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JP2021074737A - Manufacturing method for resistance spot welded joint - Google Patents

Manufacturing method for resistance spot welded joint Download PDF

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JP2021074737A
JP2021074737A JP2019202259A JP2019202259A JP2021074737A JP 2021074737 A JP2021074737 A JP 2021074737A JP 2019202259 A JP2019202259 A JP 2019202259A JP 2019202259 A JP2019202259 A JP 2019202259A JP 2021074737 A JP2021074737 A JP 2021074737A
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resistance spot
welded joint
manufacturing
energization
spot welded
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JP7368716B2 (en
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古迫 誠司
Seiji Furusako
誠司 古迫
真二 児玉
Shinji Kodama
真二 児玉
直明 嶋田
Naoaki Shimada
直明 嶋田
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Nippon Steel Corp
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Abstract

To provide a manufacturing method for a resistance spot welded joint which is excellent in strength of joint, and is excellent in delayed fracture resistance.SOLUTION: A manufacturing method for a resistance spot welded joint is provided that comprises: a preliminary electric conduction step in which a steel plate surface is electrically conducted in a pressurized state by use of a pair of DR electrodes having a distal end diameter of 8 mm to 12 mm; a main electric conduction step in which the steel plate surface is electrically conducted in a pressurized state by use of a pair of DR electrodes; and a holding step in which pressurization with the pair of DR electrodes is held. When a pressurizing force is represented by P1(kN), an electric current is represented by I1(kA) and an electric conduction time is represented by t1(s) in the preliminary electric conduction step, and a pressurizing force is represented by P2(kN) and an electric current is represented by I2(kA) in the main electric conduction step, the following formulae (1) to (5) are satisfied: formula (1) 2≤I1/h<4; formula (2) 1.5≤P1/h<2.5; formula (3) 0.1≤t1/h≤1.5; formula (4) 4≤I2/h; formula (5) 2.5≤P2/h where h is a value which is 1/2 of total plate thickness (mm) of a plurality of steel plates.SELECTED DRAWING: Figure 1

Description

本発明は、抵抗スポット溶接継手に関する。本発明は、特に、継手強度に優れるとともに、耐遅れ破壊特性に優れた抵抗スポット溶接継手の製造方法に関する。 The present invention relates to resistance spot welded joints. The present invention particularly relates to a method for manufacturing a resistance spot welded joint having excellent joint strength and delayed fracture resistance.

自動車の分野では、環境保全のため、車体の軽量化による燃費の向上とともに、衝突安全性の向上が求められている。そのため、高強度鋼板を使用して薄肉化するとともに、車体構造を最適化して、車体の軽量化と衝突安全性の向上を図るために、これまで種々の取組みがなされている。
自動車等の部品の製造や車体の組立における溶接では、抵抗スポット溶接(以下、「スポット溶接」ということもある)が主に使用されている。スポット溶接により形成された溶接継手の品質指標としては、引張強さがある。溶接継手の引張強さには、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 field of automobiles, in order to protect the environment, it is required to improve fuel efficiency by reducing the weight of the vehicle body and improve collision safety. Therefore, various efforts have been made so far in order to reduce the wall thickness by using a high-strength steel plate, optimize the vehicle body structure, reduce the weight of the vehicle body, and improve the collision safety.
Resistance spot welding (hereinafter, also referred to as "spot welding") is mainly used in welding in the manufacture of parts such as automobiles and the assembly of vehicle bodies. Tensile strength is a quality index of welded joints formed by spot welding. The tensile strength of a welded joint includes a tensile shear strength (TSS) measured by applying a tensile load in the shear direction and a cross tensile strength (CTS) measured by applying a tensile load in the peeling direction.

ここで、高強度鋼板をスポット溶接した場合においては、遅れ破壊(水素脆化)の問題がある。
高強度鋼板は、その強度を達成するために、C以外にもSi、Mn等の焼き入れ性の高い元素を多く含有しており、高強度鋼板にスポット溶接して形成された溶接継手の溶接部は、溶接の加熱冷却過程を経て焼きが入り、マルテンサイト組織となり、硬くなっている。また、溶接部では、局部的に生じる変態膨張と収縮により、溶接継手の引張残留応力が大きくなっている。特に鋼板強度が上昇するほどプレス成形後のスプリングバックが生じやすく、これは溶接フランジ部の隙間を増大させる。こうした隙間は、鋼板同士をスポット溶接する際、引張残留応力をさらに増大させる要因となる。
このため、高強度鋼板にスポット溶接して形成された溶接継手の溶接部は、硬度が高く、引張残留応力が大きくなっているので、水素侵入が起これば、遅れ破壊を引き起こしやすい部位である。このような遅れ破壊が発生すると、前述の溶接継手の品質指標である引張強さにおいて、十分な強さが得られず、また、その部分(割れ)に水分が浸入すると、腐食が発生して強度がさらに低下するという問題が生じる。これらの問題が、高強度鋼板の適用による車体の軽量化(薄肉化)を阻害する一因である。
このような状況のもと、スポット溶接の通電が終了して一定時間が経過した後にテンパー通電を行ったり、高周波で加熱したりして、溶接部を焼戻して、溶接部の硬さを低下させる技術が知られている。しかし、この技術では、溶接工程が長時間となり、生産性が低下することや、焼戻しにより溶接部が軟化し、溶接金属(ナゲット)内での剥離破断が起こりやすいこと等があった。
Here, when a high-strength steel plate is spot-welded, there is a problem of delayed fracture (hydrogen embrittlement).
In order to achieve the strength of the high-strength steel plate, a large amount of highly hardenable elements such as Si and Mn are contained in addition to C, and the welded joint formed by spot welding the high-strength steel plate is welded. The part is hardened by quenching through the heating and cooling process of welding to form a martensite structure. Further, in the welded portion, the tensile residual stress of the welded joint is increased due to the locally generated transformation expansion and contraction. In particular, as the strength of the steel sheet increases, springback after press forming is likely to occur, which increases the gap in the welded flange portion. Such a gap becomes a factor that further increases the tensile residual stress when spot welding the steel sheets to each other.
For this reason, the welded portion of the welded joint formed by spot welding to a high-strength steel plate has a high hardness and a large tensile residual stress, and therefore, if hydrogen intrusion occurs, it is a portion that is likely to cause delayed fracture. .. When such delayed fracture occurs, sufficient strength cannot be obtained in the tensile strength, which is the above-mentioned quality index of the welded joint, and when moisture infiltrates the portion (crack), corrosion occurs. The problem arises that the strength is further reduced. These problems are one of the factors that hinder the weight reduction (thinning) of the vehicle body by applying the high-strength steel plate.
Under such circumstances, after a certain period of time has passed since the energization of spot welding was completed, temper energization was performed or heating was performed at a high frequency to temper the welded portion and reduce the hardness of the welded portion. The technology is known. However, in this technique, the welding process takes a long time, the productivity is lowered, the welded portion is softened by tempering, and the weld metal (nugget) is liable to be peeled and broken.

