JP5633483B2 - Prediction method of cross tensile strength in spot welding of quenched steel sheet and spot welding method using the prediction method - Google Patents
Prediction method of cross tensile strength in spot welding of quenched steel sheet and spot welding method using the prediction method Download PDFInfo
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本発明は、自動車分野等で使用される焼き入れ処理された鋼板のスポット溶接に関するものである。 The present invention relates to spot welding of a quenched steel plate used in the automotive field and the like.
自動車業界では、燃費向上のため、車体などの鋼板により高強度の鋼板を適用し、その板厚を薄くして車体の軽量化が図られている。一方、自動車の衝突時においても安全性をより確保する観点から、車体を軽量化しつつ、高い車体強度を維持する必要性も出てきている。
しかし、鋼板の高強度化はプレス成形性の低下を招き、特にスプリングバック等により製品精度の確保がより難しくなってくる。
In the automobile industry, in order to improve fuel efficiency, a high-strength steel plate is applied to a steel plate such as a vehicle body, and the plate thickness is reduced to reduce the weight of the vehicle body. On the other hand, from the viewpoint of ensuring safety even in the event of a car collision, there is a need to maintain high vehicle body strength while reducing the weight of the vehicle body.
However, increasing the strength of the steel sheet causes a decrease in press formability, and in particular, it becomes more difficult to ensure product accuracy due to springback or the like.
このような要求を満たすために、鋼板の高強度化と加工性、製品精度を同時に満足する手法として、加熱した鋼板をプレス成形する熱間プレス法（プレスクエンチ法）が開発され、それに適する鋼板も、特許文献１に示すように開発されている。
In order to satisfy such requirements, a hot press method (press quench method) that press-forms heated steel plates has been developed as a method that satisfies the requirements of high strength, workability, and product accuracy of steel plates at the same time. Has also been developed as shown in
この熱間プレス法は、鋼板を約８００℃以上のオーステナイト域まで加熱した後、プレス成形し、同時に成形後の冷却により焼き入れを行い高強度の材質を得るものである。この熱間プレス法では、鋼板が高温で軟質、高延性になっているため、成形時の割れ発生等の加工性が改善され、かつ、比較的良好な製品精度を有する部品の製造が可能となる。 In this hot pressing method, a steel sheet is heated to an austenite region of about 800 ° C. or higher, then press-formed, and at the same time, quenched by cooling after forming to obtain a high-strength material. In this hot pressing method, the steel sheet is soft and highly ductile at high temperatures, so that workability such as cracking during molding is improved, and parts with relatively good product accuracy can be manufactured. Become.
ところで、プレス成形された部材の接合には、スポット溶接が使用される場合が多い。熱間プレス法で所定形状に成形された鋼板は、焼入れ組織により高強度化されているため、スポット溶接時の入熱条件によっては、熱影響部（ＨＡＺ）が焼き戻され、熱間プレス・焼入れ後の母材硬度よりも硬度が低い軟化部が熱影響部に形成される場合がある。
そのような軟化部が形成されると、スポット溶接部の引張強さに影響を与えることが予想される。
By the way, spot welding is often used for joining press-formed members. Since the steel sheet formed into a predetermined shape by the hot pressing method is strengthened by the quenching structure, the heat affected zone (HAZ) may be tempered depending on the heat input conditions during spot welding, A softened portion having a lower hardness than the base material hardness after quenching may be formed in the heat affected zone.
When such a softened portion is formed, it is expected to affect the tensile strength of the spot welded portion.
スポット溶接部の引張強さは、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）によって評価されているが、熱間プレス法で成形され、焼入れ処理された鋼板のような引張強度が９８０ＭＰａ以上の高強度鋼板をスポット溶接した場合には、溶接継手部において溶接端部への力学的拘束が強まることでの応力集中と、靱性の低下が生じやすい。そのような溶接継手部における靱性の不足はＣＴＳの試験で顕著に現れるため、高強度鋼板のスポット溶接継手においては、ＣＴＳが設定した値を超えるように管理されている。
しかし、焼入れ処理された鋼板で形成される上記軟化部の軟化度合とＣＴＳとの関連については十分に明らかにされていない。
The tensile strength of spot welds is evaluated by the tensile shear strength (TSS) measured by applying a tensile load in the shear direction and the cross tensile strength (CTS) measured by applying a tensile load in the peeling direction. However, when spot-welding a high-strength steel sheet having a tensile strength of 980 MPa or more, such as a steel sheet formed by a hot press method and quenched, there is a mechanical constraint on the weld end at the weld joint. Stress concentration due to strengthening and a decrease in toughness are likely to occur. Since such a lack of toughness in the welded joint portion appears remarkably in the CTS test, the spot welded joint of the high-strength steel sheet is managed so that the CTS exceeds the set value.
However, the relation between the degree of softening of the softened portion formed of the quenched steel plate and CTS has not been sufficiently clarified.
上記軟化部の軟化度合は、素材の強度やスポット溶接の通電条件によって、様々に変化するため、従来は溶接後の硬さ測定でしかわからなかった。
そこで、本発明では、熱間プレス法などの適用によって焼き入れ処理された鋼材や鋼板（以下、「焼入鋼板」と記載する。）のスポット溶接において、熱影響部に生じる軟化部のＣＴＳに対する影響を評価して、溶接条件から軟化部の軟化度合（軟化度）を予測し、軟化度からＣＴＳを予測できるようにして、ＣＴＳの予測値に基づいて溶接条件を変更し、ＣＴＳが設定した値を超えることができるようにすることを課題とする。
Since the degree of softening of the softened portion varies depending on the strength of the material and the energization conditions of spot welding, conventionally, it has been known only by measuring the hardness after welding.
