね合わせ面のコロナボンド直外の割れ６が生じていた。また、コロナボンドのナゲット際の割れ７が生じていることもあった。 The inventors of the present invention investigated why a desired joint strength could not be obtained in a spot welded joint. In FIG. 2, the cross section of the plate | board thickness direction containing a nugget in the welding location after performing spot welding to a zinc-based plated steel plate is shown. As shown in FIG. 2, in the spot welded joint where the desired tensile strength cannot be obtained, cracks 6 immediately outside the corona bond on the overlapping surface of the steel plates were generated. Moreover, the crack 7 at the time of the corrugated bond nugget sometimes occurred.

そして、スポット溶接の直前における溶接電極と被溶接部材との位置関係において、次の（ａ）〜（ｄ）のような外乱因子が存在する状態で溶接を行う場合に、鋼板が重ね合わされた内面側で生じるコロナボンド直外及びコロナボンドのナゲット際の割れが発生し易いことがわかった。
（ａ）溶接電極の軸芯と、該溶接電極と接触する鋼板表面の垂線とが平行でない状態
（ｂ）一方の溶接電極の先端部から該先端部に最も近い鋼板表面までの距離と、他方の溶接電極の先端部から該先端部に最も近い鋼板表面までの距離が異なる状態
（ｃ）一方の前記溶接電極の軸芯の延長線上に他方の前記溶接電極の軸芯がない状態
（ｄ）溶接箇所の重ね合わせ面の間に隙間を有した状態 And in the positional relationship between the welding electrode and the member to be welded immediately before spot welding, the inner surface on which the steel plates are overlapped when welding is performed in the presence of disturbance factors such as the following (a) to (d) It was found that cracks occurred immediately outside the corona bond and on the side of the corona bond nugget.
(A) A state in which the axis of the welding electrode is not parallel to the perpendicular to the surface of the steel sheet in contact with the welding electrode (b) The distance from the tip of one welding electrode to the surface of the steel plate closest to the tip, and the other (C) State in which the distance from the tip of the welding electrode to the surface of the steel plate closest to the tip is different (c) State in which the axis of the other welding electrode is not on the extension line of the axis of the welding electrode (d) A state with a gap between the overlapping surfaces of the welds

上記のような外乱因子が存在する場合、溶接電極の加圧保持終了後（電極開放後）において、コロナボンド直外及びコロナボンドのナゲット際に、残留応力が高くなる箇所が生じる。この現象について、本発明者らは、当該箇所の鋼板の結晶粒界に、溶融した亜鉛系めっきが侵入し、粒界強度を低下させ、コロナボンド直外又はコロナボンドのナゲット際の割れを発生させていると考えた。 When the disturbance factors as described above are present, after the pressurization and holding of the welding electrode (after the electrode is opened), there are places where the residual stress becomes high immediately outside the corona bond and when the corona bond is nuggeted. About this phenomenon, the present inventors, the molten zinc-based plating penetrates into the grain boundary of the steel sheet of the relevant location, reduces the grain boundary strength, and generates cracks at the corona bond directly outside or corona bond nugget I thought that I was letting.

従来、溶接電極間の通電終了後、直ちに、鋼板と溶接電極を非接触としていた。しかし、通電終了後に溶接電極の加圧保持を継続し、電極開放前に溶融した亜鉛系めっきを凝固させたところ、コロナボンド直外又はコロナボンドのナゲット際の割れが生じない場合があることを見出した。また、加圧保持を継続すると、継手強度が低下する傾向を見出し、割れを抑制でき、かつ継手強度も落とさない加圧保持の時間を検討したところ、溶接電極の加圧保持の時間を総板厚の関数とすることで、割れを抑制できるとともに十分な継手強度が得られることを知見した。 Conventionally, the steel sheet and the welding electrode are not contacted immediately after the energization between the welding electrodes is completed. However, when the welding electrode is held under pressure after the end of energization and the molten zinc-based plating is solidified before the electrode is opened, cracks may not occur on the corona bond directly outside or on the corona bond nugget. I found it. In addition, when pressure holding was continued, the joint strength was found to decrease, and cracking was suppressed and the pressure holding time during which the joint strength was not reduced was examined. It was found that by using the thickness as a function, cracks can be suppressed and sufficient joint strength can be obtained.

本発明は、亜鉛系めっき鋼板を含む板組のスポット溶接において、種々の外乱因子が存在する場合であっても、鋼板の重ね合わせ面のコロナボンド直外及びコロナボンドのナゲット際に発生する割れを抑制し、高品質のスポット溶接継手を形成できるスポット溶接方法の提供ができる。 In the spot welding of a plate assembly including a galvanized steel sheet, the present invention is a crack that occurs at the time of corona bond directly outside the corona bond and on the corona bond nugget even when various disturbance factors exist. It is possible to provide a spot welding method that suppresses the above and can form a high-quality spot welded joint.

更に、溶接継手の強度と靭性を確保するために、前述の本溶接の前後に更に通電を行う技術を適用し、その際に、コロナボンド直外又はコロナボンドのナゲット際の割れを回避することについて検討したところ、溶接終了の際（後通電を行わない場合は本溶接の終了の際、後通電を行う場合は該後通電の終了の際）の溶接電極の加圧保持の時間を総板厚の関数とすることで、割れを回避できるとともに、十分な継手強度及び靭性の溶接継手が得られることを知見した。 Furthermore, in order to ensure the strength and toughness of the welded joint, apply the technology to further energize before and after the above-mentioned main welding, and at that time, avoid cracks at the corona bond directly outside or at the corona bond nugget. When the welding was completed (when no post-energization was performed, the main welding was completed, and when post-energization was performed, the post-energization was terminated) It was found that by using the thickness as a function, cracks can be avoided and a weld joint with sufficient joint strength and toughness can be obtained.

以下に、本発明の実施形態を説明するが、本発明がこれらの実施形態のみに限られないことは自明である。 Although embodiments of the present invention will be described below, it is obvious that the present invention is not limited to these embodiments.

本実施形態に係るスポット溶接方法は、少なくとも溶接箇所が重ね合わされた複数枚の鋼板で構成される被溶接部材を溶接電極により加圧して通電する本溶接を行い、更に、前通電及び後通電の少なくとも一方の通電工程を行うスポット溶接方法であって、前記複数枚の鋼板の少なくとも一つについて、少なくとも前記溶接箇所の重ね合わせ面が亜鉛系めっきで被覆され、前記複数枚の鋼板の総板厚ｔ（ｍｍ）が１．３５ｍｍ以上であり、前記溶接電極間の通電開始時から、溶接終了の際の当該溶接電極間の通電終了時まで、当該溶接電極による前記被溶接部材の加圧を保持したままとし、溶接終了の際、前記溶接電極間の通電終了時から当該溶接電極と前記被溶接部材とを非接触とするまでの通電後保持時間Ｈｔ（秒）を下記（１）式の範囲内とする。 In the spot welding method according to the present embodiment, main welding is performed in which a welded member composed of a plurality of steel plates on which at least welding locations are overlapped is pressed with a welding electrode and energized, and further, pre-energization and post-energization are performed. A spot welding method for performing at least one energization step, wherein at least one of the plurality of steel plates is coated with a zinc-based plating on at least the overlapping surface of the welding locations, and the total thickness of the plurality of steel plates t (mm) is 1.35 mm or more, and pressurization of the welded member by the welding electrode is maintained from the start of energization between the welding electrodes to the end of energization between the welding electrodes at the end of welding. When the welding is completed, the post-energization holding time Ht (seconds) from the end of energization between the welding electrodes until the welding electrode and the member to be welded are brought into contact with each other is expressed by the following formula (1). To the inner.

０．０１５ｔ^２＋０．０２０≦Ｈｔ≦０．１６ｔ^２−０．４０ｔ＋０．７０・・・（１） 0.015t ² + 0.020 ≦ Ht ≦ 0.16t ² −0.40t + 0.70 (1)

次に、（１）式について説明するが、この式は、実験的に求められた式である。
図３に、コロナボンド直外又はコロナボンドのナゲット際の割れと継手強度における、総板厚ｔに対する通電後保持時間Ｈｔの関係を示す。図３は、本溶接の前後に、通電する工程を実施せずに、亜鉛系めっきが被覆された種々の鋼種の鋼板を２枚用いてスポット溶接する際に、鋼板の総板厚ｔと、溶接電極間の溶接通電終了時から溶接電極と被溶接部材とを非接触とするまでの通電後保持時間Ｈｔとを種々変更して実施したものである。 Next, the expression (1) will be described. This expression is an expression obtained experimentally.
FIG. 3 shows the relationship between the post-energization holding time Ht with respect to the total sheet thickness t in the crack and joint strength immediately outside the corona bond or in the corona bond nugget. FIG. 3 shows the total thickness t of the steel plate when spot welding is performed using two steel plates of various steel types coated with zinc-based plating without carrying out the step of energizing before and after the main welding. The post-energization holding time Ht from when the welding energization between the welding electrodes is completed until the welding electrode and the member to be welded are brought into contact with each other is variously changed.

なお、総板厚１．３５ｍｍ未満の被溶接部材においては、コロナボンド直外又はコロナボンドのナゲット際の割れの発生率が少ないため、総板厚１．３５ｍｍ以上の場合について検討した。 In addition, in the member to be welded having a total thickness of less than 1.35 mm, the occurrence rate of cracks at the outside of the corona bond or at the nugget of the corona bond is small, so the case where the total thickness is 1.35 mm or more was examined.

そして、得られた溶接継手について、コロナボンド直外又はコロナボンドのナゲット際の割れの有無と、剥離方向に引張荷重を負荷して測定する十字引張強さ（ＣＴＳ）を確認した。コロナボンド直外又はコロナボンドのナゲット際の割れは、ナゲットを含む板厚方向の断面を観察して確認し、十字引張強さ（ＣＴＳ）は、ＪＩＳＺ３１３７に規定されている方法により確認した。 And about the obtained welded joint, the presence or absence of the crack at the time of the corona bond right outside or corona bond nugget, and the cross tensile strength (CTS) measured by applying a tensile load in the peeling direction were confirmed. Cracks at the outside of the corona bond or the corona bond nugget were confirmed by observing a cross section in the thickness direction including the nugget, and the cross tensile strength (CTS) was confirmed by the method defined in JIS Z3137.

