JP2008229720A - Spot-welded joint of high-strength steel sheets excellent in tensile strength, automotive component having the same joint, and spot-welding method of high-strength steel sheets - Google Patents
Spot-welded joint of high-strength steel sheets excellent in tensile strength, automotive component having the same joint, and spot-welding method of high-strength steel sheets Download PDFInfo
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Description
本発明は、高炭素含有量の高張力鋼板をスポット溶接にて接合する技術に関する。 The present invention relates to a technique for joining high-tensile steel sheets having a high carbon content by spot welding.
近年、自動車車体の軽量化や高剛性化を図るため、高張力鋼（いわゆるハイテン材）の使用が進められているが、さらなる軽量化や高剛性化の要請に答えるべく、より高強度の高張力鋼の使用技術が求められている。高張力鋼に含まれる各種合金元素のうち、Ｃはハイテン化に最も有用な元素のひとつであり、鋼板を容易に高強度化できるとともに、延性などを向上させるための調質も容易である。 In recent years, the use of high-tensile steel (so-called high-tensile material) has been promoted to reduce the weight and rigidity of automobile bodies. There is a demand for the use of tensile steel. Among various alloy elements contained in high-strength steel, C is one of the most useful elements for high tempering, and the steel sheet can be easily increased in strength and tempered for improving ductility and the like.
しかしながら、Ｃ含有量の高い高張力鋼板をスポット溶接すると、接合部が急速に加熱・冷却されるため過度に硬化してしまい、素材鋼板の機械的特性には遠く及ばない接合部となる欠点があった。 However, when spot-welding a high-tensile steel plate with a high C content, the joint is rapidly heated and cooled so that it is hardened excessively, resulting in a disadvantage that the joint is not far from the mechanical properties of the material steel plate. there were.
このため、従来から、高張力鋼をスポット溶接して得られる溶接継手の疲労強度等を改善する有力な手段として、溶接通電（本通電）の後に本通電より低い電流量でテンパ通電を行ってテンパ処理（焼戻し）を行う２段通電方式のスポット溶接方法が採用されている（例えば、特許文献１、２参照）。
For this reason, as a powerful means to improve the fatigue strength of welded joints obtained by spot welding high-strength steel, temper energization has been performed at a lower current than main energization after welding energization (main energization). A two-stage energization spot welding method that performs tempering (tempering) is employed (for example, see
しかしながら、近年の高張力鋼板のさらなる高強度化（高Ｃ化）に伴い、上記２段通電方式のスポット溶接によっても十分な溶接継手の引張強度が得られなくなってきている。 However, with the further increase in strength (high C) of high-strength steel sheets in recent years, sufficient tensile strength of welded joints cannot be obtained even by spot welding using the above-described two-stage energization method.
そこで、例えば、特許文献３には、溶接電極チップの外周に絶縁材を介して配設したテンパ専用電極チップによりテンパ電流を通電してナゲット外周の熱影響部を軟化処理するスポット溶接方法が開示されている。 Therefore, for example, Patent Document 3 discloses a spot welding method in which a temper current is applied by a temper-dedicated electrode tip disposed on an outer periphery of a welding electrode tip via an insulating material to soften a heat-affected portion on the outer periphery of the nugget. Has been.
また、特許文献４には、上記２段通電のあとで、溶接電極による加圧力を増大させて溶接部の周囲の残留応力を減少させるスポット溶接方法が開示されている。 Patent Document 4 discloses a spot welding method in which, after the above-described two-stage energization, the pressure applied by the welding electrode is increased to reduce the residual stress around the welded portion.
さらに、特許文献５では、予備通電と本通電とからなる２段通電方式において、予備通電の初期に電流量を漸変的に増加させるとともに、本通電の終期に電流量を漸変的に減少させることで、十分なナゲット径を確保するとともに、熱影響部の硬度傾斜の緩斜面化を図り、応力つまり歪みの集中を緩和して割れの発生を防止するスポット溶接方法が開示されている。 Furthermore, in Patent Document 5, in the two-stage energization method including preliminary energization and main energization, the current amount is gradually increased at the initial stage of the pre-energization, and the current amount is gradually decreased at the end of the main energization. By doing so, a spot welding method is disclosed in which a sufficient nugget diameter is secured, the hardness slope of the heat affected zone is made gentle, and the concentration of stress, that is, strain, is alleviated to prevent the occurrence of cracks.
しかしながら、上記特許文献３に開示されたスポット溶接方法は、従来のスポット溶接機を用いることができず、新たな装置を必要とするため、設備コストが高くなる問題がある。 However, the spot welding method disclosed in Patent Document 3 cannot use a conventional spot welder, and requires a new device, resulting in a problem of increased equipment costs.
また、上記特許文献４および５に記載されたスポット溶接方法は、従来の本通電とテンパ通電との組合せからなる単純な２段通電方式に比べて、制御項目が多いため、接合対象の鋼種やその板厚が変わるごとに煩雑な条件設定作業を行う必要があり、実用性に問題があった。
そこで、本発明は、従来の、本通電とテンパ通電との組合せからなる単純な２段通電方式のスポット溶接にて形成しうる、引張強度に優れた高張力鋼板スポット溶接継手、および、その溶接継手を有する自動車部品、ならびに、高張力鋼板のスポット溶接方法を提供することを目的とする。 Therefore, the present invention provides a conventional high-strength steel spot-welded joint excellent in tensile strength that can be formed by spot welding of a simple two-stage energization method composed of a combination of main energization and temper energization, and welding thereof. It is an object of the present invention to provide an automotive part having a joint and a spot welding method for a high-tensile steel plate.