Therefore, in the present invention, in spot welding of a steel material or a steel plate (hereinafter referred to as “quenched steel plate”) that has been quenched by application of a hot press method or the like, the CTS of a softened portion that occurs in a heat-affected zone. The impact was evaluated, the degree of softening of the softened part (softening degree) was predicted from the welding conditions, the CTS was predicted from the softening degree, the welding conditions were changed based on the predicted value of CTS, and CTS was set The problem is to be able to exceed the value.
本発明者らは、スポット溶接の際の溶接熱影響部の温度履歴について検討した。温度履歴の数値解析の結果、溶接時の入熱によっては、ナゲットの溶融境界線（ＦＬ）近傍でＡ３点を超えて加熱される領域が存在するのに対し、ＦＬからの距離によってはＡ１点以下の温度までしか加熱されない領域が存在することが知見された。
Ａ３点を超えて加熱される領域では冷却の際に再度焼きが入って硬化するが、Ａ１点以下の温度までしか加熱されない領域では焼入相の焼戻しが起こり軟化することが予想される。
そこで、熱影響部の軟化量を、焼戻しパラメータで評価することを試みた。そして、スポット溶接の際の温度履歴の数値解析を行い、その結果から熱影響部の焼きが入らない領域における焼き戻しパラメータを導出し、焼き戻しパラメータから軟化量を予測できること見出した。また、実際に測定した軟化量と十字引張強さＣＴＳの関係から、溶接条件毎にＣＴＳを予測できることを見出した。
The present inventors examined the temperature history of the weld heat affected zone during spot welding. As a result of numerical analysis of temperature history, depending on the heat input during welding, there is a region heated beyond the A3 point in the vicinity of the nugget melting boundary line (FL), whereas depending on the distance from the FL, the A1 point It has been found that there are regions that can only be heated to the following temperatures.
In the region heated beyond the A3 point, it is rebaked and hardened during cooling, but in the region heated only to a temperature below the A1 point, it is expected that the tempered phase will be tempered and softened.
Therefore, an attempt was made to evaluate the amount of softening of the heat-affected zone using tempering parameters. Then, numerical analysis of the temperature history at the time of spot welding was performed, and from the results, a tempering parameter in a region where the heat-affected zone was not tempered was derived, and the softening amount was predicted from the tempering parameter. Moreover, it discovered that CTS was predictable for every welding condition from the relationship between the softening amount actually measured and the cross tensile strength CTS.
そのような知見に基づいてなされた本発明の要旨は次のとおりである。
（１） 焼き入れ処理された鋼板のスポット溶接において、
（Ａ）本溶接の前に、あらかじめ、
（a1）板厚ｓの焼き入れ処理された鋼板を種々の通電条件でスポット溶接し、通電条件毎に、ナゲット径Ｌと溶接熱影響部に発生する軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳを測定し、
（a2）前記の通電条件毎に溶接熱影響部の温度履歴を計算し、該温度履歴から焼戻しパラメータλを計算して、焼戻しパラメータλと軟化量ΔＨｖとの関係を求め、
（a3）軟化量ΔＨｖから下記（１）式を用いて通電条件毎の軟化度合Ｎを計算し、
（a4）十字引張強さＣＴＳを、下記（２）式を用いて軟化度合Ｎと板厚ｓとナゲット径Ｌの関数として予測しておき、
（Ｂ）ついで、板厚ｓの鋼板の本溶接にあたり、
（b1）設定した本溶接の通電条件から前記（a2）と同様にして焼戻しパラメータλを計算し、
（b2）焼戻しパラメータλから前記（a2）で求めた関係を用いて軟化量ΔＨｖを求め、
（b3）軟化量ΔＨｖから下記（１）式を用いて軟化度合Ｎを求め、
（b4）軟化度合Ｎから下記（２）式を用いて設定した通電条件で溶接したスポット溶接継手の十字引張強さＣＴＳを求める、
ことを特徴とする焼き入れ処理された鋼板のスポット溶接における十字引張強さＣＴＳの予測方法。
ここで、Ａ、α、β、γは定数であり、それぞれ０．１０≦Ａ≦０．１５、０．６≦α≦０．７、β＝０．５である。
The gist of the present invention made based on such knowledge is as follows.
(1) In spot welding of quenched steel plates,
(A) Before the main welding,
(A1) A steel sheet having a thickness s that has been quenched is spot-welded under various energization conditions, and for each energization condition, the nugget diameter L, the softening amount ΔHv of the softened portion generated in the weld heat affected zone, and the cross of the welded joint Measure the tensile strength CTS,
(A2) Calculate the temperature history of the welding heat-affected zone for each energization condition, calculate the tempering parameter λ from the temperature history, and obtain the relationship between the tempering parameter λ and the softening amount ΔHv,
(A3) The softening degree N for each energization condition is calculated from the softening amount ΔHv using the following equation (1),
(A4) The cross tensile strength CTS is predicted as a function of the softening degree N, the plate thickness s, and the nugget diameter L using the following equation (2).