図３に示す、三角又は丸のマーカーの総板厚ｔと通電後保持時間Ｈｔにおいては、溶接継手にコロナボンド直外又はコロナボンドのナゲット際の割れは無く、十分なＣＴＳを有していた。それに対して、図３の線Ａよりも下に位置する四角のマーカーの総板厚ｔと通電後保持時間Ｈｔにおいては、コロナボンド直外又はコロナボンドのナゲット際の割れを有していた。これより、コロナボンド直外又はコロナボンドのナゲット際の割れが発生する境界（三角又は丸のマーカーと四角のマーカーとの境界）は、線Ａ（Ｈｔ＝０．０１５ｔ^２＋０．０２０）となることが判明した。 In the total thickness t of the triangular or round marker shown in FIG. 3 and the holding time after energization Ht, the welded joint had no cracks immediately outside the corona bond or corona bond nugget, and had sufficient CTS. . On the other hand, the total thickness t of the square markers located below the line A in FIG. 3 and the energization holding time Ht had cracks directly outside the corona bond or at the corona bond nugget. Accordingly, the boundary (the boundary between the triangular or round marker and the square marker) where the crack occurs immediately outside the corona bond or at the corona bond nugget is the line A (Ht = 0.015t ² +0.020). It has been found.

総板厚ｔに対して、溶接後保持時間Ｈｔを線Ａ以上（（１）式の左辺以上）の数値となるように設定することで、コロナボンド直外又はコロナボンドのナゲット際の割れが無くなる理由は、次のように考えられる。 By setting the retention time Ht after welding to a value equal to or greater than the line A (greater than or equal to the left side of the formula (1)) with respect to the total thickness t, cracks directly outside the corona bond or when the corona bond is nuggeted. The reason for disappearance is considered as follows.

コロナボンド直外及びコロナボンドのナゲット際の溶接残留応力が高くなる箇所は、スポット溶接の際に溶接電極により押しつぶされる過程で圧縮状態にあるが、溶接電極が鋼板表面から離れると引張り状態になり引張応力が生じる。コロナボンド直外及びコロナボンドのナゲット際の割れの発生は、コロナボンド直外で溶融した亜鉛系めっき金属が、溶接後にコロナボンド直外及びコロナボンドのナゲット際の溶接残留応力が高い箇所の鋼板の結晶粒界に侵入して、粒界強度を低下させることにより引き起こされる。 The area where the welding residual stress increases immediately outside the corona bond and during the corona bond nugget is in a compressed state in the process of being crushed by the welding electrode during spot welding, but becomes tensile when the welding electrode moves away from the steel plate surface. Tensile stress is generated. The occurrence of cracks in the corona bond directly outside and on the corona bond nugget is caused by the fact that the zinc-plated metal melted directly outside the corona bond has a high residual welding stress in the corona bond directly outside and on the corona bond nugget after welding. It is caused by intruding into the crystal grain boundary and lowering the grain boundary strength.

残留応力が高くなる箇所が生じるのは、溶接電極が鋼板表面から離れた後（電極開放後）であるため、通電終了後に溶接電極の加圧保持を継続（通電後保持時間Ｈｔを延長）し、電極開放前に溶融した亜鉛系めっきを凝固させることで、溶融した亜鉛系めっきが溶接残留応力の高い箇所の鋼板の結晶粒界に侵入せず、割れを抑制できる。そして、亜鉛系めっきの凝固は、鋼板の冷却のされやすさ、つまり、被溶接部材の総板厚ｔに関係するため、溶接電極の通電後保持時間Ｈｔを総板厚ｔの関数として調整することで、コロナボンド直外又はコロナボンドのナゲット際の割れを抑制できる。 The location where the residual stress increases occurs after the welding electrode is separated from the steel plate surface (after the electrode is opened), so the pressure holding of the welding electrode is continued after energization (the retention time Ht after energization is extended). By solidifying the molten zinc-based plating before opening the electrode, the molten zinc-based plating does not enter the crystal grain boundaries of the steel sheet at the location where the welding residual stress is high, and cracks can be suppressed. Since the solidification of the zinc-based plating is related to the ease of cooling of the steel sheet, that is, the total thickness t of the member to be welded, the post-energization holding time Ht of the welding electrode is adjusted as a function of the total thickness t. Thereby, the crack at the time of the nugget of corona bond right outside or a corona bond can be suppressed.

また、図３の線Ｂよりも上に位置する四角のマーカーの総板厚ｔと通電後保持時間Ｈｔにおいては、溶接継手にコロナボンド直外又はコロナボンドのナゲット際の割れは無いが、ＣＴＳの低下が大きかった。これより、十分なＣＴＳが得られる境界（四角のマーカーと三角又は丸のマーカーとの境界）は、図３に示す線Ｂ（Ｈｔ＝０．１６ｔ^２−０．４０ｔ＋０．７０）となることが判明した。 In addition, in the total thickness t of the square marker positioned above the line B in FIG. 3 and the holding time Ht after energization, the welded joint is not cracked immediately outside the corona bond or corona bond nugget, but CTS. The drop in was large. Thus, the boundary (a boundary between the square marker and the triangle or circle marker) at which sufficient CTS is obtained may be a line B (Ht = 0.16t ² −0.40t + 0.70) shown in FIG. found.

総板厚ｔに対して、通電後保持時Ｈｔを線Ｂ以下（（１）式の右辺以下）の数値となるように設定することで、十分なＣＴＳを得ることができる理由は、溶接部の冷却速度が速くなりすぎず、ナゲットと熱影響部の硬さが増加しないため、十字引張強さが低下しないものと考えられる。 The reason why a sufficient CTS can be obtained by setting Ht during holding after energization to a value less than line B (below the right side of equation (1)) with respect to the total thickness t is as follows. It is considered that the cross tensile strength does not decrease because the cooling rate of the material does not become too high and the hardness of the nugget and the heat affected zone does not increase.

よって、総板厚ｔ（ｍｍ）を関数として、通電後保持時間Ｈｔ（秒）を、（０．０１５ｔ^２＋０．０２０）以上、かつ、（０．１６ｔ^２−０．４０ｔ＋０．７０）以下に設定すると、コロナボンド直外又はコロナボンドのナゲット際の割れを抑制することができ、十分なＣＴＳを得ることができることを知見した。 Therefore, as a function of the total thickness t (mm), the post-energization holding time Ht (seconds) is not less than (0.015t ² +0.020) and not more than (0.16t ² −0.40t + 0.70). It has been found that, when set, it is possible to suppress cracks immediately outside the corona bond or at the time of corona bond nugget, and a sufficient CTS can be obtained.

上記実施形態に係るスポット溶接方法では、溶接後保持時間Ｈｔ（秒）が、さらに下記（２）式を満たすことがより好ましい。
Ｈｔ≦０．１０ｔ^２−０．４０ｔ＋０．５３・・・（２） In the spot welding method according to the embodiment, it is more preferable that the post-weld holding time Ht (seconds) further satisfies the following expression (2).
Ht ≦ 0.10t ² −0.40t + 0.53 (2)

溶接後保持時間Ｈｔが、総板厚ｔの関数である（２）式の条件を満たすことで、より高い十字引張強度（ＣＴＳ）を得ることができ、より信頼性の高いスポット溶接継手を提供できる。 When the post-weld holding time Ht satisfies the condition of equation (2), which is a function of the total thickness t, a higher cross tensile strength (CTS) can be obtained, and a more reliable spot welded joint is provided. it can.

さらに、上記実施形態に係るスポット溶接方法では、溶接後保持時間Ｈｔが、さらに下記（３）式を満たすことがより好ましい。
０．０１５ｔ^２＋０．１１０≦Ｈｔ・・・（３） Furthermore, in the spot welding method according to the embodiment, it is more preferable that the post-weld holding time Ht further satisfies the following expression (3).
0.015 t ² + 0.110 ≦ Ht (3)

溶接電極による冷却は、電極の材料や冷却水の水量によって異なり、冷却の態様にバラツキが生じる場合がある。溶接後保持時間Ｈｔが、総板厚ｔの関数である（３）式の条件を満たすことで、電極の材料や冷却性能によるバラツキを抑え、生産性を確保しながらも所望の継手強度を得ることができる。 Cooling by the welding electrode varies depending on the material of the electrode and the amount of cooling water, and the cooling mode may vary. When the post-weld holding time Ht satisfies the condition of the expression (3), which is a function of the total plate thickness t, a desired joint strength is obtained while suppressing variations due to electrode materials and cooling performance and ensuring productivity. be able to.

上記実施形態に係るスポット溶接方法では、溶接後保持時間Ｈｔが、さらに下記（４）式を満たすことがさらに好ましい。
０．０１５ｔ^２＋０．１１≦Ｈｔ≦０．１０ｔ^２−０．４０ｔ＋０．５３・・・（４） In the spot welding method according to the above embodiment, it is further preferable that the post-weld holding time Ht further satisfies the following expression (4).
0.015t ² + 0.11 ≦ Ht ≦ 0.10t ² −0.40t + 0.53 (4)

本実施形態に係るスポット溶接方法では、溶接電極を被溶接部材に接触させる直前において、以下の外乱因子（ａ）〜（ｄ）の条件のうち、１又は２以上の条件を満たす場合であっても、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制し、かつ継手強度を確保することができる。 In the spot welding method according to the present embodiment, immediately before the welding electrode is brought into contact with the member to be welded, one or two or more conditions among the following disturbance factors (a) to (d) are satisfied. In addition, it is possible to suppress cracks immediately outside the corona bond and at the time of corona bond nugget, and to secure joint strength.