このような状況のもと、耐遅れ破壊の発生を抑制するための種々の提案がされている。
例えば、特許文献1は、高強度の亜鉛系めっき鋼板が小径ナゲットの形成により接合される場合であっても耐遅れ破壊特性に優れた溶接部を形成することができる抵抗スポット溶接方法を開示している。
Under such circumstances, various proposals have been made to suppress the occurrence of delayed fracture.
For example, Patent Document 1 discloses a resistance spot welding method capable of forming a welded portion having excellent delayed fracture resistance even when a high-strength galvanized steel sheet is joined by forming a small-diameter nugget. ing.

特開2018−171649号公報Japanese Unexamined Patent Publication No. 2018-171649

しかしながら、特許文献1の技術では、使用する鋼板の厚さや溶接条件によっては優れた継手強度と耐遅れ破壊特性を両立することは困難であった。 However, with the technique of Patent Document 1, it is difficult to achieve both excellent joint strength and delayed fracture resistance depending on the thickness of the steel sheet used and the welding conditions.

本発明は、上述の実情に鑑みてなされたものであり、継手強度に優れるとともに、耐遅れ破壊特性に優れた抵抗スポット溶接継手の製造方法を提供することを課題とする。 The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a resistance spot welded joint having excellent joint strength and delayed fracture resistance.

本発明の具体的方法は以下のとおりである。 The specific method of the present invention is as follows.

[1]本発明の第一の態様は、引張強度が780MPa以上である鋼板を少なくとも一枚含む複数枚の鋼板を重ね合わせて抵抗スポット溶接により接合し、抵抗スポット溶接継手を製造する方法であって、先端径が8mm以上12mm以下である一対のDR電極を用いて前記鋼板の表面に加圧した状態で通電を行う予備通電工程と、前記予備通電工程の後に、前記一対のDR電極を用いて前記鋼板の表面に加圧した状態で通電を行う本通電工程と、前記本通電工程の後に前記一対のDR電極での加圧を保持する保持工程と、を備え、前記予備通電工程における加圧力をP1(kN)、電流をI1(kA)、通電時間をt1(s)とし、前記本通電工程における加圧力をP2(kN)、電流をI2(kA)としたとき、下記(1)式から(5)式を満足する。
2≦I1/h<4 ・・・(1)式
1.5≦P1/h<2.5 ・・・(2)式
0.1≦t1/h≦1.5 ・・・(3)式
4≦I2/h ・・・(4)式
2.5≦P2/h ・・・(5)式
ただし、hは、前記複数枚の鋼板の合計板厚(mm)の1/2の値である。
[2]上記[1]に記載の抵抗スポット溶接継手の製造方法では、前記保持工程における保持時間Ht(s)が0.4×h以下であってもよい。
[3]上記[1]又は[2]に記載の抵抗スポット溶接継手の製造方法では、前記複数枚の鋼板の少なくとも一枚が亜鉛めっき鋼板であってもよい。
[4]上記[3]に記載の抵抗スポット溶接継手の製造方法では、前記保持工程における保持時間Ht(s)が0.08×h以上であってもよい。
[1] The first aspect of the present invention is a method of manufacturing a resistance spot welded joint by superimposing a plurality of steel plates containing at least one steel plate having a tensile strength of 780 MPa or more and joining them by resistance spot welding. A pre-energization step in which the surface of the steel plate is pressurized by using a pair of DR electrodes having a tip diameter of 8 mm or more and 12 mm or less, and a pre-energization step are followed by the pair of DR electrodes. A main energization step of energizing the surface of the steel plate in a pressurized state and a holding step of holding the pressurization by the pair of DR electrodes after the main energization step are provided, and the addition in the preliminary energization step is provided. When the pressure is P1 (kN), the current is I1 (kA), the energization time is t1 (s), the pressing force in the main energization step is P2 (kN), and the current is I2 (kA), the following (1) From the equation, the equation (5) is satisfied.
2≤I1 / h <4 ... (1) Equation 1.5≤P1 / h <2.5 ... (2) Equation 0.1≤t1 / h≤1.5 ... (3) Equation 4 ≦ I2 / h ・ ・ ・ (4) formula 2.5 ≦ P2 / h ・ ・ ・ (5) formula However, h is a value of 1/2 of the total thickness (mm) of the plurality of steel plates. is there.
[2] In the method for manufacturing a resistance spot welded joint according to the above [1], the holding time Ht (s) in the holding step may be 0.4 × h 2 or less.
[3] In the method for manufacturing a resistance spot welded joint according to the above [1] or [2], at least one of the plurality of steel sheets may be a galvanized steel sheet.
[4] In the method for manufacturing a resistance spot welded joint according to the above [3], the holding time Ht (s) in the holding step may be 0.08 × h 2 or more.