(B) Next, in the main welding of the steel sheet having a thickness of s,
(B1) The tempering parameter λ is calculated in the same manner as in (a2) from the set current-carrying conditions,
(B2) The softening amount ΔHv is obtained from the tempering parameter λ using the relationship obtained in (a2) above,
(B3) The softening degree N is obtained from the softening amount ΔHv using the following equation (1),
(B4) Obtain the cross tensile strength CTS of the spot welded joint welded from the softening degree N under the energization conditions set using the following equation (2).
A method for predicting a cross tensile strength CTS in spot welding of a quenched steel sheet.
Here, A, α, β, and γ are constants, and 0.10 ≦ A ≦ 0.15, 0.6 ≦ α ≦ 0.7, and β = 0.5, respectively.
（２） 焼き入れ処理された鋼板のスポット溶接において、鋼板の板厚と設定した通電条件から請求項１に記載の予測方法によって十字引張強さＣＴＳを予測し、予測された十字引張強さＣＴＳの値があらかじめ設定した基準値を下回るときは、再度通電条件を設定して十字引張強さＣＴＳの値を予測し、その予測値が基準値を上回る通電条件でスポット溶接を実施することを特徴とする焼き入れ処理された鋼板のスポット溶接方法。
(2) In spot welding of a quenched steel plate, the cross tensile strength CTS is predicted by the prediction method according to
本発明によれば、焼入鋼板のスポット溶接において、溶接条件から溶接継手のＣＴＳを予測することができるので、必要なＣＴＳの値に応じた溶接条件を採用してスポット溶接することができる。このため、設定されたＣＴＳの値を満たす継手強度を有し、信頼性の高いスポット溶接継手を得ることができる。 According to the present invention, in spot welding of a hardened steel plate, the CTS of a welded joint can be predicted from the welding conditions, so that it is possible to perform spot welding by employing welding conditions according to the required CTS value. For this reason, it is possible to obtain a spot-welded joint having a joint strength satisfying the set CTS value and having high reliability.
以下、添付の図面を参照して本発明の実施の形態を説明する。
本発明では、本溶接の前に、種々の組成や強度を有する焼入鋼板を用いて溶接試験を行い、様々なサンプルを得て、そのサンプルの熱影響部の硬さ分布及びＣＴＳなどを測定し、用いた溶接条件の温度履歴から、焼入鋼板ごとに軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳの関係をあらかじめ求めておき、実際の本溶接に当たり、用いる溶接条件の温度履歴から、その溶接条件で溶接して得られる溶接継手のＣＴＳを溶接前に予測できるようにする。
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
In the present invention, prior to the main welding, a welding test is performed using hardened steel sheets having various compositions and strengths, various samples are obtained, and the hardness distribution and CTS of the heat affected zone of the samples are measured. Then, from the temperature history of the welding conditions used, the relationship between the softening amount ΔHv of the softened portion and the cross tensile strength CTS of the welded joint is obtained in advance for each quenched steel sheet, and the temperature of the welding conditions used in actual actual welding. From the history, the CTS of the welded joint obtained by welding under the welding conditions can be predicted before welding.
そのような軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳを予測する手順について、図１、２を参照して詳細に説明する。
（Ａ）板厚ｓの焼き入れ処理された鋼板のスポット溶接において、本溶接の前に、あらかじめ、（（ａ１）軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳを測定し、（ａ２）溶接熱影響部の温度履歴から焼戻しパラメータλを計算して、焼戻しパラメータλと軟化量ΔＨｖとの関係を求め、（ａ３）軟化量ΔＨｖから通電条件毎の軟化度合Ｎを計算し、十字引張強さＣＴＳと軟化度合Ｎの関係を求め、さらに（ａ４）軟化度合Ｎと板厚ｓと通電条件から決め合うナゲット径Ｌの関数としてのＣＴＳ予測式の作成しておく。
これら（ａ１）〜（ａ４）のステップについて、次に説明する。
A procedure for predicting the softening amount ΔHv of the softened portion and the cross tensile strength CTS of the welded joint will be described in detail with reference to FIGS.
(A) In spot welding of a steel sheet having a thickness s that has been quenched, prior to the main welding, ((a1) the softening amount ΔHv of the softened portion and the cross tensile strength CTS of the welded joint are measured, a2) Calculate the tempering parameter λ from the temperature history of the weld heat affected zone to determine the relationship between the tempering parameter λ and the softening amount ΔHv. (a3) Calculate the softening degree N for each energizing condition from the softening amount ΔHv, The relationship between the tensile strength CTS and the softening degree N is obtained, and (a4) a CTS prediction formula as a function of the nugget diameter L determined from the softening degree N, the plate thickness s, and the energization conditions is prepared.
Next, the steps (a1) to (a4) will be described.