外乱因子（ａ）：溶接電極の軸芯と、該溶接電極と接触する鋼板表面の垂線とが平行でない（非平行である）状態
スポット溶接において、鋼板表面に対して溶接電極を垂直に当てるのが基本である。しかし、被溶接部材に溶接箇所が複数あり、様々な溶接姿勢での溶接を要する場合がある。このような場合、スポット溶接機のティーチング不良や作業時間の制約などのため、溶接姿勢を正す時間を確保できず、溶接電極の軸芯と、当該溶接電極と接触する鋼板表面とが垂直から傾いたままスポット溶接している場合がある。図４の（ａ）に、溶接電極８の軸芯９と、該溶接電極８と接触する鋼板１の表面の垂線１０とが平行でない状態の概略図を示す。 Disturbance factor (a): A state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet contacting the welding electrode are not parallel (non-parallel) In spot welding, the welding electrode is applied perpendicularly to the surface of the steel sheet. Is the basic. However, there are a plurality of welding locations on the member to be welded, and welding in various welding postures may be required. In such a case, due to poor teaching of the spot welder or restrictions on working time, the time for correcting the welding posture cannot be secured, and the axis of the welding electrode and the steel plate surface in contact with the welding electrode are inclined from the vertical. Spot welding may be performed as is. FIG. 4A is a schematic view showing a state in which the axis 9 of the welding electrode 8 and the perpendicular 10 on the surface of the steel plate 1 in contact with the welding electrode 8 are not parallel.

上記のように、溶接電極８の軸芯９と、該溶接電極８と接触する鋼板１の表面との角度が垂直から外れた状態で溶接する場合には、該溶接電極８と接触する鋼板１の表面の垂線１０に対して溶接電極８の軸芯９が打角という角度を有する。この打角が０度より大きい状態（溶接電極８の軸芯９が鋼板１の表面に対して垂直ではない状態）で溶接電極８を鋼板１の表面に接触させると、図４の（ｂ）のように鋼板１が変形する。このような変形により、コロナボンドの近傍に比較的応力が高くなる箇所が生じ、この箇所に割れが生じやすくなる。 As described above, when welding is performed in a state where the angle between the axis 9 of the welding electrode 8 and the surface of the steel plate 1 in contact with the welding electrode 8 deviates from vertical, the steel plate 1 in contact with the welding electrode 8. The axis 9 of the welding electrode 8 has an angle called a striking angle with respect to the normal 10 on the surface. If the welding electrode 8 is brought into contact with the surface of the steel plate 1 in a state where the angle of attack is greater than 0 degrees (a state where the axis 9 of the welding electrode 8 is not perpendicular to the surface of the steel plate 1), FIG. Thus, the steel plate 1 is deformed. Due to such deformation, a portion where the stress is relatively high is generated in the vicinity of the corona bond, and a crack is easily generated in this portion.

打角が３度以上の場合に、特にコロナボンド直外及びコロナボンドのナゲット際の割れが生じやすいが、本実施形態のスポット溶接方法では、これら割れの抑制効果がある。また、打角が５度以上の場合、さらにコロナボンド直外及びコロナボンドのナゲット際の割れが生じて継手強度が著しく低下する傾向があるが、本実施形態のスポット溶接方法では、このような品質低下も抑制することができる。なお、溶接電極８と鋼板１との間に感圧紙を挟み、溶接電極８を空打ち（通電をせずに加圧のみをかける）をすることで、溶接電極８の軸芯９と、該溶接電極８と接触する鋼板１の表面との角度が垂直から外れた状態であるか否か検知することができる。 When the striking angle is 3 ° or more, cracks are particularly likely to occur immediately outside the corona bond and during the corona bond nugget, but the spot welding method of this embodiment has an effect of suppressing these cracks. In addition, when the striking angle is 5 degrees or more, there is a tendency that the joint strength further decreases due to cracks in the corona bond directly outside and the corona bond nugget, but in the spot welding method of the present embodiment, Quality deterioration can also be suppressed. In addition, by sandwiching a pressure sensitive paper between the welding electrode 8 and the steel plate 1 and punching the welding electrode 8 (only applying pressure without energization), the shaft core 9 of the welding electrode 8 and the It is possible to detect whether or not the angle with the surface of the steel plate 1 in contact with the welding electrode 8 deviates from the vertical.

外乱因子（ｂ）：一方の溶接電極の先端部から該先端部に最も近い鋼板表面までの距離と、他方の溶接電極の先端部から該先端部に最も近い鋼板表面までの距離が異なる状態
スポット溶接中に、溶接電極挟み込みに対し、保持された板の中心位置を保つために、イコライジング機構を設けることがある。イコライジング機構を設ける場合、その分だけスポット溶接ガンが大形化し、溶接ロボットをそれに対応したものにしなければならず、溶接ロボットのコストが上がる。そのため、スポット溶接ガンにイコライジング機構を設けずに溶接する場合がある。 Disturbance factor (b): State in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip During welding, an equalizing mechanism may be provided to maintain the center position of the held plate against the sandwiching of the welding electrode. When the equalizing mechanism is provided, the spot welding gun is increased in size by that amount, and the welding robot must be adapted to the size, which increases the cost of the welding robot. Therefore, the spot welding gun may be welded without providing an equalizing mechanism.

この場合、図５の（ａ）に示すように、溶接電極８を被溶接部材（鋼板１）に接触させる直前において、一方の溶接電極８の先端部からこの先端部に近い側の鋼板１の表面までの距離と、他方の溶接電極８の先端部からこの先端部に近い側の鋼板１の表面までの距離が異なる状態にある。このような状態で溶接電極８の接触・加圧を行うと、図５の（ｂ）に示すように、溶接箇所において、一方の鋼板１（図５の場合には、上側の鋼板１）が他方の鋼板１側へ変形させられる。このように変形した状態で溶接を行うと、コロナボンドの近傍に局所的な応力が発生する。そして、この箇所に割れが生じやすくなる。 In this case, as shown in FIG. 5 (a), immediately before the welding electrode 8 is brought into contact with the member to be welded (steel plate 1), the steel plate 1 on the side close to the tip from the tip of one welding electrode 8 is provided. The distance to the surface is different from the distance from the tip of the other welding electrode 8 to the surface of the steel plate 1 on the side close to the tip. When contact / pressurization of the welding electrode 8 is performed in such a state, as shown in FIG. 5B, one steel plate 1 (in the case of FIG. 5, the upper steel plate 1) is present at the welding location. The other steel plate 1 is deformed. When welding is performed in such a deformed state, local stress is generated in the vicinity of the corona bond. And it becomes easy to produce a crack in this location.

上記のように、スポット溶接ガンにイコライジング機構を設けずに溶接する場合や、被溶接部材の構成上、溶接電極と鋼板表面との距離を適切に設定できない場合には、溶接電極の軸芯方向において、スポット溶接ガンと被溶接部材との位置ズレが吸収されず、コロナボンド直外やコロナボンドのナゲット際の割れが発生し易い。
イコライジング機構が無く、クリアランスが１ｍｍ以上ある場合に、特にコロナボンド直外及びコロナボンドのナゲット際の割れが生じやすいが、このような場合であっても、本実施形態のスポット溶接方法では、コロナボンド直外やコロナボンドのナゲット際の割れの抑制効果がある。 As mentioned above, when welding without providing an equalizing mechanism to the spot welding gun, or when the distance between the welding electrode and the steel plate surface cannot be set appropriately due to the construction of the welded member, the axial direction of the welding electrode In this case, the positional deviation between the spot welding gun and the member to be welded is not absorbed, and cracks are likely to occur immediately outside the corona bond or at the time of the corona bond nugget.
In the case where there is no equalizing mechanism and the clearance is 1 mm or more, cracks are particularly likely to occur immediately outside the corona bond and when the corona bond is nuggeted. It has the effect of suppressing cracks on the outside of the bond and the corona bond nugget.

外乱因子（ｃ）：一方の前記溶接電極の軸芯の延長線上に他方の前記溶接電極の軸芯がない状態（一方の前記溶接電極の軸芯の延長線に対し、他方の前記溶接電極の軸芯がずれている状態）
多数の打点を溶接すると、溶接電極軸の挫屈やスポット溶接ガンの可動部の摩耗により、図６の（ａ）に示すように、一方の前記溶接電極８の軸芯９の延長線上に他方の前記溶接電極８の軸芯９がなく、ズレ（以下、「電極芯ズレ」という）が発生することがある。このような電極芯ズレが発生したまま溶接を行うと、図６（ｂ）に示すような変形が加わり、コロナボンドの近傍で応力が生じる場合がある。 Disturbance factor (c): State in which there is no axial center of the other welding electrode on the extension line of the axial center of the one welding electrode (with respect to the extension line of the axial center of the one welding electrode, (Shaft is misaligned)
When a large number of hit points are welded, the other of the welding electrodes 8 on the extension line of the axis 9 of the welding electrode 8 as shown in FIG. The axial center 9 of the welding electrode 8 may be absent, and a deviation (hereinafter referred to as “electrode core deviation”) may occur. When welding is performed with such electrode misalignment, deformation as shown in FIG. 6B is applied, and stress may occur in the vicinity of the corona bond.

溶接電極の電極芯ズレが０．２ｍｍ以上である場合、特に、コロナボンド直外やコロナボンドのナゲット際の割れが発生し易い。しかし、本実施形態のスポット溶接方法では、これら割れの抑制効果がある。また、溶接電極の電極芯ズレが１ｍｍ以上であり、より大きな応力が生じる場合にも、本実施形態のスポット溶接方法による割れの抑制効果がある。 When the electrode misalignment of the welding electrode is 0.2 mm or more, cracks are particularly likely to occur immediately outside the corona bond or at the time of the corona bond nugget. However, the spot welding method of this embodiment has an effect of suppressing these cracks. Further, even when the electrode misalignment of the welding electrode is 1 mm or more and a larger stress is generated, there is an effect of suppressing cracking by the spot welding method of the present embodiment.