本発明に係る抵抗スポット溶接継手の製造方法によれば、継手強度に優れるとともに、耐遅れ破壊特性に優れた抵抗スポット溶接継手を製造することができる。 According to the method for manufacturing a resistance spot welded joint according to the present invention, it is possible to manufacture a resistance spot welded joint having excellent joint strength and delayed fracture resistance.

本実施形態に係る抵抗スポット溶接継手の製造方法で用いる電極と鋼板の概略図である。It is the schematic of the electrode and the steel plate used in the manufacturing method of the resistance spot welded joint which concerns on this embodiment. 同実施形態で用いる電極の概略図である。It is the schematic of the electrode used in the same embodiment.

本発明者等は、上述した課題を解決するための方策について鋭意検討した結果、
(A)ナゲット径が同一の溶接継手であっても、高い加圧力で通電して得られた溶接継手の方が、低い加圧力で通電して得られた溶接継手に比べ、鋼板間や鋼板表面に付着した水素源としての水と油に起因してスポット溶接部に水素が侵入しやすくなる傾向にあること、
(B)スポット溶接部に水素が侵入した場合、引張応力が集中するナゲット端部において引張応力と水素量が臨界値に達した際に遅れ破壊が発生すること、
(C)遅れ破壊は、板間隙間が増大するほど(プレス精度が悪化するほど)、かつナゲット径が小さい時に生じやすいこと、
(D)従って、電流、加圧力、及び通電時間を適正範囲とした予備通電を行うことにより鋼板同士の接触面積を徐々に増やしつつ、水素源となる水と油を広い範囲(面積)で蒸発させ、その後、本通電を行うことにより、優れた継手強度と耐遅れ破壊特性を両立できること、
を新たに見出した。
尚、(A)に関し、加圧力が高い場合に溶接部に侵入する水素量が増加傾向にある理由としては、加圧力が高い場合には鋼板同士の接触面積や電極と鋼板の接触面積が増加し電流密度が減少するため、同等のナゲット径を得るためには電流も高くする必要があり、初期の接触面積と電流が増加するためであると推察される。
As a result of diligent studies on measures for solving the above-mentioned problems, the present inventors have conducted diligent studies.
(A) Even if the welded joints have the same nugget diameter, the welded joints obtained by energizing with a high pressing force are more inter-steel or steel plates than the welded joints obtained by energizing with a low pressing force. There is a tendency for hydrogen to easily enter the spot weld due to water and oil as hydrogen sources adhering to the surface.
(B) When hydrogen invades the spot weld, delayed fracture occurs when the tensile stress and the amount of hydrogen reach the critical value at the nugget end where the tensile stress is concentrated.
(C) Delayed fracture is more likely to occur when the gap between plates increases (the press accuracy deteriorates) and the nugget diameter is small.
(D) Therefore, water and oil, which are hydrogen sources, evaporate in a wide range (area) while gradually increasing the contact area between the steel plates by performing pre-energization with the current, pressing force, and energizing time in the appropriate range. After that, by performing the main energization, both excellent joint strength and delayed fracture resistance can be achieved.
Was newly found.
Regarding (A), the reason why the amount of hydrogen invading the weld tends to increase when the pressing force is high is that the contact area between the steel plates and the contact area between the electrodes and the steel plate increase when the pressing force is high. Since the current density decreases, it is necessary to increase the current in order to obtain the same nugget diameter, which is presumed to be due to the increase in the initial contact area and current.

本発明は上述の知見に基づきなされたものである。以下、本発明の実施形態に係る抵抗スポット溶接継手の製造方法について、詳細に説明する。 The present invention has been made based on the above findings. Hereinafter, a method for manufacturing a resistance spot welded joint according to an embodiment of the present invention will be described in detail.

図1に示すように、本実施形態に係る抵抗スポット溶接継手の製造方法では、互いに重ね合わせた第一鋼板1と第二鋼板2を、第一電極10と第二電極20により挟み込んで加圧した状態で、予備通電工程、本通電工程、及び保持工程を行うことで、抵抗スポット溶接を実施する。 As shown in FIG. 1, in the method for manufacturing a resistance spot welded joint according to the present embodiment, the first steel plate 1 and the second steel plate 2 which are overlapped with each other are sandwiched between the first electrode 10 and the second electrode 20 and pressurized. In this state, resistance spot welding is performed by performing the preliminary energization step, the main energization step, and the holding step.

(鋼板)
本実施形態においては、第一鋼板1と第二鋼板2の二枚を重ね合わせて抵抗スポット溶接を行うが、更に他の鋼板を重ね合わせて、三枚以上重ね合わせて抵抗スポット溶接を行ってもよい。
第一鋼板1と第二鋼板2は、少なくとも一方が引張強度780MPa以上であればよく、両方が引張強度780MPa以上であってもよい。鋼板が三枚以上の場合、少なくとも一枚の鋼板の引張強度が780MPa以上であればよい。
更には、第一鋼板1と第二鋼板2の少なくとも一方が、亜鉛めっき鋼板であってもよい。鋼板が三枚以上の場合、少なくとも一枚の鋼板が亜鉛めっき鋼板であってよい。亜鉛めっきの付着量は、片面あたり30〜100g/mであればよい。
重ね合わせる鋼板のそれぞれの板厚は特に限定されるものではなく、例えば0.8mm〜3.2mmであればよい。
尚、本明細書においては、重ね合わせる鋼板の合計板厚(mm)の1/2の値をhと呼称する。例えば、板厚1mmの鋼板を三枚重ね合わせる場合、hの値は1.5である。
(Steel plate)
In the present embodiment, two steel plates 1 and 2 are superposed to perform resistance spot welding, but other steel plates are further superposed and three or more are superposed to perform resistance spot welding. May be good.
At least one of the first steel plate 1 and the second steel plate 2 may have a tensile strength of 780 MPa or more, and both may have a tensile strength of 780 MPa or more. When there are three or more steel plates, the tensile strength of at least one steel plate may be 780 MPa or more.
Further, at least one of the first steel plate 1 and the second steel plate 2 may be a galvanized steel plate. When there are three or more steel sheets, at least one steel sheet may be a galvanized steel sheet. The amount of zinc plating attached may be 30 to 100 g / m 2 per side.
The thickness of each of the steel plates to be overlapped is not particularly limited, and may be, for example, 0.8 mm to 3.2 mm.
In this specification, the value of 1/2 of the total thickness (mm) of the steel sheets to be overlapped is referred to as h. For example, when three steel plates having a thickness of 1 mm are laminated, the value of h is 1.5.