（ａ１）軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳの測定
種々の板厚ｓの焼き入れ処理された鋼板を種々の通電条件でスポット溶接し、多数のスポット溶接継手のサンプルを作成し、得られたサンプルからナゲットを含む試験片を採取し、通電条件毎に、溶接熱影響部に発生する軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳを測定する。
溶接部の硬度は、得られたサンプルからナゲットを含む断面を切り出し、図２に示すように、鋼板の圧接面より０．２ｍｍずれた線に沿って、ナゲットの境界（溶融境界線ＦＬ）からほとんど軟化しない位置までのビッカース硬度Ｈｖを測定して求める。そして、図１(ａ)に示すようなＦＬからの距離に対するＨｖの変化の図を作成し、図１(ｂ)に示すように、焼入部平均硬さ（母材平均硬さ）と測定した硬さの差を軟化量ΔＨｖとして求める。
また、ＣＴＳは、得られたサンプルからJIS Z 3137に基づく引張り試験を実施して求める。
(A1) Measurement of softening amount ΔHv of softened part and cross tensile strength CTS of welded joint Spot-welded steel sheets with various thicknesses s were spot-welded under various energizing conditions, and a number of spot welded joint samples were obtained. A test piece including a nugget is collected from the prepared sample, and the softening amount ΔHv of the softened portion generated in the weld heat affected zone and the cross tensile strength CTS of the welded joint are measured for each energization condition.
The hardness of the welded portion is obtained by cutting a cross-section including the nugget from the obtained sample and, as shown in FIG. 2, from the nugget boundary (melting boundary line FL) along a line shifted by 0.2 mm from the pressure contact surface of the steel plate. The Vickers hardness Hv up to a position where it hardly softens is determined by measurement. And the figure of the change of Hv with respect to the distance from FL as shown to Fig.1 (a) was created, and as shown in FIG.1 (b), it was measured with the quenching part average hardness (base material average hardness). The difference in hardness is obtained as the softening amount ΔHv.
CTS is obtained by conducting a tensile test based on JIS Z 3137 from the obtained sample.
（ａ２）焼戻しパラメータλと軟化量ΔＨｖとの関係の算出
焼戻しパラメータλと軟化量ΔＨｖとの関係を求めるには、
（ｉ）前記の通電条件毎に溶接熱影響部の温度履歴を計算し、
（ｉｉ）該温度履歴から通電条件毎の焼戻しパラメータλを計算し、
（iｉi）焼戻しパラメータλと前記（ａ１）で求めた通電条件毎の軟化量ΔＨｖから、焼戻しパラメータλと軟化量ΔＨｖとの関係を求める。
(A2) Calculation of the relationship between the tempering parameter λ and the softening amount ΔHv To obtain the relationship between the tempering parameter λ and the softening amount ΔHv,
(I) Calculate the temperature history of the weld heat affected zone for each of the energization conditions,
(Ii) calculating a tempering parameter λ for each energization condition from the temperature history;
(Iii) The relationship between the tempering parameter λ and the softening amount ΔHv is obtained from the tempering parameter λ and the softening amount ΔHv for each energization condition obtained in the above (a1).
通電条件毎の溶接熱影響部の温度履歴の計算は、例えば次のように行う。
スポット溶接時の温度履歴解析は、市販ソフト、例えばQucik Spot（商品名）を用いて行うことができる。この解析では、まず、スポット溶接しようとする焼入鋼板の物性値（抵抗値、熱伝導率、高温強度）を、焼入鋼板を用いて予め実測しておき、その物性値に基づいて電磁場解析を行なう。そして、その結果から得られる電流分布による抵抗発熱を熱源とした熱伝導解析を行ない、その結果から得られる温度分布による熱膨張と電極加圧とについての弾塑性解析を行ない、さらに、これらの解析結果に対応するように物性値を書き換えるというステップを指定した時間刻み毎に実行し、溶接時の温度履歴を導出する。その際、１５００℃に達した領域をナゲットとみなして計算を行う。
数値解析は、スポット溶接の通電パターンを設定し、鋼板圧接面より０．２ｍｍずれた線上の、ナゲット中心、ＦＬ内側０．５ｍｍから外側３．０ｍｍまでの位置において行い、図１（ｃ）に示すような各位置での溶接時の温度履歴を導出する。
Calculation of the temperature history of the welding heat affected zone for each energization condition is performed as follows, for example.
The temperature history analysis at the time of spot welding can be performed using commercially available software such as Qucik Spot (trade name). In this analysis, first, the physical properties (resistance value, thermal conductivity, high temperature strength) of a hardened steel plate to be spot welded are measured in advance using a hardened steel plate, and an electromagnetic field analysis is performed based on the physical property values. To do. Then, heat conduction analysis was performed using resistance heat generation due to the current distribution obtained from the results as a heat source, and elastoplastic analysis was performed on the thermal expansion and electrode pressurization based on the temperature distribution obtained from the results. The step of rewriting the physical property value so as to correspond to the result is executed at every specified time step, and the temperature history at the time of welding is derived. At that time, the calculation is performed by regarding the region reaching 1500 ° C. as a nugget.
Numerical analysis is performed by setting a spot welding current pattern, and is performed at a position from the nugget center, 0.5 mm inside FL to 3.0 mm outside, on a line shifted 0.2 mm from the steel plate pressure contact surface, as shown in FIG. The temperature history during welding at each position as shown is derived.
求めた温度履歴から焼戻しパラメータλを求める計算は次のように行う。
軟化領域は、焼き戻しにより軟化していると考えられる。焼き戻しによる軟化の程度を表わすパラメータとして、式（３）で定義される焼き戻しパラメータが用いられている。
そこで、焼戻しパラメータλを計算して、焼戻しパラメータλと軟化量ΔＨｖとの関係を求めるようにする。
λ ＝ Ｔ（logｔ＋Ｄ） ・・・（３）
なお、Ｔは絶対温度で表される温度、ｔは加熱時間であり、Ｄは定数であり、通常は２０が用いられる。
Calculation for obtaining the tempering parameter λ from the obtained temperature history is performed as follows.