外乱因子（ｄ）：溶接箇所の重ね合わせ面の間に隙間を有した状態
図７の（ａ）には、２枚の鋼板１の右側に大きな隙間がある例を示している。隙間は、左右で偏りがない場合もある。これらの現象は、たとえば、プレス成形した際、スプリングバックが起こった場合や、スポット溶接を隣り合う位置で順番に行なわないときに、１枚の鋼板が部分的に盛り上がったりする場合に起こる。図７（ａ）では、鋼板１の間に他の部材１１が挿入されている場合を図示している。溶接箇所の重ね合わせ面の鋼板１間に隙間（以下、「板隙」ともいう）を有したままスポット溶接する場合がある。このような板隙が存在する状態で接触・加圧をすると、図７の（ｂ）に示すように、溶接箇所において、鋼板１の局所的な変形が見られる。このように変形した状態で溶接を行うと、コロナボンドの近傍に局所的な応力が発生する。そして、この箇所に割れが生じやすくなる。
被溶接部材において、溶接電極の打点位置における軸芯方向の板隙が１ｍｍ以上である場合、特に、コロナボンド直外やコロナボンドのナゲット際の割れが発生し易い。このような場合であっても、本実施形態のスポット溶接方法では、このような割れの抑制効果がある。 Disturbance factor (d): state in which there is a gap between the overlapped surfaces of the welded portions FIG. 7A shows an example in which there is a large gap on the right side of the two steel plates 1. The gap may be unbalanced on the left and right. These phenomena occur, for example, when a springback occurs during press molding, or when a single steel plate partially rises when spot welding is not performed in order at adjacent positions. FIG. 7A shows a case where another member 11 is inserted between the steel plates 1. Spot welding may be performed with a gap (hereinafter also referred to as “plate gap”) between the steel plates 1 on the overlapped surface of the welded portions. When contact / pressurization is performed in a state where such a sheet gap exists, local deformation of the steel sheet 1 is observed at the welded portion, as shown in FIG. When welding is performed in such a deformed state, local stress is generated in the vicinity of the corona bond. And it becomes easy to produce a crack in this location.
In the member to be welded, when the plate gap in the axial center direction at the hitting position of the welding electrode is 1 mm or more, cracks are particularly likely to occur immediately outside the corona bond or at the corrugated nugget. Even in such a case, the spot welding method of this embodiment has an effect of suppressing such cracks.

更に、本発明者らは、溶接継手の強度と靭性を確保するために、本溶接の前後に通電する工程を実施する技術を適用し、その際に、ロナボンド直外又はコロナボンドのナゲット際の割れを抑制することについて検討した。 Furthermore, in order to ensure the strength and toughness of the welded joint, the present inventors applied a technique of conducting a process of energizing before and after the main welding, and at that time, at the time of nonaget immediately outside the Lonabond or coronabond. The suppression of cracking was studied.

上述のように、コロナボンド直外及びコロナボンドのナゲット際の割れの発生は、溶接電極が鋼板表面から離れ、引張の溶接残留応力が高くなった箇所に、溶融した亜鉛系めっき金属が侵入して引き起こされる。 As described above, the occurrence of cracks immediately outside the corona bond and during the corona bond nugget is caused by the molten zinc-based plated metal penetrating into the location where the welding electrode has moved away from the steel plate surface and the tensile welding residual stress has increased. Is caused.

そこで、通電後保持時間Ｈｔ（秒）を、溶接終了の際（後通電を行わない場合は本溶接の終了の際、後通電を行う場合は該後通電の終了の際）の溶接電極間の通電終了時から溶接電極と被溶接部材とを非接触とするまでの保持時間として、本溶接の前後に通電する工程を実施しない場合に得られた総板厚ｔ（ｍｍ）に対する通電後保持時間Ｈｔ（秒）の関係に、そのまま適用することを試みた。 Accordingly, the holding time after energization Ht (seconds) is set between the welding electrodes at the end of welding (when the post-energization is not performed, at the end of the main welding, when post-energization is performed, at the end of the post-energization). Holding time after energization with respect to the total plate thickness t (mm) obtained when the energization process is not performed before and after the main welding as the holding time from the end of energization until the welding electrode and the member to be welded are brought into non-contact An attempt was made to apply it directly to the relationship of Ht (seconds).

その結果、総板厚ｔ（ｍｍ）を関数として、通電後保持時間Ｈｔ（秒）を、（１）式の範囲に設定すると、コロナボンド直外又はコロナボンドのナゲット際の割れを抑制することができ、十分なＣＴＳを得ることができることを知見した。 As a result, setting the post-energization holding time Ht (seconds) as a function of the total thickness t (mm) as a function of the range of the formula (1) suppresses cracks immediately outside the corona bond or at the corona bond nugget. It was found that sufficient CTS can be obtained.

＜前通電及び後通電＞
本実施形態に係るスポット溶接法は、溶接終了の際（後通電を行わない場合は本溶接の終了の際、後通電を行う場合は該後通電の終了の際）の溶接電極間の通電終了時から溶接電極と被溶接部材とを非接触とするまでの通電後保持時間Ｈｔ（秒）を総板厚ｔ（ｍｍ）との関係で特定するものであり、以降、後通電を行わない場合と後通電を行う場合に分けて説明する。 <Pre-energization and post-energization>
In the spot welding method according to the present embodiment, the end of energization between the welding electrodes at the end of welding (at the end of main welding when no post-energization is performed, or at the end of post-energization when post-energization is performed) When the post-energization holding time Ht (seconds) from when the welding electrode and the member to be welded are brought into non-contact is specified in relation to the total thickness t (mm). This will be described separately in the case of performing post-energization.

図８に、後通電を行わない場合の溶接電極への通電パターンの例を示す。後通電を行わない場合は、被溶接部材である２枚の鋼板を重ね合わせて、両側から２枚の鋼板を挟み込むように、銅合金などからなる溶接電極により加圧して、電流値を前通電電流Ｉ_ｆ（ｋＡ）とし、電流値が前通電電流Ｉ_ｆ（ｋＡ）の状態を前通電時間ｔ_ｆ（ｓｅｃ）保持する前通電を行う。 FIG. 8 shows an example of an energization pattern to the welding electrode when post-energization is not performed. When post-energization is not performed, the two steel plates that are the members to be welded are overlapped and pressed with welding electrodes made of copper alloy so that the two steel plates are sandwiched from both sides, and the current value is pre-energized. Current energization is performed with current I _f (kA) and the current value maintained at the pre-energization current _If (kA) for the pre-energization time t _f (sec).

前通電電流Ｉ_ｆ（ｋＡ）及び前通電時間ｔ_ｆ（ｓｅｃ）は、特に限定されるものでなく、散りの発生を抑制するために、前通電電流Ｉ_ｆ（ｋＡ）は、本溶接電流Ｉ_Ｗ（ｋＡ）の０．４倍以上、本溶接電流Ｉ_Ｗ（ｋＡ）未満、前通電時間ｔ_ｆ（ｓｅｃ）は、０．０２ｓｅｃ以上が例示される。 The pre-energization current I _f (kA) and the pre-energization time t _f (sec) are not particularly limited. In order to suppress the occurrence of scattering, the pre-energization current I _f (kA) is the main welding current I _W (kA) of 0.4 times or more, the welding current _I W less than (kA), before energization time _t f (sec) is more than 0.02sec are exemplified.

前通電時間ｔ_ｆ（ｓｅｃ）が経過すると、溶接電極の加圧を保持したまま、電流値を０（ゼロ）にし、電流値が０（ゼロ）の状態を前通電後冷却時間ｔ_Ｃ（ｓｅｃ）保持する。前通電後冷却時間ｔ_Ｃ（ｓｅｃ）は、０．００ｓｅｃ以上０．２５ｓｅｃ未満が例示される。なお、前通電の開始時に、電流値を直ちに前通電電流Ｉ_ｆ（ｋＡ）とせず、電流値が前通電電流Ｉ_ｆ（ｋＡ）になるまで、電流値を０（ゼロ）から漸増（アップスロープ）させてもよい。 When the front energization time t _{f (sec)} has elapsed, while maintaining the pressure of the welding electrodes, the current value to 0 (zero), after pre-energized state of the current value 0 (zero) cooling time t _{C (sec} )Hold. The cooling time after pre-energization t _C (sec) is exemplified by 0.00 sec or more and less than 0.25 sec. At the start of pre-energization, the current value is not immediately set to the pre-energization current _If (kA), and the current value is gradually increased from 0 (zero) until the current value becomes the pre-energization current _If (kA). ).

冷却の後、溶接電極の加圧を保持したまま、電流値を本溶接電流Ｉ_Ｗ（ｋＡ）にして本溶接を行い、溶融金属を形成する。この際の本溶接の条件は、特に限定されるものでない。たとえば、電極をドームラジアス型の先端直径６〜８ｍｍのものとし、加圧力１５００〜６０００Ｎ、通電時間５〜５０サイクル（電源周波数５０Ｈｚ）、通電電流４〜１５ｋＡとすることができる。 After cooling, with the pressure of the welding electrode maintained, the current value is set to the main welding current I _W (kA) to perform main welding to form a molten metal. The conditions for main welding at this time are not particularly limited. For example, the electrode may have a dome radius type tip diameter of 6 to 8 mm, a pressing force of 1500 to 6000 N, an energization time of 5 to 50 cycles (power frequency 50 Hz), and an energization current of 4 to 15 kA.

本溶接が終了すると、溶接電極の加圧を保持したまま、電流値を０（ゼロ）とし、電流値が０（ゼロ）の状態で、上記（１）式を満足する通電後保持時間Ｈｔ（秒）保持する。たとえば、総板厚ｔが４．０ｍｍの場合、通電後保持時間Ｈｔを０．２４〜１．６６秒とすることができる。 When the main welding is completed, the post-energization holding time Ht () satisfying the above equation (1) in a state where the current value is 0 (zero) while maintaining the pressurization of the welding electrode and the current value is 0 (zero). Seconds) hold. For example, when the total thickness t is 4.0 mm, the post-energization holding time Ht can be set to 0.24 to 1.66 seconds.