(電極)
図1に示すように、第一電極10はDR電極であり、先端部11と、先端部11に連続部12を介して連なる本体部13とを有する。
先端部11は、鋼板への加圧力に応じて鋼板との接触面積が変化する部位であり、図2に示すように先端R(電極先端の曲面の曲率半径)が一定値である部位を意味する。先端Rは30mm以上であればよく、例えば40mmである。先端Rの最大値は100mmであればよい。
連続部12は、先端部11よりも曲率半径が小さい部位である。連続部12の曲率半径は例えば6mm以上であればよい。
本体部13は略円柱状の部位であり、一端が連続部12に連接し、他端は図示しない電極上部の構造体に接続される。本体部13の直径Dは、12mm〜20mmであればよい。
(electrode)
As shown in FIG. 1, the first electrode 10 is a DR electrode, and has a tip portion 11 and a main body portion 13 connected to the tip portion 11 via a continuous portion 12.
The tip portion 11 is a portion where the contact area with the steel plate changes according to the pressing force on the steel plate, and means a portion where the tip R (radius of curvature of the curved surface of the electrode tip) is a constant value as shown in FIG. To do. The tip R may be 30 mm or more, for example 40 mm. The maximum value of the tip R may be 100 mm.
The continuous portion 12 is a portion having a smaller radius of curvature than the tip portion 11. The radius of curvature of the continuous portion 12 may be, for example, 6 mm or more.
The main body portion 13 is a substantially columnar portion, one end of which is connected to the continuous portion 12 and the other end of which is connected to a structure above an electrode (not shown). The diameter D of the main body 13 may be 12 mm to 20 mm.

第一電極10は、先端径dが8mm以上であることにより、後述する予備通電工程において鋼板表面の水素源である油や水を十分な面積で除去することができるため、遅れ破壊を防止することができる。
従って、第一電極10の先端径dは8mm以上であり、好ましくは9mm以上である。
一方、第一電極10の先端径dが12mm超である場合、接触面積が大きくなることに起因して所望の電流密度を得ることが困難となる。従って、第一電極10の先端径dは12mm以下であり、好ましくは11mm以下である。
Since the tip diameter d of the first electrode 10 is 8 mm or more, oil and water, which are hydrogen sources on the surface of the steel sheet, can be removed in a sufficient area in the preliminary energization step described later, so that delayed fracture is prevented. be able to.
Therefore, the tip diameter d of the first electrode 10 is 8 mm or more, preferably 9 mm or more.
On the other hand, when the tip diameter d of the first electrode 10 is more than 12 mm, it becomes difficult to obtain a desired current density due to the large contact area. Therefore, the tip diameter d of the first electrode 10 is 12 mm or less, preferably 11 mm or less.

第二電極20は、第一電極10と同様に、先端部21と、先端部21に連続部22を介して連なる本体部23とを有する。第二電極20の形状や寸法は第一電極10と同じであればよいため説明は省略する。 Like the first electrode 10, the second electrode 20 has a tip portion 21 and a main body portion 23 connected to the tip portion 21 via a continuous portion 22. Since the shape and dimensions of the second electrode 20 may be the same as those of the first electrode 10, the description thereof will be omitted.

本実施形態に係る抵抗スポット溶接継手の製造方法では、上記の第一電極10と第二電極20とを用いて、予備通電工程と、本通電工程と、保持工程とを行う。 In the method for manufacturing a resistance spot welded joint according to the present embodiment, the preliminary energization step, the main energization step, and the holding step are performed using the first electrode 10 and the second electrode 20 described above.

(予備通電工程)
予備通電工程では、一対の電極1で第一鋼板1、第二鋼板2を加圧した状態で通電を行う。
ここで、予備通電工程における加圧力をP1(kN)、電流をI1(kA)、通電時間をt1(s)としたとき、下記(1)式から(3)式を満足する。
2≦I1/h<4(kA/mm) ・・・(1)式
1.5≦P1/h<2.5(kN/mm) ・・・(2)式
0.1≦t1/h≦1.5 (s/mm)・・・(3)式
(Preliminary energization process)
In the pre-energization step, energization is performed in a state where the first steel plate 1 and the second steel plate 2 are pressurized by the pair of electrodes 1.
Here, when the pressing force in the pre-energization step is P1 (kN), the current is I1 (kA), and the energizing time is t1 (s), the following equations (1) to (3) are satisfied.
2≤I1 / h <4 (kA / mm) ... (1) Equation 1.5≤P1 / h <2.5 (kN / mm) ... (2) Equation 0.1≤t1 / h≤ 1.5 (s / mm) ・ ・ ・ Eq. (3)

(1)式は、予備通電工程における、hに対する電流I1の範囲を規定するものである。I1/hが2未満である場合、(2)式と(3)式を満たしていても、入熱不足により鋼板表面の水素源である油や水を十分に除去することが困難となる。このため、本通電工程において水素が溶接部に侵入し、遅れ破壊が発生する場合がある。従って、I1/hは2以上である。
一方、I1/hが4以上である場合は予備通電工程を行わずに本通電工程を行うことと等しいため、鋼板表面の水素源である油や水を十分に除去できていない状態でスポット溶接が実施されることになる。従って、水素が溶接部に侵入し、遅れ破壊が発生する場合がある。従って、I1/hは4未満である。
Equation (1) defines the range of the current I1 with respect to h in the pre-energization step. When I1 / h is less than 2, even if the equations (2) and (3) are satisfied, it becomes difficult to sufficiently remove oil and water which are hydrogen sources on the surface of the steel sheet due to insufficient heat input. Therefore, hydrogen may enter the welded portion in the main energization step, resulting in delayed fracture. Therefore, I1 / h is 2 or more.
On the other hand, when I1 / h is 4 or more, it is equivalent to performing the main energization process without performing the pre-energization process. Therefore, spot welding is performed in a state where oil and water, which are hydrogen sources on the surface of the steel sheet, cannot be sufficiently removed. Will be implemented. Therefore, hydrogen may invade the welded portion and cause delayed fracture. Therefore, I1 / h is less than 4.