本発明者らは、後記実施例２で示すように、スポット溶接継手のナゲット中心から熱影響部を経て母相に至るビッカース硬さ分布曲線が、前記熱影響部において極大値を有する場合に、該スポット溶接継手の引張強度が格段に向上することを見出し、この知見に基づいて以下の発明を完成させるに至った。 When the Vickers hardness distribution curve from the nugget center of the spot welded joint through the heat-affected zone to the parent phase has a maximum value in the heat-affected zone, as shown in Example 2 below, It has been found that the tensile strength of the spot welded joint is remarkably improved, and the following invention has been completed based on this finding.
請求項１に記載の発明は、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせてスポット溶接して得られたスポット溶接継手であって、当該溶接継手のナゲット中心から熱影響部を経て母相に至るビッカース硬さ分布曲線が、熱影響部において極大値Ｈｖｍａｘを有するとともに、この極大値Ｈｖｍａｘと、この極大値を示す点よりナゲット側での、熱影響部におけるビッカース硬さの最小値Ｈｖｍｉｎとの差（Ｈｖｍａｘ−Ｈｖｍｉｎ）が、５Ｈｖ以上であることを特徴とする引張強度に優れた高張力鋼板スポット溶接継手である。
ただし、前記ビッカース硬さは、ＪＩＳ２２４４に基づき、荷重０．９８０６６５Ｎにて測定した値である。
The invention according to
However, the Vickers hardness is a value measured with a load of 0.980665N based on JIS2244.
請求項２に記載の発明は、前記極大値Ｈｖｍａｘと母相の平均ビッカース硬さＨｖＢＭとの差（Ｈｖｍａｘ−ＨｖＢＭ）が、５０Ｈｖ以上である請求項１に記載の引張強度に優れた高張力鋼板スポット溶接継手である。
The invention according to
請求項３に記載の発明は、ナゲットの中心部におけるビッカース硬さＨｖＮＣと母相の平均ビッカース硬さＨｖＢＭとの差（ＨｖＮＣ−ＨｖＢＭ）が、０Ｈｖ以上である請求項１または２に記載の引張強度に優れた高張力鋼板スポット溶接継手請求項１または２に記載の引張強度に優れた高張力鋼板スポット溶接継手である。
In the invention described in claim 3, the difference between the Vickers hardness Hv NC at the center of the nugget and the average Vickers hardness Hv BM of the parent phase (Hv NC −Hv BM ) is 0 Hv or more. A high-strength steel spot welded joint with excellent tensile strength according to
請求項４に記載の発明は、請求項１〜３のいずれか1項に記載のスポット溶接継手を有する自動車部品である。
The invention according to claim 4 is an automotive part having the spot welded joint according to any one of
請求項５に記載の発明は、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせてスポット溶接する方法であって、前記高張力鋼板の重ね合わせ部に本通電を行った後、１．０ｓ以上３．０ｓ以下の無通電時間を置き、さらにその後に本通電の電流量の５０％以上１００％未満の電流量を０．５ｓ以上１．５ｓ以下通電するテンパ通電を行い、請求項１〜３のいずれか1項に記載のスポット溶接継手を得ることを特徴とする高張力鋼板のスポット溶接方法である。
The invention according to claim 5 is a method for performing spot welding by superimposing high-tensile steel sheets having a C content of 0.10% by mass or more and 0.30% by mass or less, wherein the overlapping part of the high-tensile steel sheet After conducting the energization, a non-energization time of 1.0 s or more and 3.0 s or less is set, and then the current amount of 50% or more and less than 100% of the current amount of the main energization is 0.5 s or more and 1.5 s or less It is a spot welding method for a high-tensile steel sheet, characterized in that a temper energization is performed to obtain the spot welded joint according to any one of
本発明によれば、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせてスポット溶接して得られたスポット溶接継手であっても、そのナゲット中心から熱影響部を経て母相に至るビッカース硬さ分布曲線が、熱影響部において極大値を有するとともに、この極大値と、この極大値を示す点（極大点）よりナゲット側での、熱影響部におけるビッカース硬さの最小値との差を所定値（５Ｈｖ）以上とすることで、当該スポット溶接継手に引張荷重が掛ったときに、該極大点の両側に歪が分散するので、従来のように低い引張荷重で破断することが防止され、引張強度に優れたスポット溶接継手を提供できるようになった。 According to the present invention, even if it is a spot welded joint obtained by spot welding with high-tensile steel sheets having a C content of 0.10 mass% or more and 0.30 mass% or less overlapped from the center of the nugget, The Vickers hardness distribution curve that reaches the parent phase through the heat-affected zone has a maximum value in the heat-affected zone, and the heat-affected zone on the nugget side from this maximum value and the point indicating this maximum value (maximum point) By making the difference from the minimum value of Vickers hardness at a predetermined value (5 Hv) or more, when a tensile load is applied to the spot welded joint, strain is dispersed on both sides of the local maximum point. Therefore, it is possible to provide a spot welded joint which is prevented from breaking at a low tensile load and has excellent tensile strength.
以下、本発明をさらに詳細に説明する。 Hereinafter, the present invention will be described in more detail.