It is considered that the softened region is softened by tempering. As a parameter representing the degree of softening due to tempering, the tempering parameter defined by equation (3) is used.
Therefore, the tempering parameter λ is calculated to obtain the relationship between the tempering parameter λ and the softening amount ΔHv.
λ = T (logt + D) (3)
T is a temperature expressed in absolute temperature, t is a heating time, D is a constant, and 20 is usually used.
式（３）は一定温度での焼戻しについてのものであり、焼き戻しパラメータに時間ｔはlogの形で寄与するため、スポット溶接のような時間で変化する温度履歴においては、そのまま加算して適用することはできない。そこで、土山の提案した一定温度での焼き戻しに換算する方法（「熱処理」Vol.42, No.3, 163-168頁(2002.06.28)参照）を用いる。 Equation (3) is for tempering at a constant temperature, and since time t contributes to the tempering parameter in the form of log, it is applied as it is in the temperature history that changes with time such as spot welding. I can't do it. Therefore, a method of conversion to tempering at a constant temperature proposed by Tsuchiyama (see “Heat Treatment” Vol. 42, No. 3, pp. 163-168 (2002.06.28)) is used.
例として、まず２段階の温度履歴を考える。ｔ0からｔ1の時間までを温度Ｔ1で、続けてｔ1からｔ2の時間までを温度Ｔ2で焼き戻すことを考える。
この場合、前半だけの焼き戻しにおける焼き戻しパラメータは、
λ1＝ Ｔ1｛log（ｔ1−ｔ0）＋Ｄ｝ ・・・（３ａ）
になり、温度Ｔ2で同等の焼き戻しを加えるには、式（３ｂ）を満たす時間ｔ2´が必要になる。
λ1＝ Ｔ2（logｔ'2 ＋Ｄ） ・・・（３ｂ）
そこで、この２段の焼き戻しを、時間（ｔ2−ｔ1＋ｔ'2 ）の一定温度Ｔ2での焼き戻しとして、式（３Ｃ）のように決定することができる。
λ＝ Ｔ2｛log（ｔ2−ｔ1＋ｔ'2 ）＋Ｄ｝ ・・・（３ｃ）
この論法を逐次繰り返し、温度履歴の計算結果から得られるn点の時間と温度の系列を温度変化をn段階(n>１)の焼き戻しとして、ｋ番目(ｋ>１)での焼き戻しパラメータλｋ
λｋ＝Ｔk｛log（ｔk−ｔk-1＋ｔ'k-1）＋Ｄ｝ ・・・（３ｄ）
を逐次計算し、温度履歴全体に対する焼き戻しパラメータを、最終温度Ｔnにおける一定温度での焼き戻しに換算して焼き戻しパラメータλとして以下の式（３ｅ）のように決定する。
λ＝λn＝Ｔn｛log（ｔn−ｔn-1＋ｔ'n-1）＋Ｄ｝ ・・・（３ｅ）
As an example, first consider a two-stage temperature history. at a temperature T 1 until the time t 1 from t 0, consider that the tempering at a temperature T 2 from t 1 to time t 2 to continue.
In this case, the tempering parameters in the tempering only in the first half are
λ 1 = T 1 {log (t 1 −t 0 ) + D} (3a)
Thus, in order to perform the same tempering at the temperature T 2 , a time t 2 ′ satisfying the expression (3b) is required.
λ 1 = T 2 (logt ′ 2 + D) (3b)
Therefore, this two-stage tempering can be determined as shown in Expression (3C) as tempering at a constant temperature T 2 for a time (t 2 −t 1 + t ′ 2 ).
λ = T 2 {log (t 2 −t 1 + t ′ 2 ) + D} (3c)
Repeating this theory sequentially, the n-point time and temperature series obtained from the temperature history calculation results are tempered at the nth stage (n> 1), and the tempering parameters at the kth (k> 1) λ k
λ k = T k {log (t k −t k−1 + t ′ k−1 ) + D} (3d)
Sequentially calculates the tempering parameter for the entire temperature history is determined by the following equation (3e) as a parameter λ tempering in terms of tempering at a constant temperature at the final temperature T n.
λ = λ n = T n {log (t n −t n−1 + t ′ n−1 ) + D} (3e)
そして、温度履歴の数値解析の結果から上記式（３ｅ）に基づいて求めた焼き戻しパラメータλと前記（ａ１）のステップで求めた軟化量ΔＨｖとから、図１（ｄ）に示されるような軟化量ΔＨｖと焼き戻しパラメータλとの間の関係を求める。 Then, from the result of numerical analysis of the temperature history, the tempering parameter λ obtained based on the above equation (3e) and the softening amount ΔHv obtained in the step (a1) as shown in FIG. A relationship between the softening amount ΔHv and the tempering parameter λ is obtained.
（ａ３）軟化量ΔＨｖから軟化度合Ｎの算出
軟化部の軟化度合Ｎを、図１(ｂ)に示すように、ＦＬからの距離に対するＨｖの変化を表す線における焼入部平均硬さより硬さが低い部分の占める面積と定義する。そして、軟化量ΔＨｖから下記（１）式を用いて通電条件毎の軟化度合Ｎを計算し、前記（ａ１）のステップで得られた通電条件毎の十字引張強さＣＴＳの値を用いて十字引張強さＣＴＳと軟化度合Ｎの関係を求める。
なお、（１）式の積分はナゲット中心から母材方向へ向かう硬さ測定経路上において、ほとんど軟化せずに焼入母材の硬さに一致するところまで行なう。
It should be noted that the integration of equation (1) is performed up to the hardness of the hardened base material with almost no softening on the hardness measurement path from the nugget center toward the base material.