(2)式は、予備通電工程における、hに対する加圧力P1の範囲を規定するものである。P1/hが1.5未満である場合、加圧力が低いことにより鋼板(第一鋼板1、第二鋼板2)と電極(第一電極10、第二電極20)との接触面積及び鋼板同士の接触面積が小さくなる。従って、(1)式と(3)式を満たしていても、鋼板表面の水素源である油や水を除去できる範囲が狭くなる。このため、本通電工程において水素が溶接部に侵入し、遅れ破壊が発生する場合がある。従って、P1/hは1.5以上であり、より好ましくは1.7以上である。
一方、P1/hが2.5超である場合、加圧力が高いことにより鋼板と電極との接触面積及び鋼板同士の接触面積が大きくなる。このため、電流密度が小さくなり、(1)式と(2)式を満たしていても、鋼板表面の水素源である油や水を十分に除去することが困難となる。このため、本通電工程において水素が溶接部に侵入し、遅れ破壊が発生する場合がある。従って、P1/hは2.5以下であり、より好ましくは2.3以下である。
Equation (2) defines the range of the pressing force P1 with respect to h in the pre-energization step. When P1 / h is less than 1.5, the contact area between the steel plates (first steel plate 1, second steel plate 2) and the electrodes (first electrode 10, second electrode 20) and the steel plates are due to the low pressing force. Contact area becomes smaller. Therefore, even if the equations (1) and (3) are satisfied, the range in which oil and water, which are hydrogen sources on the surface of the steel sheet, can be removed is narrowed. Therefore, hydrogen may enter the welded portion in the main energization step, resulting in delayed fracture. Therefore, P1 / h is 1.5 or more, more preferably 1.7 or more.
On the other hand, when P1 / h is more than 2.5, the contact area between the steel plate and the electrode and the contact area between the steel plates become large due to the high pressing force. Therefore, the current density becomes small, and even if the equations (1) and (2) are satisfied, it becomes difficult to sufficiently remove oil and water which are hydrogen sources on the surface of the steel sheet. Therefore, hydrogen may enter the welded portion in the main energization step, resulting in delayed fracture. Therefore, P1 / h is 2.5 or less, more preferably 2.3 or less.

(3)式は、予備通電工程における、hに対する通電時間t1の範囲を規定するものである。t1/hが0.1未満である場合、(1)式と(2)式を満たしていても、通電時間が短いことにより鋼板表面の水素源である油や水を十分に除去することが困難となる。このため、本通電工程において水素が溶接部に侵入し、遅れ破壊が発生する場合がある。従って、t1/hは0.1以上であり、より好ましくは0.5以上である。
一方、t1/hが1.5超であっても、鋼板表面の水素源である油や水を除去する効果は飽和し、寧ろ生産性の低下が懸念される。従って、t1/hは1.5以下である。
予備通電と本通電との間は、クール時間を設けず、すぐに本通電をすることが、遅れ破壊抑制のために好ましい。
予備通電をアップスロープにしてもよい。この場合、予備通電の初期の電流は2≦I1/h≦3(kA/mm)とし、徐々に電流値を上げ、予備通電の終了時の電流値と初期の電流値との平均値が、2≦I1/h<4(kA/mm)となるようにする。
Equation (3) defines the range of the energization time t1 with respect to h in the pre-energization step. When t1 / h is less than 0.1, even if equations (1) and (2) are satisfied, oil and water, which are hydrogen sources on the surface of the steel sheet, can be sufficiently removed due to the short energization time. It will be difficult. Therefore, hydrogen may enter the welded portion in the main energization step, resulting in delayed fracture. Therefore, t1 / h is 0.1 or more, more preferably 0.5 or more.
On the other hand, even if t1 / h is more than 1.5, the effect of removing oil and water which are hydrogen sources on the surface of the steel sheet is saturated, and there is a concern that the productivity is rather lowered. Therefore, t1 / h is 1.5 or less.
It is preferable to immediately perform the main energization without providing a cool time between the pre-energization and the main energization in order to suppress delayed fracture.
The pre-energization may be an upslope. In this case, the initial current of the pre-energization is set to 2 ≦ I1 / h ≦ 3 (kA / mm), the current value is gradually increased, and the average value of the current value at the end of the pre-energization and the initial current value becomes 2 ≦ I1 / h <4 (kA / mm).

(本通電工程)
本通電工程では、上述の予備通電工程の後に、電極1を用いて前記鋼板の表面に加圧した状態で通電を行う。
ここで、本通電工程における加圧力をP2(kN)、電流をI2(kA)としたとき、下記(4)式と(5)式を満足する。
4≦I2/h ・・・(4)式
2.5≦P2/h ・・・(5)式
(Main energization process)
In the main energization step, after the pre-energization step described above, energization is performed in a state where the surface of the steel sheet is pressurized using the electrode 1.
Here, when the pressing force in the main energization step is P2 (kN) and the current is I2 (kA), the following equations (4) and (5) are satisfied.
4 ≦ I2 / h ・ ・ ・ ・ Eq. (4) 2.5 ≦ P2 / h ・ ・ ・ ・ ・ Eq. (5)

(4)式は、本通電工程における、hに対する電流I2の範囲を規定するものであり、(5)式は、本通電工程における、hに対する加圧力P2の範囲を規定するものである。
本願では、上述のような予備通電により鋼板表面の水素源である油や水を十分に除去した状態で(4)式と(5)式を満たす電流及び加圧力で本通電を行うことで、耐遅れ破壊特性に優れたスポット溶接継手を得ることができる。
Equation (4) defines the range of the current I2 with respect to h in the main energization step, and equation (5) defines the range of the pressing force P2 with respect to h in the main energization step.
In the present application, the main energization is performed with the current and the pressing force satisfying the equations (4) and (5) in a state where the oil and water which are the hydrogen sources on the surface of the steel sheet are sufficiently removed by the pre-energization as described above. A spot welded joint having excellent delayed fracture resistance can be obtained.