これにより、溶接残留応力の高い箇所に溶融した亜鉛系めっきが侵入しなくなるので、コロナボンド直外及びコロナボンドのナゲット際において液体金属割れの発生を防ぐことができ、十分なＣＴＳを得ることができる。 As a result, the molten zinc-based plating does not invade the place where the welding residual stress is high, so that the occurrence of liquid metal cracks can be prevented immediately outside the corona bond and at the corona bond nugget, and sufficient CTS can be obtained. it can.

次に、後通電を行う場合は、被溶接部材に対して、前述の前通電及び本溶接を行い、後通電を行うか、又は、前述の本溶接を行い、後通電を行う。次に、後通電を行う場合について、本溶接を行い、後通電を行う場合（前通電を行わない場合）を例に説明する。図５に、後通電を行う場合の溶接電極への通電パターンの例を示す。 Next, when post-energization is performed, the above-described pre-energization and main welding are performed on the member to be welded, and post-energization is performed, or the above-described main welding is performed and post-energization is performed. Next, a case where post-energization is performed will be described by taking as an example a case where main welding is performed and post-energization is performed (when pre-energization is not performed). In FIG. 5, the example of the electricity supply pattern to the welding electrode in the case of performing post-energization is shown.

本溶接において、被溶接部材である２枚の鋼板を重ね合わせて、両側から２枚の鋼板を挟み込むように、銅合金などからなる溶接電極により加圧して、電流値が本溶接電流Ｉ_Ｗ（ｋＡ）になるまで、電流値を０（ゼロ）から漸増（アップスロープ）させる。そして、電流値を本溶接電流Ｉ_Ｗ（ｋＡ）にして、本溶接を行う。なお、電流値が本溶接電流Ｉ_Ｗ（ｋＡ）になるまで、電流値を０（ゼロ）から漸増（アップスロープ）させずに、電流値を直ちに本溶接電流Ｉ_Ｗ（ｋＡ）にしてもよい。 In the main welding, the two steel plates as the members to be welded are overlapped and pressed by a welding electrode made of a copper alloy or the like so as to sandwich the two steel plates from both sides, and the current value becomes the main welding current I _W ( The current value is gradually increased (upslope) from 0 (zero) until kA). Then, the main welding is performed with the current value set to the main welding current I _W (kA). Note that until the current value becomes the welding current I _{W (kA),} the current value without causing 0 increasing from (zero) (up-slope), may immediately present the welding current I _W a current value _(kA) .

本溶接が終了すると、溶接電極の加圧を保持したまま、電流値を０（ゼロ）にし、電流値が０（ゼロ）の状態を本溶接後冷却時間（凝固時間）ｔ_Ｓ（ｓｅｃ）保持し、溶融金属の外周から凝固させて、内側に未凝固域が残る殻状の凝固域を形成する。冷却の時間は、特に限定されるものでなく、後述する後通電によって熱影響部の靱性を向上させるために、０．００１〜０．３００ｓｅｃが例示される。 When the main welding is completed, the current value is set to 0 (zero) while maintaining the pressurization of the welding electrode, and the current value is maintained at 0 (zero) after the main welding cooling time (solidification time) t _S (sec). Then, the molten metal is solidified from the outer periphery to form a shell-shaped solidified region in which an unsolidified region remains inside. The cooling time is not particularly limited, and 0.001 to 0.300 sec is exemplified in order to improve the toughness of the heat affected zone by post-energization described later.

本溶接後冷却時間ｔ_Ｓ（ｓｅｃ）が経過すると、溶接電極の加圧を保持したまま、未凝固域が存在しているときに、電流値を後通電電流Ｉ_Ｐ（ｋＡ）とし、電流値が後通電電流Ｉ_Ｐ（ｋＡ）の状態を後通電時間ｔ_Ｐ（ｓｅｃ）保持し、後通電を行う。後通電電流Ｉ_Ｐ（ｋＡ）及び後通電時間ｔ_Ｐ（ｓｅｃ）は、特に限定されるものでなく、熱影響部の靱性に大きく影響するＰ、Ｓ等の偏析を軽減させるために、後通電電流Ｉ_Ｐ（ｋＡ）は、本溶接電流Ｉ_Ｗ（ｋＡ）の０．６倍以上、本溶接電流Ｉ_Ｗ（ｋＡ）以下、後通電時間ｔ_Ｐ（ｓｅｃ）は、０．００１〜０．５００ｓｅｃが例示される。 When the cooling time t _S (sec) after the main welding elapses, the current value is defined as a post-energization current I _P (kA) when an unsolidified region exists while maintaining the pressurization of the welding electrode. Holds the state of the post-energization current I _P (kA) for the post-energization time t _P (sec) and performs post-energization. The post-energization current I _P (kA) and the post-energization time t _P (sec) are not particularly limited. In order to reduce segregation of P, S, etc., which greatly affects the toughness of the heat affected zone, post-energization current _I P (kA), the main welding current _I W (kA) of 0.6 times or more, the welding current _I W (kA) hereinafter, rear energization time _t P (sec) is 0.001～0.500Sec Is exemplified.

なお、スポット溶接の連続打点性の向上のため、電流値を０（ゼロ）にする冷却と後通電とを一つの工程とし、２回以上繰り返してもよい。２回以上繰り返す際は、１回目工程の冷却時間ｔ_Ｓ、後通電電流Ｉ_Ｐ、後通電時間ｔ_Ｐと、それ以降の工程におけるこれらの条件とを同じにすると、作業効率上好ましい。また、後通電における各後通電時間ｔ_Ｐの合計は、０．０８０〜２．０００ｓｅｃが好ましい。 In addition, in order to improve the continuous spot property of spot welding, the cooling and the post-energization with a current value of 0 (zero) may be performed as one process and repeated two or more times. When the process is repeated two or more times, it is preferable in terms of working efficiency that the cooling time t _S , the post-energization current I _P and the post-energization time t _{P in} the first process are the same as those conditions in the subsequent processes. The total of each rear energization time _{t P} in the rear energization, 0.080～2.000Sec is preferred.

後通電が終了すると、溶接電極の加圧を保持したまま、電流値を０（ゼロ）とし、電流値が０（ゼロ）の状態で、上記（１）式を満足する通電後保持時間Ｈｔ（秒）保持する。たとえば、総板厚ｔが４．０ｍｍの場合、通電後保持時間Ｈｔを０．２４〜１．６６秒とすることができる。 When the post-energization is completed, the post-energization holding time Ht () satisfying the above equation (1) in a state where the current value is 0 (zero) while maintaining the pressurization of the welding electrode and the current value is 0 (zero). Seconds) hold. For example, when the total thickness t is 4.0 mm, the post-energization holding time Ht can be set to 0.24 to 1.66 seconds.

また、本発明の溶接法は、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れの少なくとも一方の割れが発生する溶接箇所に対して採用することが効率的で望ましい。そこで、後通電を行わない場合は、本溶接における溶接電極間の通電終了後、後通電を行う場合は、後通電における溶接電極間の通電終了後、直ちに、溶接電極と被溶接部材とを非接触とするスポット溶接を予め行い、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れを確認し、割れが発生したとき、以降、その溶接箇所に対する溶接において、通電後保持時間を上記（１）式の範囲内としてスポット溶接すると良い。 Moreover, it is efficient and desirable to employ the welding method of the present invention for a welded portion where at least one of a crack directly outside the corona bond and a crack in the corona bond nugget occurs. Therefore, when post-energization is not performed, after energization between the welding electrodes in main welding is completed, when post-energization is performed, immediately after energization between the welding electrodes in post-energization is completed, the welding electrode and the member to be welded are immediately removed. Spot welding to be contacted is performed in advance, cracks immediately outside the corona bond and cracks at the time of corona bond nugget are confirmed, and when a crack occurs, the holding time after energization is set to the above (1 ) Spot welding should be performed within the range of the formula.

被溶接部材に予め行うスポット溶接は、溶接終了の際、溶接電極間の通電終了後、直ちに、溶接電極と被溶接部材とを非接触にして、通電後保持時間を延長せず、上述のスポット溶接と同様に行う。この際の本溶接、前通電及び後通電の条件は、上述と同様の条件を採用することができる。 When spot welding is performed in advance on a member to be welded, immediately after the end of energization between welding electrodes at the end of welding, the welding electrode and the member to be welded are brought into non-contact immediately and the holding time after energization is not extended. The same as welding. The conditions similar to the above can be adopted as the conditions for the main welding, pre-energization and post-energization at this time.

そして、重ね合わせ面のコロナボンド直外の割れ及びコロナボンドのナゲット際の割れの発生の有無を確認する。割れの確認は、たとえば、ナゲットを含むように、板厚方向に切断して、その断面を確認して行う。図２に示すような、重ね合わせ面のコロナボンド直外及びコロナボンドのナゲット際の少なくとも一方に割れを確認したとき、総板厚ｔに対する通電後保持時間Ｈｔが上記（１）式を満足するようにして、以降のスポット溶接を実施する。 And the presence or absence of the generation | occurrence | production of the crack in the corona bond right outside of the overlapping surface and the crack at the time of corona bond nugget is confirmed. For example, the crack is confirmed by cutting in the thickness direction so as to include the nugget and confirming the cross section. As shown in FIG. 2, when a crack is confirmed in at least one of the corona bond immediately outside the corona bond and the nugget of the corona bond as shown in FIG. 2, the retention time Ht after energization with respect to the total thickness t satisfies the above formula (1). Thus, the subsequent spot welding is performed.