〔本発明に係る高張力鋼板スポット溶接継手の構成〕
本発明に係る、引張強度に優れた高張力鋼板スポット溶接継手は、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせてスポット溶接して得られたスポット溶接継手であって、当該溶接継手のナゲット中心から熱影響部を経て母相に至るビッカース硬さ分布曲線が、熱影響部において極大値Ｈｖｍａｘを有するとともに、この極大値Ｈｖｍａｘと、この極大値を示す点よりナゲット側での、熱影響部におけるビッカース硬さの最小値Ｈｖｍｉｎとの差（Ｈｖｍａｘ−Ｈｖｍｉｎ）が、５Ｈｖ以上であることを特徴とする。
[Configuration of Spot Welded Joint of High Strength Steel Plate According to the Present Invention]
The high-tensile steel spot welded joint with excellent tensile strength according to the present invention was obtained by spot welding with overlapping high-tensile steel sheets having a C content of 0.10% by mass to 0.30% by mass. It is a spot welded joint, and the Vickers hardness distribution curve from the nugget center of the welded joint through the heat affected zone to the parent phase has a maximum value Hv max in the heat affected zone, and this maximum value Hv max The difference (Hv max −Hv min ) from the minimum value Hv min of the Vickers hardness in the heat-affected zone on the nugget side from the point showing the maximum value is 5 Hv or more.
ここで、高張力鋼板のＣ含有量を０．１０質量％以上０．３０質量％以下としたのは、鋼板のＣ含有量が高くなるほど、スポット溶接後の焼入れ作用により継手部が硬化して割れやすくなるため、このような従来、スポット溶接が困難であったＣ含有量の高い高張力鋼板を対象としたものである。なお、Ｃ含有量を高くしすぎると母材自体の硬さが高くなりすぎて成形性が劣化するので、０．３質量％を上限とした。より適したＣ含有量は０．１５質量％以上０．２５質量％である。 Here, the C content of the high-tensile steel plate is set to 0.10% by mass or more and 0.30% by mass or less, as the C content of the steel plate increases, the joint portion hardens due to the quenching action after spot welding. Since it becomes easy to break, it is intended for such a high-tensile steel plate having a high C content, which has been difficult to spot weld. If the C content is too high, the hardness of the base material itself becomes too high and the formability deteriorates, so 0.3 mass% was made the upper limit. A more suitable C content is 0.15% by mass or more and 0.25% by mass.
そして、当該溶接継手のナゲット中心から熱影響部を経て母相に至るビッカース硬さ分布曲線が、熱影響部において極大値を有するとともに、その極大値と、その極大点よりナゲット側での、熱影響部における最小値との差が所定値（５Ｈｖ）以上になるようにすることで、当該溶接継手に引張荷重が掛ったときに、該極大点の両側に応力つまり歪が分散するので、従来のように低い引張荷重で破断することが防止され、引張強度に優れたスポット溶接継手が得られる。 The Vickers hardness distribution curve from the nugget center of the welded joint through the heat-affected zone to the parent phase has a maximum value in the heat-affected zone, the maximum value, and the heat on the nugget side from the maximum point. By making the difference from the minimum value in the affected part equal to or greater than a predetermined value (5 Hv), when a tensile load is applied to the welded joint, stress or strain is dispersed on both sides of the maximum point. As described above, it is possible to prevent breakage at a low tensile load, and to obtain a spot welded joint excellent in tensile strength.
ここで、従来、Ｃ含有量が高い高張力鋼板同士をスポット溶接すると低い引張荷重にて溶接継手部で破断割れが発生しやすかったのは、以下の理由によると想定される。 Here, conventionally, when high-strength steel plates having a high C content are spot-welded together, it is assumed that the fractures are likely to occur in the welded joint portion with a low tensile load because of the following reasons.
すなわち、Ｃ含有量が高い高張力鋼板同士を１段通電方式でスポット溶接すると、図１（ａ）に模式的に示すように、溶接継手部の硬さ分布は、ナゲット部から熱影響部の途中までほぼ一定の高い値を示した後、急激に低下して母相（母材）の値に近づく（後記実施例２の図６（ａ）参照）。つまり、通電による抵抗加熱にてナゲットと熱影響部が形成されるが、ナゲット部と、熱影響部のうちナゲット部に近い側の一定領域では、通電後の無通電時に、水冷構造の溶接電極で急冷され、その焼き入れ作用により著しく硬化して母材（母相）に比べて大幅に高い、ほぼ一定のレベルの硬さを示す。しかしながら、熱影響部のうちナゲット部から一定距離以上離れて母相に近づくにつれて、上記加熱および急冷作用が緩和されて、硬さが急激に低下して母材（母相）の硬さに近づくこととなる。 That is, when high-strength steel sheets having a high C content are spot-welded with a one-stage energization method, as schematically shown in FIG. 1 (a), the hardness distribution of the welded joint portion is from the nugget portion to the heat affected zone. After showing a substantially constant high value halfway, it rapidly decreases and approaches the value of the parent phase (base material) (see FIG. 6A of Example 2 described later). In other words, the nugget and the heat-affected zone are formed by resistance heating by energization, but in the nugget zone and a certain area near the nugget portion of the heat-affected zone, a water-cooled welding electrode is used when there is no energization after energization. It is rapidly cooled and is hardened by its quenching action and exhibits a substantially constant level of hardness that is significantly higher than that of the base material (matrix). However, the heating and quenching actions are alleviated and the hardness is drastically lowered to approach the hardness of the base material (matrix phase) as it approaches the parent phase away from the nugget portion in the heat affected zone. It will be.