（ａ４）軟化度合Ｎと板厚ｓとナゲット径Ｌの関数としてのＣＴＳ予測式の作成
以上の結果を種々の板厚で得て、軟化度合Ｎと板厚ｓとを関数として回帰分析を行ない、ＣＴＳと軟化度合Ｎと板厚ｓとの関係を式（４）の形で得る。
ＣＴＳ＝Ａ・Ｎα・ｔβ ・・・（４）
ここで、Ａ、α、βは定数であり、以上のような溶接試験結果から実験的に決定される値である。
(A4) Creation of CTS prediction formula as a function of softening degree N, thickness s, and nugget diameter L The above results are obtained with various thicknesses, and regression analysis is performed with softening degree N and thickness s as functions. The relationship among CTS, softening degree N and plate thickness s is obtained in the form of equation (4).
CTS = A · N α · t β (4)
Here, A, α, and β are constants, and are values determined experimentally from the above-described welding test results.
通電条件から決められるナゲット径ＬはＣＴＳに二乗で効くと考えられるので、ＣＴＳを軟化度合Ｎと板厚ｓとナゲット径Ｌの関数として、式（２）から予測できる。
ＣＴＳ＝Ａ・Ｎα・ｓβ・Ｌ２ ・・・（２）
本発明らは、複数の板厚ｓの鋼板を、ナゲット径Ｌ＝５√（ｓ）の複数の通電条件で溶接試験を実施し、得られたＣＴＳと軟化度合Ｎの値から、それぞれ０．１０≦Ａ≦０．１５、０．６≦α≦０．７、β＝０．５であると決定した。
Since the nugget diameter L determined from the energization condition is considered to be effective in the square of the CTS, the CTS can be predicted from the equation (2) as a function of the softening degree N, the plate thickness s, and the nugget diameter L.
CTS = A · N α · s β · L 2 (2)
The present inventors conducted welding tests on steel sheets having a plurality of thicknesses s under a plurality of energizing conditions with a nugget diameter L = 5√ (s), and from the obtained values of CTS and softening degree N, 0. It was determined that 10 ≦ A ≦ 0.15, 0.6 ≦ α ≦ 0.7, and β = 0.5.
（Ｂ）ついで、板厚ｓの焼入鋼板を本溶接するにあたり、（ｂ１）設定した本溶接の通電条件から焼戻しパラメータλを計算し、（ｂ２）焼戻しパラメータλから前記（ａ２）のステップで求めた関係を用いて軟化量ΔＨｖを求め、（ｂ３）軟化量ΔＨｖから軟化度合Ｎを求め、（ｂ４）軟化度合Ｎから前記（ａ３）のステップで求めた関係を用いて設定した通電条件で溶接したスポット溶接継手のＣＴＳを求めるようにする。
これら（ｂ１）〜（ｂ４）のステップについて、次に説明する。
(B) Next, in the main welding of the hardened steel sheet having a thickness of s, (b1) the tempering parameter λ is calculated from the set current welding conditions, and (b2) the tempering parameter λ is used in the step (a2). Using the obtained relationship, the softening amount ΔHv is obtained, (b3) the softening degree N is obtained from the softening amount ΔHv, and (b4) the energization condition set by using the relation obtained in the step (a3) from the softening degree N. The CTS of the welded spot welded joint is obtained.
Next, the steps (b1) to (b4) will be described.
（ｂ１）設定した本溶接の通電条件から焼戻しパラメータλの算出
設定した本溶接の通電条件から前記（ａ２）のステップと同様にして焼戻しパラメータλを計算する。
（ｂ２）焼戻しパラメータλから軟化量ΔＨｖの算出
焼戻しパラメータλから前記（ａ２）のステップで求めた図１（ｄ）で示すような関係を用いて軟化量ΔＨｖを求める。
(B1) Calculation of the tempering parameter λ from the set current welding energization condition The tempering parameter λ is calculated from the set current welding energization condition in the same manner as in the step (a2).
(B2) Calculation of the softening amount ΔHv from the tempering parameter λ The softening amount ΔHv is obtained from the tempering parameter λ using the relationship shown in FIG. 1 (d) obtained in the step (a2).
（ｂ３）軟化量ΔＨｖから軟化度合Ｎの算出
前記（ｂ２）で求めた軟化量ΔＨｖから下記（１）式を用いて軟化度合Ｎを求める。
（ｂ４）設定した通電条件で溶接したスポット溶接継手のＣＴＳの算出
上記（ｂ３）で得られた軟化度合Ｎ、鋼板板厚ｓ、設定した通電条件から決定されるナゲット径Ｌを用いて、前記（ａ４）のステップで求めた、下記（２）式を用いて設定した通電条件で溶接したスポット溶接継手のＣＴＳを求める。
ＣＴＳ＝Ａ・Ｎα・ｓβ・Ｌ２ ・・・（２）
(B4) Calculation of CTS of spot welded joint welded under set energization condition Using the nugget diameter L determined from the softening degree N, the steel plate thickness s, and the set energization condition obtained in (b3) above, The CTS of the spot welded joint welded under the energization conditions set by using the following equation (2) obtained in the step (a4) is obtained.