次に、本発明の溶接法について、さらに、必要な要件や好ましい要件について順次説明する。 Next, regarding the welding method of the present invention, necessary requirements and preferable requirements will be sequentially described.

＜被溶接部材＞
（被溶接部材の構成）
スポット溶接される被溶接部材は、少なくとも溶接箇所が重ね合わされた複数枚の鋼板で構成され、そのうちの少なくとも１枚以上の鋼板の重ね合わせ面に亜鉛系めっきが被覆されていれば、特に限定されるものでない。たとえば、全てのスポット溶接される鋼板の重ね合わせ面に亜鉛系めっきが被覆された複数枚の鋼板や、スポット溶接される鋼板の重ね合わせ面に亜鉛系めっきが被覆された鋼板とスポット溶接される鋼板に亜鉛系めっきが被覆されていない鋼板を含む複数枚の鋼板などが例示される。 <Welded member>
(Configuration of welded member)
A member to be welded to be spot welded is particularly limited as long as it is composed of a plurality of steel plates on which at least welded portions are overlapped, and at least one of the steel plates is covered with zinc-based plating. It is not something. For example, spot welding is performed with a plurality of steel plates in which zinc-plating is coated on the overlapping surfaces of all spot-welded steel plates or steel plates in which zinc-plating is coated on the overlapping surfaces of spot-welded steel plates. Examples of the steel sheet include a steel sheet that is not coated with zinc-based plating.

たとえば、亜鉛系めっきが被覆された、引張強度７８０ＭＰａ以上、Ｃｅｑが０．１５質量％以上、板厚０．５〜３．０ｍｍの鋼板を２枚準備する。なお、Ｃｅｑは下記（５）式に示すものである。 For example, two steel plates having a tensile strength of 780 MPa or more, a Ceq of 0.15 mass% or more, and a plate thickness of 0.5 to 3.0 mm are prepared. Ceq is shown in the following formula (5).

Ｃｅｑ＝［Ｃ］＋［Ｓｉ］／３０＋［Ｍｎ］／２０＋２［Ｐ］＋４［Ｓ］・・・（５）
ただし、［Ｃ］、「Ｓｉ］、［Ｍｎ］、［Ｐ］、［Ｓ］は、Ｃ、Ｓｉ、Ｐ、及びＳの含有量（質量％）である。
軽量化及び高強度化の観点からは、好ましいＣｅｑは０．１８質量％以上である、より好ましくは０．２０質量％以上である。 Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] (5)
However, [C], “Si], [Mn], [P], and [S] are the contents (mass%) of C, Si, P, and S.
From the viewpoint of weight reduction and strength enhancement, the preferred Ceq is 0.18% by mass or more, more preferably 0.20% by mass or more.

また、スポット溶接される重ね合わせ面に亜鉛系めっきが被覆された鋼板において、スポット溶接される鋼板の重ね合わせ面と反対側の面、つまり、溶接電極との接触面に亜鉛系めっきが、被覆されていても、被覆されていなくもよい。ただし、スポット溶接継手の耐食性を考慮すれば、溶接電極との接触面にも亜鉛系めっきが被覆されていることが好ましい。 In addition, in a steel plate with a zinc-plated coating on the overlapped surface to be spot-welded, the surface opposite to the overlapped surface of the steel plate to be spot-welded, that is, the contact surface with the welding electrode is coated with zinc-based plating. It may or may not be coated. However, in consideration of the corrosion resistance of the spot welded joint, it is preferable that the contact surface with the welding electrode is also coated with zinc-based plating.

（鋼板の枚数、板厚）
スポット溶接される複数枚の鋼板として、図２等では、２枚の鋼板を記載しているが、接合する構造部品の形態に応じて、３枚以上の複数枚の鋼板とすることができる。スポット溶接される各鋼板の板厚は、特に限定されるものでなく、０．５〜３．０ｍｍとすることができる。また、複数枚の鋼板の全体の総板厚ｔは、１．３５ｍｍ以上とし、上限は特に限定されるものでなく、総板厚ｔを７．０ｍｍ以下とすることができる。 (Number of steel plates, thickness)
In FIG. 2 and the like, two steel plates are described as the plurality of steel plates to be spot-welded, but three or more steel plates can be used depending on the form of the structural parts to be joined. The plate | board thickness of each steel plate spot-welded is not specifically limited, It can be 0.5-3.0 mm. The total thickness t of the plurality of steel plates is 1.35 mm or more, and the upper limit is not particularly limited, and the total thickness t can be 7.0 mm or less.

より好ましい総板厚ｔの範囲は、２．４ｍｍ以上３．２ｍｍ以下であり、さらに好ましい範囲は、２．７ｍｍ以上３．２ｍｍ以下である。総板厚ｔをこの範囲とすることで、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制し、継手強度を確保し、さらには被溶接部材の軽量化と高強度化が図れる。 A more preferable range of the total thickness t is 2.4 mm or more and 3.2 mm or less, and a more preferable range is 2.7 mm or more and 3.2 mm or less. By setting the total sheet thickness t within this range, cracks at the outside of the corona bond and at the time of the corona bond nugget can be suppressed, the joint strength can be ensured, and further the weight and strength of the welded member can be increased.

（鋼板の形態）
また、鋼板は、少なくとも一部に板状部を有し、当該板状部が互いに積み重ね合わされる部分を有するものであればよく、全体が板でなくともよい。また、複数枚の鋼板は、別々の鋼板から構成されるものに限定されず、１枚の鋼板を管状などの所定の形状に成形したものを重ね合わせたものでもよい。 (Plate form)
Moreover, the steel plate should just have a plate-shaped part in at least one part, and the said plate-shaped part should have a part by which each other is piled up, and the whole may not be a board. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and may be a superposition of a single steel plate formed into a predetermined shape such as a tubular shape.

（鋼板の種類）
また、スポット溶接される被溶接部材の鋼板は、成分組成や、金属組織など、特に限定されるものでない。ただし、溶接される重ね合わせ面に亜鉛系めっきが被覆された鋼板、又は、亜鉛系めっきが被覆された鋼板と亜鉛系めっきを介して重ね合わされている鋼板に対して、ＴＲＩＰ鋼板などでは特に上記の割れが発生しやすいが、それ以外の高強度鋼板でＣｅｑが０．１５質量％以上の鋼板を用いたときなどにも、コロナボンド直外及びコロナボンドのナゲット際での割れが発生しやすいため、本発明の溶接法において、特に、このような鋼板を対象としてもよい。 (Type of steel sheet)
Moreover, the steel plate of the member to be welded that is spot-welded is not particularly limited, such as a component composition or a metal structure. However, in the case of a TRIP steel plate, etc., especially with respect to a steel plate in which the overlapped surface to be welded is coated with zinc-based plating, or a steel plate coated with zinc-based plating and a steel plate overlapped via zinc-based plating. However, when using other high-strength steel sheets with a Ceq of 0.15% by mass or more, cracks are likely to occur on the outside of the corona bond and on the corona bond nugget. Therefore, in the welding method of the present invention, such a steel plate may be particularly targeted.

（亜鉛系めっき）
溶接される鋼板に被覆される亜鉛系めっきは、亜鉛を含むめっきであれば、特に限定するものでなく、たとえば、めっき種として、合金化溶融亜鉛めっき、溶融亜鉛めっき、電気亜鉛めっき、亜鉛・ニッケルめっきが例示される。また、亜鉛・アルミニウム・マグネシウム系のめっきも含むことができる。 (Zinc-based plating)
The zinc-based plating coated on the steel sheet to be welded is not particularly limited as long as it contains zinc. For example, as a plating type, alloyed hot dip galvanizing, hot dip galvanizing, electrogalvanizing, zinc Nickel plating is exemplified. Also, zinc / aluminum / magnesium plating can be included.

＜予め行うスポット溶接＞
上記実施形態に係るスポット溶接方法は、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れの少なくとも一方の割れが発生する溶接箇所に対して採用することが効率的で望ましい。そこで、被溶接部材に溶接電極間の溶接通電終了後、直ちに、溶接電極と被溶接部材とを非接触とするスポット溶接を予め行い、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れを確認し、割れが発生したとき、以降、その溶接箇所に対する溶接において、溶接後保持時間を上記（１）式の範囲内としてスポット溶接すると良い。 <Pre-spot spot welding>
The spot welding method according to the above-described embodiment is efficient and desirable to be applied to a weld location where at least one of the cracks immediately outside the corona bond and the crack at the corona bond nugget occurs. Therefore, immediately after the end of the welding energization between the welding electrodes to the member to be welded, spot welding is performed in advance so that the welding electrode and the member to be welded are not in contact with each other. After confirming that a crack has occurred, it is preferable to perform spot welding with the post-weld retention time within the range of the above formula (1) in the welding to the welded portion.

被溶接部材に予め行うスポット溶接は、溶接電極間の溶接通電終了後、直ちに、溶接電極と被溶接部材とを非接触にして、溶接後保持時間Ｈｔを延長せず、上述のスポット溶接と同様に、２枚の鋼板を重ね合わせて、両側から、２枚の鋼板を挟み込むように、電極を押し付けつつ通電して、断面楕円形状のナゲットを形成する。この際のスポット溶接の条件は、上述と同様の条件を採用することができる。 The spot welding to be performed on the welded member in advance is the same as the above-mentioned spot welding without making the welding electrode and the welded member non-contact immediately after the end of the welding energization between the welding electrodes and extending the holding time Ht after welding. In addition, the two steel plates are overlapped and energized while pressing the electrodes so as to sandwich the two steel plates from both sides, thereby forming a nugget having an elliptical cross section. The conditions similar to the above can be adopted as the spot welding conditions at this time.