尚、I2/hの上限値は規定する必要はないが、8以下であることが好ましい。I2/hが8超であると加圧力が適正範囲であっても散りが発生する確率が高まるからである。本通電の時間は、所望のナゲット径が得られるよう適宜設定すればよい。 The upper limit of I2 / h does not need to be specified, but is preferably 8 or less. This is because if I2 / h is more than 8, the probability that scattering will occur increases even if the pressing force is in the appropriate range. The main energization time may be appropriately set so that a desired nugget diameter can be obtained.

(保持工程)
保持工程では、上述の本通電工程の後に、所定の時間、電極による鋼の表面への加圧を保持する。保持を行うことにより溶融金属の凝固を進ませることができ、鋼板強度が高い場合や板間に隙間がある場合でも溶接部の強度を高めることができる。保持時間Htは0秒超であればよいが、0.04秒以上であることが好ましい。
保持時間Htの上限は、生産性の観点、および継手強度が低下する可能性を考慮し、0.4×hを上限とすることが好ましい。保持時間Htを0.4×hより長くすると、例えば十字引張継手の強度が低下するためである。この理由は、保持時間Htが所定範囲を超えると、電極の抜熱によって溶接部が急速冷却され溶接部が硬化するためである。硬化によって溶接部のじん性は低下し、継手も低下すると考えられる。さらに、溶接部の硬化は水素脆化の感受性を高める。遅れ破壊抑制の効果を一層確実とするためにも0.4×hを保持時間Htの上限とすることが好ましい。一方、保持時間Htが0.4×h以下であれば、電極解放後の冷却が比較的緩やかとなるため、オートテンパ―(自己焼戻し)が進行し、溶接部のじん性が改善されると共に、水素脆化の感受性を改善できる。従って、保持時間Htの上限は0.4×hである。
(Holding process)
In the holding step, the pressurization of the steel surface by the electrodes is held for a predetermined time after the above-mentioned main energization step. By holding the molten metal, solidification of the molten metal can be promoted, and the strength of the welded portion can be increased even when the strength of the steel plate is high or there is a gap between the plates. The holding time Ht may be more than 0 seconds, but is preferably 0.04 seconds or more.
The upper limit of the holding time Ht is preferably 0.4 × h 2 in consideration of productivity and the possibility that the joint strength may decrease. This is because if the holding time Ht is longer than 0.4 × h 2 , for example, the strength of the cross tension joint is lowered. The reason for this is that when the holding time Ht exceeds a predetermined range, the welded portion is rapidly cooled by the heat removal of the electrode and the welded portion is hardened. It is considered that the toughness of the welded portion decreases due to hardening, and the joint also decreases. In addition, hardening of the weld increases the sensitivity to hydrogen embrittlement. In order to further ensure the effect of suppressing delayed fracture, it is preferable to set 0.4 × h 2 as the upper limit of the holding time Ht. On the other hand, when the holding time Ht is 0.4 × h 2 or less, the cooling after the electrode is released becomes relatively slow, so that auto tempering (self-tempering) proceeds and the toughness of the welded portion is improved. At the same time, the sensitivity to hydrogen embrittlement can be improved. Therefore, the upper limit of the holding time Ht is 0.4 × h 2 .

ここで、複数の鋼板の少なくとも一枚が亜鉛めっき層を有する場合においては、保持時間Htは0.08×h以上であることが好ましい。
複数の鋼板の少なくとも一枚が亜鉛めっき層を有する場合には、予備通電工程と本通電工程の条件によっては、溶接中に溶融した亜鉛が固体の鋼板と接触することに起因してLME割れ(Liquid Metal Embrittlement Crack)が発生する場合がある。LME割れは、溶接中に溶融した亜鉛が固体の鋼板と接触すること、またその部位に引張応力(ひずみ)が働くことで発生する。従って、保持時間Htを、0.08×h以上とすることで、溶融している亜鉛が保持時間中に凝固する(溶融亜鉛が減少する、または完全に無くなる)ため、保持が終了し電極が解放されるときに引張応力が発生しても、LME割れを抑制することができる。従って、複数の鋼板の少なくとも一枚が亜鉛めっき層を有する場合においては、保持時間Htの下限は0.08×hとすることが好ましい。
尚、複数の鋼板の少なくとも一枚が亜鉛めっき層を有する場合においては、予備通電工程により鋼板表層の亜鉛めっき層を除去したり、あるいは亜鉛めっき層の合金化を進めたりする効果があるため、予備通電工程と保持工程の適正化の組合せによってLME割れの抑制をより確実にできる。
Here, when at least one of the plurality of steel sheets has a zinc-plated layer, the holding time Ht is preferably 0.08 × h 2 or more.
When at least one of the plurality of steel sheets has a galvanized layer, depending on the conditions of the pre-energization process and the main energization process, LME cracking due to contact of zinc melted during welding with the solid steel sheet (LME cracking ( Liquid Metal Embrittlement Crack) may occur. LME cracking occurs when zinc melted during welding comes into contact with a solid steel sheet and tensile stress (strain) acts on the site. Therefore, by setting the holding time Ht to 0.08 × h 2 or more, the molten zinc solidifies during the holding time (the molten zinc decreases or completely disappears), so that the holding is completed and the electrode Even if tensile stress is generated when is released, LME cracking can be suppressed. Therefore, when at least one of the plurality of steel sheets has a zinc-plated layer, the lower limit of the holding time Ht is preferably 0.08 × h 2.
When at least one of the plurality of steel sheets has a galvanized layer, the pre-energization step has the effect of removing the galvanized layer on the surface of the steel sheet or advancing the alloying of the galvanized layer. LME cracking can be suppressed more reliably by combining the pre-energization process and the proper holding process.