CTS = A · N α · s β · L 2 (2)
（Ｃ）焼き入れ処理された鋼板のスポット溶接において、設定した通電条件から上記（Ａ）及び（Ｂ）の方法に基づいて予測された十字引張強さＣＴＳの値があらかじめ設定した基準値を下回るときは、設定されたＣＴＳの値から、そのＣＴＳを満たす軟化度合Ｎの条件を求め、再度通電条件を設定してその通電条件での温度履歴解析を行って、焼戻しパラメータを求め、焼戻しパラメータから上記（ｂ２）及び（ｂ３）のステップを通じて軟化度合Ｎを求め、その軟化度合Ｎが前述の条件を満たしているかどうかを判断し、最終的に、設定されたＣＴＳの基準値を上回ることができる通電条件を得て、その条件でスポット溶接を実施するようにする。 (C) In spot welding of a steel sheet that has been quenched, the value of the cross tensile strength CTS predicted based on the set energization conditions based on the above methods (A) and (B) is lower than a preset reference value. When the value of the softening degree N satisfying the CTS is obtained from the set CTS value, the energization condition is set again, the temperature history analysis is performed under the energization condition, the tempering parameter is obtained, and the tempering parameter is obtained. The degree of softening N can be obtained through the steps (b2) and (b3) above, and it can be determined whether the degree of softening N satisfies the above-mentioned conditions. Finally, it can exceed the set reference value of CTS. Obtain the energization conditions and perform spot welding under those conditions.
本発明は、以上説明したように構成されるものであるが、以下、実施例を用いて、本発明の実施可能性及び効果についてさらに説明する。 Although the present invention is configured as described above, the feasibility and effects of the present invention will be further described below using examples.
板厚２ｍｍの１４７０ＭＰａ級焼入鋼板において、図３（ａ）〜（ｃ）に示す通電パターン条件１〜３を用いてナゲット径５√（２．０）＝７．１ｍｍのスポット溶接継手を作成し、作成したスポット溶接継手から、JIS Z 3137に基づく引張り試験を実施してそれぞれのＣＴＳを測定した。
通電パターン条件１〜３における溶接時の温度履歴の解析結果をそれぞれ図４〜６に示す。なお、図４において、１ Centerはナゲット中心、２ FLin0.5mmはＦＬ内側0.5mmの位置、３ FLはＦＬの位置、４〜９ FLout0.5〜3.0mmはＦＬ外側0.5〜3.0mmの位置での解析結果を示している。図５、６も同様である。
A spot welded joint with a nugget diameter of 5√ (2.0) = 7.1 mm is prepared using the
The analysis results of the temperature history during welding under the
図４〜６の温度履歴において、中心からＦＬより０．５ｍｍ外側の領域はＡＣ３点を超え、焼きが入る温度履歴になる。また、ＦＬから１．０ｍｍ外側の領域の最高加熱温度はＡＣ１点程度であり、一部は焼きが入り、残りは焼き戻しが起こり軟化する可能性がある。ＦＬから１．５ｍｍより外側では焼きが入らないため、溶接時の熱履歴により焼き戻しを受けるのみになると考えられる。 In the temperature history shown in FIGS. 4 to 6, the region 0.5 mm outside the FL from the center exceeds the AC3 point, and becomes a temperature history at which baking occurs. In addition, the maximum heating temperature in the area 1.0 mm outside the FL is about AC1 point, part of which is baked and the rest may be tempered and softened. It is considered that since tempering does not occur outside the FL from 1.5 mm, only tempering is performed due to the thermal history during welding.
作成したスポット溶接継手をナゲットの中心を通るように切断し、断面のビッカース硬さＨｖを前述のようにして測定した。その結果を図７に示す。図７から条件１〜３の通電パターンでの軟化量ΔＨｖを求め、前記の温度履歴解析の結果明らかになったＦＬから１．５ｍｍ以上外側の軟化領域で、前述のようにして焼戻しパラメータλを導出した。
その結果、図８に示すように、焼き戻しパラメータλと軟化量ΔＨｖはほぼ比例関係にある結果が得られた。
The prepared spot welded joint was cut so as to pass through the center of the nugget, and the Vickers hardness Hv of the cross section was measured as described above. The result is shown in FIG. The softening amount ΔHv in the energization pattern of
As a result, as shown in FIG. 8, the result that the tempering parameter λ and the softening amount ΔHv are in a substantially proportional relationship was obtained.
したがって、焼きが入らない領域での軟化量は、通電パターンに基づく温度履歴解析により推定できることが確認された。
また、図７に基づき前記式（１）を用いて軟化度合Ｎ（長さはmm単位で計算）を計算し、スポット溶接部ＣＴＳの関係を調べると、図９に示すように、Ｎが大きいほうがＣＴＳは高くなっており、図に付記した関係式で近似できる結果が得られた。
Therefore, it was confirmed that the amount of softening in the region where no quenching can be estimated by temperature history analysis based on the energization pattern.
Further, when the degree of softening N (length is calculated in mm) is calculated based on FIG. 7 and the relationship between the spot welds CTS is examined, N is large as shown in FIG. The CTS was higher, and a result that could be approximated by the relational expression attached to the figure was obtained.