そして、このような硬さ分布を有する継手に引張荷重を掛けると、図１（ｂ）に模式的に示すように、硬さが急激に低下する狭い領域Ａに歪（半径方向歪）が集中して、この領域で破断が起こりやすいため、低い引張荷重で破断が生じていたためと想定される。 When a tensile load is applied to the joint having such a hardness distribution, as schematically shown in FIG. 1B, strain (radial strain) is concentrated in a narrow region A where the hardness sharply decreases. And since it is easy to generate | occur | produce a fracture | rupture in this area | region, it is assumed that it fractured | ruptured with the low tensile load.
また、Ｃ含有量が高い高張力鋼板同士を２段通電方式でスポット溶接しても、本通電後の無通電時間が比較的短い場合は、ナゲット部で硬さがやや低下する傾向が認められるものの、硬さ分布曲線全体としては、図１（ａ）に近似した形状の曲線が得られる（後記実施例２の図６（ｂ）、（ｃ）参照）。 In addition, even when spot-welding high-tensile steel sheets having a high C content to each other by the two-stage energization method, if the non-energization time after the main energization is relatively short, the hardness tends to decrease slightly at the nugget portion. However, as a whole hardness distribution curve, a curve having a shape approximate to that in FIG. 1A is obtained (see FIGS. 6B and 6C in Example 2 described later).
したがって、２段通電方式を採用した場合でも、従来のように、本通電後の無通電時間が短いときには、上記１段通電方式と同様、硬さが急激に低下する狭い領域Ａにだけ歪が集中して、この領域で破断が起こりやすいため、低い引張荷重で破断が発生していたものと想定される。 Therefore, even when the two-stage energization method is adopted, when the non-energization time after the main energization is short as in the prior art, the distortion is applied only to the narrow region A where the hardness sharply decreases as in the case of the one-stage energization method. It is presumed that the fracture occurred at a low tensile load because it was concentrated and the fracture was likely to occur in this region.
これに対して、本発明に係る溶接継手では、図２（ａ）に模式的に示すような、熱影響部に明瞭な極大点Ｐを有するビッカース硬さ分布曲線とする（後記実施例２の図７（ａ）〜（ｃ）参照）ことで、図２（ｂ）に模式的に示すように、従来１箇所に集中していた歪を、この極大点Ｐの両側に分散させることができ、その結果、従来よりも格段に高い引張強度が得られるものと想定される。 On the other hand, in the welded joint according to the present invention, a Vickers hardness distribution curve having a clear maximum point P in the heat affected zone as schematically shown in FIG. 7 (a) to (c)), as schematically shown in FIG. 2 (b), the strain that has been concentrated in one place in the past can be dispersed on both sides of the maximum point P. As a result, it is assumed that a remarkably high tensile strength can be obtained.
したがって、歪を極大点の両側に確実に分散させることが重要であり、このためには、熱影響部に存在する極大点を明瞭にすることが必要である。すなわち、極大値Ｈｖｍａｘと、極大点よりナゲット側での、熱影響部における最小値Ｈｖｍｉｎとの差（Ｈｖｍａｘ−Ｈｖｍｉｎ）は、少なくとも、５Ｈｖ以上とする必要があり、好ましくは３０Ｈｖ以上、より好ましくは５０Ｈｖ以上とするのがよい。 Therefore, it is important to surely distribute the strain on both sides of the maximum point, and for this purpose, it is necessary to clarify the maximum point existing in the heat-affected zone. That is, the difference (Hv max −Hv min ) between the maximum value Hv max and the minimum value Hv min in the heat-affected zone on the nugget side from the maximum point needs to be at least 5 Hv, preferably 30 Hv or more. More preferably, it is good to set it as 50 Hv or more.
また、極大値Ｈｖｍａｘと母相の平均ビッカース硬さＨｖＢＭとの差（Ｈｖｍａｘ−ＨｖＢＭ）は、好ましくは５０Ｈｖ以上、さらに好ましくは８０Ｈｖ以上、特に好ましくは１００Ｈｖ以上とするのがよい。これにより、熱影響部に存在する極大点をさらに明瞭にすることができ、歪をより確実に極大点の両側に分散させることができる。 The difference between the maximum value Hv max and the average Vickers hardness Hv BM of the parent phase (Hv max −Hv BM ) is preferably 50 Hv or more, more preferably 80 Hv or more, and particularly preferably 100 Hv or more. Thereby, the local maximum point existing in the heat-affected zone can be further clarified, and the strain can be more reliably distributed on both sides of the local maximum point.
さらに、ナゲットの中心部におけるビッカース硬さＨｖＮＣと母相の平均ビッカース硬さＨｖＢＭとの差（ＨｖＮＣ−ＨｖＢＭ）は、好ましくは０Ｈｖ以上、さらに好ましくは５０Ｈｖ以上、特に好ましくは１００Ｈｖ以上とするのがよい。このように、ナゲット部の硬さを母相（母材）の強度と同等ないし高く維持することで、最小値Ｈｖｍｉｎを過度に低下させることが防止されるため、より高い引張強度の溶接継手が得られる。 Further, the difference between the Vickers hardness Hv NC at the center of the nugget and the average Vickers hardness Hv BM of the parent phase (Hv NC −Hv BM ) is preferably 0 Hv or more, more preferably 50 Hv or more, particularly preferably 100 Hv or more. It is good to do. As described above, since the hardness of the nugget portion is maintained to be equal to or higher than the strength of the parent phase (base material), the minimum value Hv min is prevented from being excessively lowered, so that a welded joint with higher tensile strength is obtained. Is obtained.