そして、重ね合わせ面のコロナボンド直外の割れ及びコロナボンドのナゲット際の割れの発生の有無を確認する。割れの確認は、たとえば、ナゲットを含むように、板厚方向に切断して、その断面を確認して行う。図２に示すような、重ね合わせ面のコロナボンド直外及びコロナボンドのナゲット際の少なくとも一方に割れを確認したとき、総板厚ｔに対する溶接後保持時間Ｈｔが上記（１）式を満足するようにして、以降のスポット溶接を実施する。 And the presence or absence of the generation | occurrence | production of the crack in the corona bond right outside of the overlapping surface and the crack at the time of corona bond nugget is confirmed. For example, the crack is confirmed by cutting in the thickness direction so as to include the nugget and confirming the cross section. As shown in FIG. 2, when a crack is confirmed in at least one of the corona bond immediately outside the corona bond and the nugget of the corona bond, the post-weld holding time Ht with respect to the total thickness t satisfies the above formula (1). Thus, the subsequent spot welding is performed.

特に、溶接電極間の通電終了後、直ちに、溶接電極と被溶接部材とを非接触とするスポット溶接を予め行い、コロナボンド直外の割れ及びコロナボンドのナゲット際の割れの少なくとも一方の割れが発生するか否かを確認し、割れが発生した溶接箇所に対して、以降、本発明の溶接法を採用することが効率的で望ましい。 In particular, immediately after the end of energization between the welding electrodes, spot welding is performed in advance so that the welding electrode and the member to be welded are not in contact with each other, and at least one of the cracks immediately outside the corona bond and the crack at the time of corona bond nugget is cracked. It is efficient and desirable that the welding method of the present invention is employed for the welded portion where the crack is generated after confirming whether or not it occurs.

予め行う被溶接部材へのスポット溶接では、溶接終了の際、溶接電極間の通電終了後、直ちに、溶接電極と被溶接部材とを非接触にして、通電後保持時間Ｈｔを延長しないこと以外、スポット溶接条件を同等にして実施する。たとえば、溶接条件は、特に限定されるものでなく、通常の溶接条件を採用することができる。たとえば、電極をドームラジアス型の先端直径６〜８ｍｍのものとし、加圧力１５００〜６０００Ｎ、通電時間５〜５０サイクル、通電電流４〜１５ｋＡとすることができる。
また、溶接電極間の通電終了後、直ちに、溶接電極と被溶接部材とを非接触にするとは、（１）式の左辺未満の通電後保持時間Ｈｔで溶接電極と被溶接部材とを非接触とすることである。 In spot welding to the member to be welded in advance, at the end of welding, immediately after the end of energization between the welding electrodes, immediately after the energization between the welding electrode and the member to be welded is made non-contact, and the holding time Ht after energization is not extended, Perform spot welding under the same conditions. For example, welding conditions are not particularly limited, and normal welding conditions can be employed. For example, the electrode may have a dome radius type tip diameter of 6 to 8 mm, a pressing force of 1500 to 6000 N, an energization time of 5 to 50 cycles, and an energization current of 4 to 15 kA.
In addition, immediately after energization between welding electrodes is completed, the welding electrode and the member to be welded are not in contact with each other. It is to do.

（コロナボンド直外及びコロナボンドのナゲット際の割れの確認）
予め行うスポット溶接によって得られたスポット溶接継手のコロナボンド直外及びコロナボンドのナゲット際の割れの発生の有無を確認する場合、コロナボンド直外及びコロナボンドのナゲット際の割れの発生の有無の確認方法は、特に限定されるものでなく、ナゲットを含む板厚方向の断面を観察して行う方法や、スポット溶接継手の引張試験を実施して所定の引張強度が得られるか否かで判定して行う方法を用いることができる。または、スポット溶接部を含む板厚方向の断面の切断位置によっては、コロナボンド直外及びコロナボンドのナゲット際の割れが観察できない場合もあるため、Ｘ線透過試験を行って割れを確認してもよい。 (Confirmation of cracks directly outside the corona bond and corona bond nuggets)
When confirming the occurrence of cracks on the corona bond directly outside the corona bond and on the corona bond nugget obtained by spot welding performed in advance, The confirmation method is not particularly limited, and is determined by observing a cross-section in the thickness direction including the nugget or by performing a tensile test of the spot welded joint to obtain a predetermined tensile strength. Can be used. Or, depending on the cutting position of the cross section in the plate thickness direction including the spot welded portion, cracks may not be observed directly outside the corona bond and when corona bond is nuggeted. Also good.

コロナボンド直外及びコロナボンドのナゲット際の割れは、亜鉛系めっきが被覆された鋼板の重ね合わせ面、又は、亜鉛系めっきが被覆された鋼板と亜鉛系めっきを介して重ね合わされている鋼板の面に発生する。そして、これらの鋼板の面のうち、コロナボンド内及びその近傍に発生する。コロナボンド内及びその近傍とは、前記鋼板の面のうち、コロナボンドのナゲット側端部から、コロナボンド直径Ｄｃのおよそ１．２倍の範囲までのことである。 Cracks on the corona bond directly outside and corona bond nugget are caused by the overlapping surface of the steel plate coated with zinc-based plating or the steel plate coated with zinc-based plating and the steel plate overlapped via zinc-based plating. Occurs on the surface. And it generate | occur | produces in the corona bond and its vicinity among the surfaces of these steel plates. In the corona bond and in the vicinity thereof, the surface of the steel sheet is from the nugget side end of the corona bond to a range approximately 1.2 times the corona bond diameter Dc.

（ａ）から（ｄ）までの１つまたは２つ以上の外乱因子がある場合に、上記実施形態のスポット溶接方法（溶接後保持時間を適正範囲とするスポット溶接方法）を行なう場合、あるいは、予めスポット溶接を行ってから上記実施形態のスポット溶接方法（溶接後保持時間を適正範囲とするスポット溶接方法）を行なう場合を説明した。しかしながら、実機の生産ラインにおいては、打角を測定したり、クリアランスを測定したり、電極の軸芯のずれ量を測定したり、鋼板板間の隙間を測定したりすることが手間である場合がある。これら前提条件なしに、最初から上記実施形態のスポット溶接方法を行なえば、コロナボンド直外及びコロナボンドのナゲット際の割れを抑制することができ、より好ましい。 When there is one or more disturbance factors from (a) to (d), when performing the spot welding method of the above embodiment (spot welding method in which the retention time after welding is within an appropriate range), or The case where the spot welding method of the above embodiment (the spot welding method in which the retention time after welding is in an appropriate range) after performing spot welding in advance has been described. However, in actual production lines, it is troublesome to measure the angle of attack, to measure the clearance, to measure the deviation of the axis of the electrode, or to measure the gap between the steel plates. There is. If the spot welding method of the above embodiment is performed from the beginning without these preconditions, it is more preferable because it can suppress cracks immediately outside the corona bond and at the time of corona bond nugget.

次に、本発明の実施例について説明するが、実施例での条件は、本発明の実施可能性及び効果を確認するために採用した一条件例であり、本発明は、この一条件例に限定されるものではない。本発明は、本発明の要旨を逸脱せず、本発明の目的を達成する限りにおいて、種々の条件を採用し得るものである。 Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

（実施例１）
表１に、試験に供した鋼板Ａ〜Ｆについて示す。表１に示す鋼板Ａ〜Ｆは、合金化溶融亜鉛めっき鋼板であり、その両面に亜鉛めっきが被覆されている。
鋼板Ａ〜Ｆに施された合金化溶融亜鉛めっきの被覆層は、焼鈍後に亜鉛めっき浴につけ合金化処理をして形成した。 Example 1
Table 1 shows steel sheets A to F subjected to the test. Steel plates A to F shown in Table 1 are alloyed hot-dip galvanized steel plates, and both surfaces thereof are coated with galvanizing.
The coating layer of the alloyed hot dip galvanizing applied to the steel plates A to F was formed by annealing in a galvanizing bath after annealing.

表２〜５に示すような鋼板の組み合わせについて、表２〜５に示す種々の溶接条件と外乱因子にて、サーボガンタイプの溶接機を用いてスポット溶接を行った。また、全てのスポット溶接において、先端の曲率半径：４０（ｍｍ）の銅製のドームラジアス型で、先端径８（ｍｍ）の電極を用いて溶接した。なお、電極間の通電開始時から溶接終了の際の電極間の通電終了時まで、電極による鋼板の加圧を保持したままとし、加圧中は、加圧力を変えないようにした。 About the combination of the steel plates as shown in Tables 2-5, spot welding was performed using a servo gun type welding machine under various welding conditions and disturbance factors shown in Tables 2-5. In all spot welding, welding was performed using an electrode having a tip diameter of 8 (mm) with a copper dome radius type having a radius of curvature of the tip of 40 (mm). It should be noted that the pressing of the steel sheet by the electrodes was kept from the start of energization between the electrodes to the end of energization between the electrodes at the end of welding, and the applied pressure was not changed during the pressurization.

そして、得られた各々の溶接継手を溶接部の中心線に沿って切断し、断面を観察することで、割れの有無を判断した。さらに、同溶接条件にてＪＩＳＺ３１３７に規定の方法で、スポット溶接継手のＣＴＳ（十字引張力）を測定した。表２〜５に、割れの有無、ＣＴＳ及び溶接継手強度比を示す。 Each of the obtained welded joints was cut along the center line of the welded portion, and the presence or absence of cracks was determined by observing the cross section. Further, the CTS (cross tensile force) of the spot welded joint was measured by the method specified in JIS Z 3137 under the same welding conditions. Tables 2 to 5 show the presence or absence of cracks, the CTS, and the weld joint strength ratio.