次に、板厚１．６ｍｍの１４７０ＭＰａ級焼入鋼板において、４種類の通電パターンについて、ナゲット径５√（１．６）＝６．３ｍｍのスポット溶接スポット溶接継手を作成し、板厚２ｍｍの場合と同様に、通電パターンごとに温度履歴解析と軟化度合Ｎを導出し、作成したスポット溶接継手から、JIS Z 3137に基づく引張り試験を実施してそれぞれのＣＴＳを測定した。
その結果、図１０に示すように、軟化度合Ｎが大きくなると、ＣＴＳが大きくなる結果が得られた。
Next, a spot weld spot welded joint with a nugget diameter of 5√ (1.6) = 6.3 mm was prepared for four kinds of energization patterns in a 1470 MPa class hardened steel plate with a plate thickness of 1.6 mm. Similarly to the case, a temperature history analysis and a softening degree N were derived for each energization pattern, and a tensile test based on JIS Z 3137 was performed from the created spot welded joint to measure each CTS.
As a result, as shown in FIG. 10, when the degree of softening N increases, the result that CTS increases is obtained.
以上の板厚２．０ｍｍと板厚１．６ｍｍのスポット溶接結果から得られた軟化度合ＮとＣＴＳの関係を、軟化度合Ｎと板厚ｓとの関数として回帰分析を行なった。その結果、図１１に示すように上記（４）式の具体的な形として、下記の式（４）’が得られた。
ＣＴＳ＝０．３２Ｎ0.64・ｓ1.5 ・・・（４）’
The relationship between the softening degree N and CTS obtained from the spot welding results of the above plate thickness 2.0 mm and plate thickness 1.6 mm was subjected to regression analysis as a function of the softening degree N and the plate thickness s. As a result, as shown in FIG. 11, the following formula (4) ′ was obtained as a specific form of the above formula (4).
CTS = 0.32N 0.64 · s 1.5 (4) '
さらに、両板厚ともナゲット径Ｌ＝５√（ｓ）で試験を行なったことから、ＣＴＳを軟化度合Ｎと板厚ｓとナゲット径Ｌの関数として上記（２）式の具体的な形として、下記式（２）’から予測できることが確認された。
Claims (2)
（Ａ）本溶接の前に、あらかじめ、
（a1）板厚ｓの焼き入れ処理された鋼板を種々の通電条件でスポット溶接し、通電条件毎に、ナゲット径Ｌと溶接熱影響部に発生する軟化部の軟化量ΔＨｖと溶接継手の十字引張強さＣＴＳを測定し、
（a2）通電条件毎に溶接熱影響部の温度履歴を計算し、該温度履歴から焼戻しパラメータλを計算して、焼戻しパラメータλと軟化量ΔＨｖとの関係を求め、
（a3）軟化量ΔＨｖから下記（１）式を用いて通電条件毎の軟化度合Ｎを計算し、
（a4）十字引張強さＣＴＳを、下記（２）式を用いて軟化度合Ｎと板厚ｓとナゲット径Ｌの関数として予測しておき、
（Ｂ）ついで、本溶接にあたり、
（b1）設定した本溶接の通電条件から前記（a2）と同様にして焼戻しパラメータλを計算し、
（b2）焼戻しパラメータλから前記（a2）で求めた関係を用いて軟化量ΔＨｖを求め、
（b3）軟化量ΔＨｖから下記（１）式を用いて軟化度合Ｎを求め、
（b4）軟化度合Ｎから下記（２）式を用いて設定した通電条件で溶接したスポット溶接継手の十字引張強さＣＴＳを求める、
ことを特徴とする焼き入れ処理された鋼板のスポット溶接における十字引張強さＣＴＳの予測方法。
ここで、Ａ、α、β、γは定数であり、それぞれ０．１０≦Ａ≦０．１５、０．６≦α≦０．７、β＝０．５である。 In spot welding of quenched steel plates,
(A) Before the main welding,
(A1) A steel sheet having a thickness s that has been quenched is spot-welded under various energization conditions, and for each energization condition, the nugget diameter L, the softening amount ΔHv of the softened portion generated in the weld heat affected zone, and the cross of the welded joint Measure the tensile strength CTS,
(A2) Calculate the temperature history of the weld heat affected zone for each energization condition, calculate the tempering parameter λ from the temperature history, and obtain the relationship between the tempering parameter λ and the softening amount ΔHv,
(A3) The softening degree N for each energization condition is calculated from the softening amount ΔHv using the following equation (1),
(A4) The cross tensile strength CTS is predicted as a function of the softening degree N, the plate thickness s, and the nugget diameter L using the following equation (2).
(B) Next, in the main welding,
(B1) The tempering parameter λ is calculated in the same manner as in (a2) from the set current-carrying conditions,
(B2) The softening amount ΔHv is obtained from the tempering parameter λ using the relationship obtained in (a2) above,
(B3) The softening degree N is obtained from the softening amount ΔHv using the following equation (1),
(B4) Obtain the cross tensile strength CTS of the spot welded joint welded from the softening degree N under the energization conditions set using the following equation (2).
A method for predicting a cross tensile strength CTS in spot welding of a quenched steel sheet.
Here, A, α, β, and γ are constants, and 0.10 ≦ A ≦ 0.15, 0.6 ≦ α ≦ 0.7, and β = 0.5, respectively.
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