〔本発明に係る高張力鋼板スポット溶接継手の適用対象部材〕
本発明に係る高張力鋼板スポット溶接継手を適用する対象部材としては、例えば、５９０ＭＰａ級以上、好ましくは７８０ＭＰａ級以上、特に好ましくは９８０ＭＰａ級以上の引張強度を有する高張力鋼板を用いてスポット溶接により組み立てられる自動車用部品が挙げられる。このような自動車部品としては、シートレール、サイドレールなどの、高強度化によって軽量化されたシート部材、センターピラー、レインフォースメント、ロッカーレインフォースメント、バンパーなどの、耐衝撃用部材や補強用部材が例示される。
[Applicable member of high-strength steel spot welded joint according to the present invention]
As a target member to which the high strength steel plate spot welded joint according to the present invention is applied, for example, spot welding using a high strength steel plate having a tensile strength of 590 MPa class or more, preferably 780 MPa class or more, particularly preferably 980 MPa class or more. There are automotive parts to be assembled. Such automobile parts include seat rails, side rails, etc., weight-reduced seat members, center pillars, reinforcements, rocker reinforcements, bumpers, etc. The member is exemplified.
〔本発明に係る、高張力鋼板のスポット溶接方法〕
本発明に係る高張力鋼板スポット溶接継手は、例えば、以下のようにして得ることができる。
[Spot Welding Method of High Tensile Steel Sheet According to the Present Invention]
The high strength steel plate spot welded joint according to the present invention can be obtained, for example, as follows.
すなわち、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせ、この重ね合わせ部に本通電を行った後、１．０ｓ以上３．０ｓ以下の無通電時間を置き、さらにその後に本通電の電流量の５０％以上１００％未満の電流量を０．５ｓ以上１．５ｓ以下通電するテンパ通電を行えばよい。 That is, after superposing high-tensile steel sheets having a C content of 0.10% by mass to 0.30% by mass and performing main energization on the overlapped portion, no energization of 1.0s to 3.0s. Temporary energization may be performed in which time is passed, and then a current amount of 50% to less than 100% of the current amount of main energization is applied for 0.5s to 1.5s.
なお、本通電の電流量（単位：Ａ）は、スポット溶接において通常行われているように、予備的な溶接試験により、ナゲット直径が４√ｔ（ｔ：板厚（単位：ｍｍ））となる電流量と、チリが発生する電流量とをそれぞれ求めておき、該ナゲット直径が４√ｔ（ｔ：板厚（単位：ｍｍ））となる電流量以上であって、該チリが発生する電流量未満の範囲で設定すればよい。また、本通電の通電時間は、鋼種や板厚、および上記のようにして設定した本通電の電流量に応じて０．２〜０．４ｓの範囲で設定すればよい。 Note that the current amount (unit: A) of the main energization is a nugget diameter of 4√t (t: plate thickness (unit: mm)) by a preliminary welding test, as is normally done in spot welding. The amount of current and the amount of current that generates dust are obtained, and the nugget diameter is equal to or greater than the amount of current that provides 4√t (t: plate thickness (unit: mm)), and the dust is generated. What is necessary is just to set in the range below current amount. Moreover, what is necessary is just to set the energization time of this energization in the range of 0.2-0.4 s according to the steel amount, the plate thickness, and the current amount of the main energization set as described above.
また、無通電時間は、鋼種や板厚に応じて１．０ｓ以上で設定すればよいが、長くしすぎると溶接作業効率が低下するので、３．０ｓ以下の範囲で設定するとよい。 Further, the non-energization time may be set at 1.0 s or more depending on the steel type and the plate thickness, but if it is too long, the welding work efficiency decreases, so it is preferable to set it within a range of 3.0 s or less.
テンパ通電の電流量は、鋼種や板厚に応じて本通電の電流量の５０％以上１００％未満の範囲で設定すればよいが、特に６０％以上８０％以下の範囲で設定するのが好ましい。 The amount of tempering current may be set in the range of 50% to less than 100% of the current amount of main current depending on the steel type and plate thickness, but is preferably set in the range of 60% to 80%. .
また、テンパ通電の通電時間は、鋼種や板厚、およびテンパ通電の電流量に応じて、本通電の通電時間より長い０．５ｓ以上の範囲で適宜設定すればよいが、通電時間を長くしすぎると、焼戻し効果が効きすぎてナゲット部の硬さ、すなわちナゲット部の強度自体が母相よりも低くなってしまうおそれが高まるとともに、溶接作業効率も低下するので、１．５ｓ以下の範囲で設定するとよい。 The energization time for temper energization may be set as appropriate within a range of 0.5 s or longer, which is longer than the energization time for main energization, depending on the steel type, plate thickness, and temper energization current amount. If it is too high, the tempering effect is too effective and the hardness of the nugget part, that is, the strength of the nugget part itself is likely to be lower than that of the parent phase, and the welding work efficiency is also reduced. It is good to set.