溶接継手強度比は、通電後保持時間Ｈｔ以外の溶接条件と外乱因子を同じにして形成したスポット溶接継手のＣＴＳを基準とした比である。例えば、番号１ａの溶接継手強度比は、通電後保持時間Ｈｔ以外同じ溶接条件にてスポット溶接継手を形成した番号１のＣＴＳから番号１ａのＣＴＳを減算し、番号１のＣＴＳで割った値に、１００を掛けて得られた数値である。同様に番号２ｂの溶接継手強度比は、番号２のＣＴＳを基準として計算される値である。なお、溶接継手強度比が３０以上の場合、ＣＴＳが低下したと判定した。表２〜５において、クリアランス１ｍｍとは、イコライジング機構を設けず、電極と鋼板間に１ｍｍのクリアランスが生じた状態を意味する。 The weld joint strength ratio is a ratio based on the CTS of a spot weld joint formed with the same disturbance factors as the welding conditions other than the post-energization holding time Ht. For example, the weld joint strength ratio of No. 1a is a value obtained by subtracting the CTS of No. 1a from the CTS of No. 1 that forms a spot welded joint under the same welding conditions except for the holding time Ht after energization and dividing by the CTS of No. 1 , 100 is a numerical value obtained by multiplying by 100. Similarly, the weld joint strength ratio of No. 2b is a value calculated on the basis of the CTS of No. 2. In addition, when the weld joint strength ratio was 30 or more, it was determined that CTS was lowered. In Tables 2 to 5, the clearance of 1 mm means a state in which an equalizing mechanism is not provided and a clearance of 1 mm is generated between the electrode and the steel plate.

表２及び３の結果では、コロナボンド直外及びコロナボンドのナゲット際に割れが発生していない。これは、総板厚ｔと溶接後保持時間Ｈｔとの関係が（１）式を満足するためであると考えられる。
また、表３の実験例よりも表２の実験例の方が比較的ＣＴＳが高い傾向にあることがわかる。これは、総板厚ｔと溶接後保持時間Ｈｔとの関係が（２）式を満足するためであると考えられる。 In the results of Tables 2 and 3, no cracks occurred in the corona bond directly outside and on the corona bond nugget. This is considered to be because the relationship between the total thickness t and the post-weld holding time Ht satisfies the equation (1).
Further, it can be seen that the experimental example in Table 2 tends to have a relatively high CTS than the experimental example in Table 3. This is presumably because the relationship between the total thickness t and the post-weld holding time Ht satisfies the equation (2).

これに対して、表４の結果では、コロナボンド直外又はコロナボンドのナゲット際に割れが発生した。この割れによって、ＣＴＳの低下も生じた場合があった。これは、総板厚ｔと溶接後保持時間Ｈｔとの関係が（１）式の左辺の関係を満足しないためであると考えられる。 On the other hand, in the result of Table 4, the crack generate | occur | produced in the case of the nugget of corona bond right outside or a corona bond. In some cases, the crack also caused a decrease in CTS. This is considered because the relationship between the total thickness t and the post-weld holding time Ht does not satisfy the relationship on the left side of the equation (1).

また、表５の結果では、上記のような割れは見られなかったものの、十分なＣＴＳが得られなかった。これは、総板厚ｔと溶接後保持時間Ｈｔとの関係が（１）式の右辺の関係を満足しないためであると考えられる。 Moreover, in the result of Table 5, although the above cracks were not seen, sufficient CTS was not obtained. This is considered because the relationship between the total thickness t and the post-weld holding time Ht does not satisfy the relationship on the right side of the equation (1).

次に、各外乱因子による、溶接箇所の変形態様について説明する。図１０は、打角５度でスポット溶接を行った溶接箇所における、板厚方向の断面を光学顕微鏡で撮影した断面写真である。打角を有することにより、ナゲットの左右で鋼板の変形の様子が異なることがわかる。
図１１は、イコライジング機構を設けずに鋼板と電極の間に１ｍｍのクリアランスを設けてスポット溶接を行った場合の断面写真である。溶接箇所において、下側の鋼板と上側の鋼板の変形の様子が異なることがわかる。図１１の場合、溶接電極の先端部から鋼板表面までの距離が、下側よりも上側の方が小さい。 Next, the deformation | transformation aspect of the welding location by each disturbance factor is demonstrated. FIG. 10 is a cross-sectional photograph of a cross section in the plate thickness direction taken with an optical microscope at a welding location where spot welding was performed at a hitting angle of 5 degrees. It can be seen that by having the hitting angle, the deformation state of the steel sheet is different on the left and right sides of the nugget.
FIG. 11 is a cross-sectional photograph when spot welding is performed with a clearance of 1 mm between the steel plate and the electrode without providing an equalizing mechanism. It can be seen that the state of deformation of the lower steel plate and the upper steel plate is different at the weld location. In the case of FIG. 11, the distance from the tip of the welding electrode to the steel sheet surface is smaller on the upper side than on the lower side.

図１２は、対抗する溶接電極の軸芯が１．５ｍｍずれた状態でスポット溶接を行った場合の断面写真である。ナゲットの上下のインデンテーションが非対称となっていることがわかる。
図１３は、溶接箇所の重ね合わせ面の間に片側に打点から２０ｍｍの場所に高さ２ｍｍの板隙がある状態でスポット溶接を行った場合の断面写真である。ナゲットの左右で鋼板の変形の様子が異なることがわかる。 FIG. 12 is a cross-sectional photograph when spot welding is performed in a state where the axis of the opposing welding electrode is shifted by 1.5 mm. It can be seen that the indentation above and below the nugget is asymmetric.
FIG. 13 is a cross-sectional photograph in the case where spot welding is performed in a state where there is a plate gap of 2 mm in height at a location 20 mm from the striking point on one side between the overlapping surfaces of the welding locations. It can be seen that the deformation of the steel sheet is different on the left and right sides of the nugget.

本発明によれば、スポット溶接において、本溶接又は後通電における溶接電極の通電後保持時間を総板厚の関数とするので、コロナボンド直外及びコロナボンドのナゲット際の割れ抑制と継手強度の確保をすることができる。特に、自動車用の亜鉛系めっき高強度鋼板を含む複数枚の鋼板を対象とし、継手の強度に直接影響を与える、鋼板の重ね合わせ面のコロナボンド直外及びコロナボンドのナゲット際に発生する液体金属割れを防ぎ、継手強度を確保することができるスポット溶接方法に関するものである。よって、本発明は、産業上の利用可能性が高いものである。 According to the present invention, in spot welding, since the retention time after energization of the welding electrode in the main welding or post-energization is a function of the total plate thickness, the crack suppression and the joint strength of the corona bond directly outside and on the corrugated nugget are reduced. It can be secured. Especially for multiple steel sheets including zinc-plated high-strength steel sheets for automobiles, which directly affects the strength of the joints. The liquid generated directly outside the corona bond and the corona bond nugget on the overlapping surface of the steel sheets. The present invention relates to a spot welding method capable of preventing metal cracking and ensuring joint strength. Therefore, the present invention has high industrial applicability.

１鋼板
２ナゲット
３鋼板の表面からナゲットに向かって進展する割れ
４熱影響部
５肩部から熱影響部に向かって進展する割れ
６コロナボンド直外の割れ
７コロナボンドのナゲット際に割れ
８溶接電極
９軸芯
１０垂線
１１他の部材
Ａ総板厚と通電後保持時間の関係における割れ発生が無くなる境界線
Ｂ総板厚と通電後保持時間の関係における十分なＣＴＳとなる境界線
Ｄｃコロナボンド直径 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Nugget 3 Crack which progresses from steel plate surface toward nugget 4 Heat-affected zone 5 Crack which progresses from shoulder to heat-affected zone 6 Crack just outside corona bond 7 Crack when corona bond nugget 8 Weld Electrode 9 Axis core 10 Perpendicular line 11 Other members A Boundary line where cracks do not occur in the relationship between total plate thickness and holding time after energization B Boundary line that provides sufficient CTS in the relationship between total plate thickness and holding time after energization Dc Coronabond diameter

Claims

A spot welding method in which at least one energization step of pre-energization and post-energization is performed, in which main welding is performed by pressurizing a welded member composed of a plurality of steel plates on which at least welding points are overlapped with a welding electrode. Because
At least one of the plurality of steel plates is a high strength steel plate having a tensile strength of 780 MPa or more,
For at least one of the plurality of steel plates, at least the overlapping surface of the welded portions is coated with zinc-based plating, and the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more,
From the start of energization between the welding electrodes to the end of energization between the welding electrodes at the end of welding, the pressurization of the welded member by the welding electrodes is kept,
At the end of welding, the post-energization holding time Ht (seconds) from the end of energization between the welding electrodes until the welding electrode and the member to be welded are brought into contact with each other is within the range of the following formula ( 4 ). A spot welding method characterized by the above.
0.015t ² + 0.110 ≦ Ht ≦ 0.16t ² −0.40t + 0.53 ( 4 )

2. The spot welding method according to claim 1 , wherein a total plate thickness t (mm) of the plurality of steel plates is 2.4 mm or greater and 3.2 mm or less.

The spot welding according to claim 1 or 2 , wherein one or more of the following conditions (a) to (d) are satisfied immediately before the welding electrode is brought into contact with the member to be welded. Method.
(A) a state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet in contact with the welding electrode are not parallel;
(B) a state in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip;
(C) a state in which there is no axial center of the other welding electrode on an extension line of the axial center of the one welding electrode; and
(D) A state having a gap between the overlapping surfaces of the welding locations

In the spot welding method according to any one of claims 1 to 3 ,
When the post-energization is not performed, spot welding is performed in advance so that the welding electrode and the welded member are not in contact with the welded member immediately after the end of energization between the welding electrodes in the main welding. The presence or absence of cracks immediately outside the corona bond and the corona bond nugget on the overlapped surface of the steel sheets was confirmed, and at least one of the cracks immediately outside the corona bond and the crack at the corona bond nugget occurred. When
Spot welding method, wherein welding is performed within the range of the holding time after energization.

In the spot welding method according to any one of claims 1 to 3 ,
When the post-energization is performed, spot welding is performed in advance so that the welding electrode and the welded member are not contacted immediately after the energization between the welding electrodes in the post-energization is completed on the welded member. When there is a crack on the overlapping surface of the corona bond and a crack on the corona bond nugget, and at least one of the crack on the corona bond and the corona bond nugget occurs ,
Spot welding method, wherein welding is performed within the range of the holding time after energization.