上述のように、本実施形態に係る抵抗スポット溶接継手の製造方法によれば、電流、加圧力、及び通電時間を適正範囲とした予備通電を行うことにより水素源となる水と油を蒸発させ、その後、本通電を行うことにより、優れた継手強度と耐遅れ破壊特性を両立できる。 As described above, according to the method for manufacturing a resistance spot welded joint according to the present embodiment, water and oil as hydrogen sources are evaporated by performing preliminary energization within an appropriate range of current, pressing force, and energizing time. After that, by performing the main energization, both excellent joint strength and delayed fracture resistance can be achieved.

(実施例)
以下、本発明の効果を実施例により具体的に説明する。
(Example)
Hereinafter, the effects of the present invention will be specifically described with reference to Examples.

表1に示す鋼板a、bを準備し、各鋼板をW40mm×L100mmに切断し、鋼板の両端にW40mm×L30mm×t2.0mmのスペーサーを挟んでこの中央をスポット溶接した。この中央の溶接点を評価の対象とした。 The steel plates a and b shown in Table 1 were prepared, each steel plate was cut into W40 mm × L100 mm, and spacers of W40 mm × L30 mm × t2.0 mm were sandwiched between both ends of the steel plate and spot welded at the center thereof. This central welding point was evaluated.

Figure 2021074737
Figure 2021074737

電極としては、本体部の直径Dが16mm、先端部の曲率半径(先端R)が40mm、連続部の曲率半径が8mmであり、先端径dが6mm又は8mmである2種類のCr−Cu製のDR電極を用いた。
表2に、各実験例で用いた鋼板と電極、及び、溶接条件を示す。本発明の範囲外の数値には下線を付した。予備通電と本通電の間のクール時間はゼロとした。
The electrodes are made of two types of Cr-Cu, in which the diameter D of the main body is 16 mm, the radius of curvature of the tip (tip R) is 40 mm, the radius of curvature of the continuous portion is 8 mm, and the tip diameter d is 6 mm or 8 mm. DR electrode was used.
Table 2 shows the steel plate and electrodes used in each experimental example, and the welding conditions. Numerical values outside the scope of the present invention are underlined. The cool time between the pre-energization and the main energization was set to zero.

Figure 2021074737
Figure 2021074737

表3に、それぞれの実験例について、遅れ破壊及び継手強度の評価結果を示す。 Table 3 shows the evaluation results of delayed fracture and joint strength for each experimental example.

遅れ破壊の評価は、継手を板表面に垂直で板長手方向に、ナゲットの中心を通る断面で切断し、この切断片からナゲットを含む試験片を切り出し、切断面を研磨し、研磨された切断面を光学顕微鏡で観察して行った。この試験を5片の試験片に実施し、試験片5片とも割れが発生しない場合を「遅れ破壊無し」とした。 In the evaluation of delayed failure, the joint is cut in the longitudinal direction of the plate perpendicular to the plate surface with a cross section passing through the center of the nugget, a test piece containing the nugget is cut out from this cut piece, the cut surface is polished, and the polished cut is performed. The surface was observed with an optical microscope. This test was carried out on 5 test pieces, and the case where no cracks occurred in any of the 5 test pieces was defined as "no delayed fracture".

継手強度は、引張せん断試験及(JIS Z3136)び十字引張試験(JIS Z3137)により測定した引張せん断強度(TSS)及び十字引張強度(CTS)により評価した。TSSが19kN以下、又は、CTSが6.2以下である場合を不合格と判断した。
尚、スポット溶接時に散りが発生した場合には「有り」と記載している。
The joint strength was evaluated by the tensile shear strength (TSS) and the cross tensile strength (CTS) measured by the tensile shear test and the cross tensile test (JIS Z3137). When the TSS was 19 kN or less or the CTS was 6.2 or less, it was judged as a failure.
If scattering occurs during spot welding, it is described as "yes".

Figure 2021074737
Figure 2021074737

本発明例に係る実験例3,4,5,10,11,12では、適切な条件で予備通電工程及び本通電工程を実施したことにより、継手強度に優れるとともに、耐遅れ破壊特性に優れた抵抗スポット溶接継手を製造することができた。 In Experimental Examples 3, 4, 5, 10, 11 and 12 according to the example of the present invention, the joint strength was excellent and the delayed fracture resistance was excellent by carrying out the preliminary energization step and the main energization step under appropriate conditions. A resistance spot welded joint could be manufactured.