上記のように、特に、無通電時間を従来より長い１．０ｓ以上（ただし３．０ｓ以下）とすることで、熱影響部に明瞭な極大値を有する硬さ分布が得られ、引張強度に優れたスポット溶接継手が得られる。 As described above, in particular, by setting the non-energization time to 1.0 s or more (however, 3.0 s or less) longer than the conventional one, a hardness distribution having a clear maximum value in the heat-affected zone can be obtained, and the tensile strength can be increased. An excellent spot welded joint can be obtained.
ここで、熱影響部に明瞭な極大値を有する硬さ分布とするために、無通電時間を、従来より長めの、１．０ｓ以上（ただし３．０ｓ以下）に延長すればよい理由を以下に説明する。 Here, in order to obtain a hardness distribution having a clear maximum value in the heat-affected zone, the reason why the non-energization time should be extended to 1.0 s or more (however, 3.0 s or less), which is longer than before, is as follows. Explained.
すなわち、上述したように、無通電の期間中に、水冷構造の電極によりナゲット部からその周辺の熱影響部へと順次冷却効果が拡大していく。 That is, as described above, the cooling effect is gradually expanded from the nugget portion to the surrounding heat affected zone by the water-cooled structure electrode during the non-energized period.
しかしながら、無通電時間（すなわち、冷却時間）が１．０ｓ未満と短い場合は、ナゲット部は、冷却が不足して高温状態に留まり他の部位より電気抵抗が高くなる。このため、テンパ通電時の再発熱は主としてナゲット部で発生する。この結果、ナゲット部は焼戻し効果を受けてやや軟化し硬さが低下するものの、熱影響部にまでは十分に焼戻し効果が及ばず、後記実施例２の図６（ｂ）、（ｃ）に示すように、熱影響部における硬さ分布に明瞭な極大値は現れない。 However, when the non-energization time (that is, the cooling time) is as short as less than 1.0 s, the nugget portion is insufficiently cooled and stays in a high temperature state and has higher electrical resistance than other parts. For this reason, re-heating when the temper is energized mainly occurs in the nugget portion. As a result, the nugget portion is slightly softened due to the tempering effect and the hardness is lowered, but the tempering effect is not sufficiently reached even to the heat affected zone, and in FIGS. 6B and 6C of Example 2 described later. As shown, no clear maximum value appears in the hardness distribution in the heat affected zone.
これに対し、無通電時間（すなわち、冷却時間）が１．０ｓ以上と長くなると、ナゲット部は十分に冷却され低温状態に至り電気抵抗が低くなる一方、熱影響部のうちナゲット部寄りの部位はナゲット部ほどには十分に冷却されずに比較的高温状態に留まり他の部位より電気抵抗が高くなる。このため、テンパ通電時の再発熱は主として熱影響部のうちナゲット寄りの部位で発生する。この結果、熱影響部のうちナゲット寄りの部位は焼き戻し効果を受けて軟化し硬さが低下するので、後記実施例２の図７（ａ）〜（ｃ）に示すように、熱影響部における硬さ分布に明瞭な極大値が現れる。 On the other hand, when the non-energization time (that is, the cooling time) is as long as 1.0 s or more, the nugget portion is sufficiently cooled to reach a low temperature state and the electric resistance is lowered, while the portion near the nugget portion in the heat affected zone. Does not cool as well as the nugget part and remains in a relatively high temperature state, and its electric resistance is higher than other parts. For this reason, re-heating when the temper is energized mainly occurs in a portion near the nugget in the heat affected zone. As a result, the portion near the nugget in the heat-affected zone is softened due to the tempering effect and decreases in hardness, so that the heat-affected zone as shown in FIGS. A clear local maximum appears in the hardness distribution at.
よって、無通電時間を１．０ｓ以上（ただし３．０ｓ以下）とすることで、熱影響部に明瞭な極大値を有する硬さ分布曲線を得ることができる。 Therefore, by setting the non-energization time to 1.0 s or more (however, 3.0 s or less), a hardness distribution curve having a clear maximum value in the heat affected zone can be obtained.
本発明の効果を確証するため、表１に示す成分組成を有する、３種類のＣ含有量レベルの高張力鋼板をスポット溶接し、得られた溶接継手について引張せん断試験および十字引張試験を行った。
スポット溶接に用いた溶接電極チップとしては、図３に示すような先端径８ｍｍのものを用い、溶接時の加圧力は５００ｋｇｆ（＝４９０３．３２５Ｎ）とした。また、本通電の電流量は、予備試験にて４．２５√ｔ（ｔ：板厚（ｍｍ））のナゲット直径（ｍｍ）が得られる電流量とした。 As the welding electrode tip used for spot welding, a tip having a tip diameter of 8 mm as shown in FIG. 3 was used, and the applied pressure during welding was 500 kgf (= 4903.325 N). In addition, the current amount of the main energization was set to a current amount at which a nugget diameter (mm) of 4.25√t (t: plate thickness (mm)) was obtained in the preliminary test.
引張せん断試験は、２枚の４０ｍｍ×１２５ｍｍ×１．４ｍｍの、同一成分組成からなる鋼板を図４に示すように片端部同士を重ね合わせてその重ね合わせ部をスポット溶接し、得られた溶接継手について、同図中の矢印の方向に引っ張って行った。 In the tensile shear test, two 40 mm × 125 mm × 1.4 mm steel plates having the same composition are overlapped with each other as shown in FIG. The joint was pulled in the direction of the arrow in the figure.
また、十字引張試験は、ＪＩＳ Ｚ３１３７に規定する試験方法に基づき、２枚の５０ｍｍ×１５０ｍｍ×１．４ｍｍの、同一成分組成からなる鋼板を図５に示すように十字に重ね合わせてその重ね合わせ部をスポット溶接し、得られた溶接継手について、同図中の矢印の方向に引っ張って行った。 The cross tension test is based on a test method specified in JIS Z3137, and two 50 mm × 150 mm × 1.4 mm steel plates having the same composition are overlapped on a cross as shown in FIG. The part was spot welded, and the obtained welded joint was pulled in the direction of the arrow in the figure.
熱影響部におけるビッカース硬さは、上記引張せん断強度試験用の溶接継手を板幅中心で板厚方向に切断し、その断面を接合界面に沿って１００μｍピッチで測定した。なお、ビッカース硬さの測定は、ＪＩＳ２２４４に基づき、荷重１００ｇｆ（＝０．９８０６６５Ｎ）にて行った。 The Vickers hardness in the heat-affected zone was measured by cutting the welded joint for the tensile shear strength test in the plate thickness direction at the plate width center, and measuring the cross section at a pitch of 100 μm along the bonding interface. In addition, the measurement of Vickers hardness was performed by load 100gf (= 0.980665N) based on JIS2244.
試験条件および試験結果を表２に示す。なお、同表中、発明例においては、［ａ］は、「極大点よりナゲット側での、熱影響部におけるビッカース硬さの最小値Ｈｖｍｉｎ」に相当し、［ｂ］は、「熱影響部におけるビッカース硬さの極大値Ｈｖｍａｘ」に相当する。 Test conditions and test results are shown in Table 2. In the table, in the invention examples, [a] corresponds to “minimum value Vv min hardness Vvmin hardness in the heat-affected zone on the nugget side from the maximum point”, and [b] Corresponds to the maximum value Hv max of the Vickers hardness in the part.
同表に示すように、（［ｂ］−［ａ］）すなわち（Ｈｖｍａｘ−Ｈｖｍｉｎ）が、５Ｈｖ以上となる場合（発明例）は、この値が５Ｈｖに満たない場合（比較例）に比べて、十字引張強度、引張せん断強度とも向上しており、特に十字引張強度で著しい向上効果が認められた。 As shown in the table, when ([b] − [a]), that is, (Hv max −Hv min ) is 5 Hv or more (invention example), this value is less than 5 Hv (comparative example). In comparison, both the cross tensile strength and the tensile shear strength were improved, and a remarkable improvement effect was recognized particularly in the cross tensile strength.
また、同表より、（Ｈｖｍａｘ−Ｈｖｍｉｎ）を５Ｈｖ以上とするには、Ｃ含有量が０．１０質量％以上０．３０質量％以下の高張力鋼板同士を重ね合わせ、この重ね合わせ部に本通電を行った後、１．０ｓ以上３．０ｓ以下の無通電時間を置き、さらにその後に本通電の電流量の５０％以上１００％未満の電流量を０．５ｓ以上１．５ｓ以下通電するテンパ通電を行えばよいことが確認できた。
次に、鋼種Ｂ（Ｃ：０．１８質量％、引張強度：７８０ＭＰａ級）の鋼板を用い、他の溶接条件は一定にして、２段通電方式にて無通電時間だけを変化させてスポット溶接を行い、溶接継手部のビッカース硬さ分布を測定し、硬さ分布曲線の変化の様子を調査した。 Next, spot welding was performed by using a steel plate of steel type B (C: 0.18 mass%, tensile strength: 780 MPa class), changing the non-energization time only in a two-stage energization method while keeping other welding conditions constant. The Vickers hardness distribution of the welded joint was measured, and the change of the hardness distribution curve was investigated.
なお、溶接条件としては、上記実施例１における引張せん断試験用の板寸法および接合方向を採用し、表１の試験Ｎｏ．４〜６と同じテンパ通電条件（電流量：本通電の８０％、通電時間：０．７ｓ）を採用した。 In addition, as welding conditions, the plate size and joining direction for the tensile shear test in Example 1 above were adopted, and the test No. 1 in Table 1 was adopted. The same temper energization conditions (current amount: 80% of main energization, energization time: 0.7 s) as in 4 to 6 were adopted.
ビッカース硬さの測定は、上記実施例１と同じ荷重（１００ｇｆ（＝０．９８０６６５Ｎ））にて、溶接継手を板幅中心で板厚方向に切断し、その断面を接合界面に沿って行った。ただし、ナゲットの中心部では、ナゲット中心点およびその左右両側それぞれ１００μｍ離れた２点の合計３点でのみ測定を行うとともに、ナゲットと熱影響部の界面からナゲット中心方向に５００μｍ入った点から、熱影響部を経て母相までの範囲を、母材の硬さと同じ硬さが３点続くまで、１００μｍピッチで測定を行った。なお、ナゲット中心部では上板と下板の接合界面上でのみ測定を行ったが、その他の部位では、接合界面から上下方向（板厚方向）にそれぞれ１００μｍ離れた高さ位置で、上板側、下板側双方の硬さ分布を測定した。
The Vickers hardness was measured by cutting the welded joint in the plate thickness direction at the plate width center at the same load (100 gf (= 0.980665N)) as in Example 1, and performing the cross section along the bonding interface. . However, at the center of the nugget, measurement is performed only at a total of three points, that is, two
調査結果を図６（従来例）および図７（発明例）に示す。なお、図６（ａ）は、テンパ通電を行わずに、本通電のみを行った１段通電方式の例である。 The investigation results are shown in FIG. 6 (conventional example) and FIG. 7 (invention example). FIG. 6A shows an example of a one-stage energization method in which only main energization is performed without performing temper energization.