比較例である実験例1では、予備通電工程を行わずに本通電工程を行ったことに起因して、水素源となる水と油が残った状態で本通電工程を行ったことにより、遅れ破壊が発生した。
比較例である実験例2では、電極の先端径が小さかったことに起因して、電極と鋼板との接触面積が小さく、水素源となる水と油を十分に取り除くことができず、遅れ破壊が発生した。
比較例である実験例6では、予備通電工程におけるP1/h、すなわち板厚に対する加圧力が小さかったことに起因して、電極と鋼板との接触面積が小さく、水素源となる水と油を十分に取り除くことができず、遅れ破壊が発生した。
比較例である実験例7では、予備通電工程におけるI1/h、すなわち板厚に対する電流が大きかったことに起因して散りが発生した。
比較例である実験例8では、本通電工程におけるP2/h、すなわち板厚に対する加圧力が小さかったことに起因して、電極と鋼板との接触面積が小さくなり、電流密度が大きくなり散りが発生した。
比較例である実験例9では、本通電工程におけるI2/h、すなわち板厚に対する電流が小さかったことに起因して入熱が不十分であり、ナゲット形成が困難となり十分な継手強度を得られなかった。
In Experimental Example 1, which is a comparative example, due to the fact that the main energization step was performed without performing the pre-energization step, the main energization step was performed with water and oil as hydrogen sources remaining, which was delayed. Destruction has occurred.
In Experimental Example 2, which is a comparative example, the contact area between the electrode and the steel plate is small due to the small tip diameter of the electrode, and water and oil, which are hydrogen sources, cannot be sufficiently removed, resulting in delayed fracture. There has occurred.
In Experimental Example 6, which is a comparative example, the contact area between the electrode and the steel plate is small due to P1 / h in the pre-energization step, that is, the pressing force with respect to the plate thickness is small, and water and oil serving as hydrogen sources are used. It could not be removed sufficiently, and delayed destruction occurred.
In Experimental Example 7, which is a comparative example, scattering occurred due to a large current with respect to I1 / h in the pre-energization step, that is, the plate thickness.
In Experimental Example 8, which is a comparative example, the contact area between the electrode and the steel plate becomes small, the current density becomes large, and the scattering occurs due to the fact that P2 / h in the main energization step, that is, the pressing force with respect to the plate thickness is small. Occurred.
In Experimental Example 9, which is a comparative example, heat input is insufficient due to the small current of I2 / h, that is, the plate thickness in the main energization step, which makes it difficult to form a nugget and obtains sufficient joint strength. There wasn't.

本発明によれば、継手強度に優れるとともに耐遅れ破壊特性に優れた抵抗スポット溶接継手を提供することができ、産業上の利用価値が高い。 According to the present invention, it is possible to provide a resistance spot welded joint having excellent joint strength and delayed fracture resistance, and has high industrial utility value.

1 第一鋼板
2 第二鋼板
10 第一電極
11 先端部
12 連続部
13 本体部
20 第二電極
21 先端部
22 連続部
23 本体部
1 1st steel plate 2 2nd steel plate 10 1st electrode 11 Tip part 12 Continuous part 13 Main body part 20 Second electrode 21 Tip part 22 Continuous part 23 Main body part

Claims (4)

引張強度が780MPa以上である鋼板を少なくとも一枚含む複数枚の鋼板を重ね合わせて抵抗スポット溶接により接合し、抵抗スポット溶接継手を製造する方法であって、
先端径が8mm以上12mm以下である一対のDR電極を用いて前記鋼板の表面に加圧した状態で通電を行う予備通電工程と、
前記予備通電工程の後に、前記一対のDR電極を用いて前記鋼板の表面に加圧した状態で通電を行う本通電工程と、
前記本通電工程の後に前記一対のDR電極での加圧を保持する保持工程と、
を備え、
前記予備通電工程における加圧力をP1(kN)、電流をI1(kA)、通電時間をt1(s)とし、前記本通電工程における加圧力をP2(kN)、電流をI2(kA)としたとき、下記(1)式から(5)式を満足する抵抗スポット溶接継手の製造方法。
2≦I1/h<4 ・・・(1)式
1.5≦P1/h<2.5 ・・・(2)式
0.1≦t1/h≦1.5 ・・・(3)式
4≦I2/h ・・・(4)式
2.5≦P2/h ・・・(5)式
ただし、hは、前記複数枚の鋼板の合計板厚(mm)の1/2の値である。
It is a method of manufacturing a resistance spot welded joint by superimposing a plurality of steel plates including at least one steel plate having a tensile strength of 780 MPa or more and joining them by resistance spot welding.
A pre-energization step in which the surface of the steel sheet is pressurized by using a pair of DR electrodes having a tip diameter of 8 mm or more and 12 mm or less.
After the pre-energization step, a main energization step in which the surface of the steel sheet is pressurized by using the pair of DR electrodes is energized.
After the main energization step, a holding step of holding the pressurization by the pair of DR electrodes and a holding step.
With
The pressing force in the pre-energization step was P1 (kN), the current was I1 (kA), the energizing time was t1 (s), the pressing force in the main energizing step was P2 (kN), and the current was I2 (kA). When, a method for manufacturing a resistance spot welded joint that satisfies the following equations (1) to (5).
2≤I1 / h <4 ... (1) Equation 1.5≤P1 / h <2.5 ... (2) Equation 0.1≤t1 / h≤1.5 ... (3) Equation 4 ≦ I2 / h ・ ・ ・ (4) formula 2.5 ≦ P2 / h ・ ・ ・ (5) formula However, h is a value of 1/2 of the total thickness (mm) of the plurality of steel plates. is there.
前記保持工程における保持時間Ht(s)が0.4×h以下である請求項1に記載の抵抗スポット溶接継手の製造方法。 The method for manufacturing a resistance spot welded joint according to claim 1, wherein the holding time Ht (s) in the holding step is 0.4 × h 2 or less. 前記複数枚の鋼板の少なくとも一枚が亜鉛めっき鋼板である請求項1又は2に記載の抵抗スポット溶接継手の製造方法。 The method for manufacturing a resistance spot welded joint according to claim 1 or 2, wherein at least one of the plurality of steel sheets is a galvanized steel sheet. 前記保持工程における保持時間Ht(s)が0.08×h以上である請求項3に記載の抵抗スポット溶接継手の製造方法。 The method for manufacturing a resistance spot welded joint according to claim 3, wherein the holding time Ht (s) in the holding step is 0.08 × h 2 or more.
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Publication number Priority date Publication date Assignee Title
JP2004358500A (en) * 2003-06-04 2004-12-24 Daihatsu Motor Co Ltd Spot welding method and spot welding device
JP2010172946A (en) * 2009-01-30 2010-08-12 Jfe Steel Corp Resistance spot welding method of high-strength steel sheet
WO2014171495A1 (en) * 2013-04-17 2014-10-23 新日鐵住金株式会社 Spot welding method
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