図６から明らかなように、１段通電方式（同図（ａ））、および２段通電方式でも無通電時間が６０サイクル（＝１ｓ）に満たない場合（同図（ｂ）、（ｃ））には、熱影響部（ＨＡＺ）に明瞭な極大値が認められないのに対し、図７から明らかなように、無通電時間が６０ｓ（＝１ｓ）以上になると、熱影響部（ＨＡＺ）に明瞭な極大値が現れるのが確認できた。 As can be seen from FIG. 6, when the non-energization time is less than 60 cycles (= 1 s) even in the one-stage energization method (FIG. 6A) and the two-stage energization method (FIG. 6B, FIG. 6C). ), A clear maximum value is not recognized in the heat affected zone (HAZ), but as is clear from FIG. 7, when the non-energization time is 60 s (= 1 s) or more, the heat affected zone (HAZ) It was confirmed that a clear maximum value appeared.
上記実施例１および２では、通常の高張力鋼板に対して本発明の適用性を検討したが、本実施例では、高強度でかつ伸びに優れた鋼板として近年特に注目されているＴＲＩＰ型ベイナイト鋼（以下、「ＴＢＦ鋼」という。）（例えば、特開２００５−２２０４４０号公報、粕谷ら:Ｒ＆Ｄ神戸製鋼技報、Ｖｏｌ．５７、Ｎｏ．２（Ａｕｇ．２００７）ｐ．２７−３０参照）に対して本発明の適用性を検討した。 In Examples 1 and 2 above, the applicability of the present invention was examined with respect to a normal high-tensile steel plate. However, in this example, TRIP-type bainite has recently attracted particular attention as a steel plate having high strength and excellent elongation. Steel (hereinafter referred to as “TBF steel”) (see, for example, JP-A-2005-220440, Kajitani et al .: R & D Kobe Steel Engineering Reports, Vol. 57, No. 2 (Aug. 2007) p. 27-30) The applicability of the present invention was examined.
本実施例では９８０〜１４７０ＭＰａ級のＴＢＦ鋼板であって、その組成が質量％で、Ｃ：０．１５〜０．２２％、Ｓｉ：１．０〜２．０％、Ｍｎ：２．０〜２．８％、Ｃｕ：０．３％以下（０％を含む）、Ｎｉ：０．３％以下（０％を含む）、Ａｌ：０．０３〜０．０４５％、Ｎｂ：０．１％以下（０％を含む）、Ｔｉ：０．１％以下（０％を含む）、Ｂ：０．０３０％以下（０％を含む）、残部Ｆｅおよび不可避的不純物であり、全組織に対する面積率で、残留オーステナイト組織（以下、「残留γ」と記す。）を０％より多く、２５％以下含有するものを対象とした。 In this example, it is a TBF steel plate of 980 to 1470 MPa class, and its composition is mass%, C: 0.15 to 0.22%, Si: 1.0 to 2.0%, Mn: 2.0 to 2.8%, Cu: 0.3% or less (including 0%), Ni: 0.3% or less (including 0%), Al: 0.03-0.045%, Nb: 0.1% Below (including 0%), Ti: 0.1% or less (including 0%), B: 0.030% or less (including 0%), remaining Fe and unavoidable impurities, and area ratio to the entire structure Thus, the residual austenite structure (hereinafter referred to as “residual γ”) was included in an amount of more than 0% and 25% or less.
そして、下記表３に示す種々の成分組成および残留γを有するＴＢＦ鋼板（製造方法については、上記特開２００５−２２０４４０号公報を参照方）を、上記実施例１の発明例である試験Ｎｏ．６、１０、１４と同様の溶接条件にてスポット溶接し、得られた溶接継手について引張せん断試験および十字引張試験を行った。
試験条件および試験結果を下記表４に示す。同表に示すように、ＴＢＦ鋼板を用いた場合にも、スポット溶接条件を適切に設定することで、（［ｂ］−［ａ］）すなわち（Ｈｖｍａｘ−Ｈｖｍｉｎ）が５Ｈｖ以上となり、十字引張強度、引張せん断強度ともに優れた溶接継手が得られることが確認できた。
Claims (5)
ただし、前記ビッカース硬さは、ＪＩＳ２２４４に基づき、荷重０．９８０６６５Ｎにて測定した値である。 A spot-welded joint obtained by spot welding by superposing high-tensile steel sheets having a C content of 0.10% by mass to 0.30% by mass, wherein the heat affected zone is formed from the center of the nugget of the welded joint. The Vickers hardness distribution curve leading to the parent phase has a maximum value Hv max in the heat affected zone, and the Vickers hardness in the heat affected zone on the nugget side from the maximum value Hv max and the point indicating the maximum value. A high-tensile steel spot-welded joint excellent in tensile strength, characterized in that the difference (Hv max -Hv min ) from the minimum value Hv min is 5 Hv or more.
However, the Vickers hardness is a value measured with a load of 0.980665N based on JIS2244.
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