JP2013057646A - Fatigue characteristic evaluation method of t joint part in t-shaped welded joint structure - Google Patents
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本発明は、実際に溶接継手を作製して疲労試験を行なわなくても、Ｔ型溶接継手構造体におけるＴ継手部の疲労特性を簡便かつ迅速に評価（予測、推定）する方法に関するものである。本発明の評価方法は、例えば造船、海洋構造物、低温タンク、ラインパイプ、土木・建築構造物などのようなＴ型溶接継手構造体が適用される様々な分野に適用可能である。 The present invention relates to a method for simply and quickly evaluating (predicting and estimating) the fatigue characteristics of a T-joint portion in a T-type welded joint structure without actually producing a welded joint and conducting a fatigue test. . The evaluation method of the present invention can be applied to various fields to which T-type welded joint structures such as shipbuilding, offshore structures, low-temperature tanks, line pipes, civil engineering / building structures and the like are applied.
船舶や機械などのように厚鋼板が使用される溶接構造物では、繰り返し応力が加わるものが多いため、疲労強度の向上が求められている。溶接構造物の破壊の大半は疲労き裂の発生に起因しており、その疲労き裂の殆どは溶接継手部より発生する。溶接継手部の疲労強度は、母材に比べて著しく低いからである。 Many welded structures using thick steel plates, such as ships and machines, are often subjected to repeated stress, and therefore there is a demand for improved fatigue strength. Most of the fracture of the welded structure is caused by the occurrence of fatigue cracks, and most of the fatigue cracks are generated from the welded joint. This is because the fatigue strength of the welded joint is significantly lower than that of the base material.
溶接継手部における疲労特性は、溶接止端部（溶接金属と母材との境界）の母材側の形状（特に止端半径）の影響が大きいことが知られている。例えばＴ継手溶接部に繰り返し負荷を加えると、溶接止端部において疲労き裂が発生するが、この疲労き裂に対して溶接止端部の曲率半径が大きな影響を及ぼしている。そこで従来では、溶接継手部の疲労き裂の発生を防止して疲労特性を向上させるため、溶接止端部をグラインダーで研磨するなどして滑らかにし（溶接部形状の平坦化）、止端部の曲率半径を大きくすることによって止端部に発生するひずみを小さくし、応力集中を低減する方法が行なわれている（例えば非特許文献１を参照）。しかしながら、グラインダー処理を行うと、溶接時の作業工程が増加するため、作業負荷低減の観点からグラインダー処理の削減が強く望まれている。 It is known that the fatigue characteristics in the welded joint are greatly affected by the shape (particularly the toe radius) of the base end side of the weld toe end (boundary between the weld metal and the base metal). For example, when a load is repeatedly applied to a T-joint weld, a fatigue crack is generated at the weld toe, and the radius of curvature of the weld toe has a great influence on the fatigue crack. Therefore, conventionally, in order to prevent the occurrence of fatigue cracks in the welded joint and improve the fatigue characteristics, the weld toe is smoothed by grinding it with a grinder (flattening the shape of the weld), and the toe A method of reducing the stress concentration by increasing the curvature radius of the toe and reducing the stress concentration has been performed (see Non-Patent Document 1, for example). However, when the grinder process is performed, the number of work steps during welding increases, and therefore reduction of the grinder process is strongly desired from the viewpoint of reducing the work load.
一方、溶接継手部の材料特性（機械的特性や組織因子）が疲労特性に及ぼす影響については明確でなく、これまで殆ど研究されていない。そのため、溶接継手部の疲労特性を評価（予測）するに当たっては、多くの溶接継手の疲労試験結果を統計処理するなどして決定されているというのが実情である。具体的には、溶接継手部の条件を変化させ、各条件を反映させた溶接継手構造試験体を作製して疲労試験を実施しており、多大なコストと時間がかかっており、非現実的である。 On the other hand, the effect of material properties (mechanical properties and structure factors) of welded joints on fatigue properties is not clear and has not been studied so far. For this reason, in evaluating (predicting) the fatigue characteristics of welded joints, the fact is that they are determined by statistically processing the fatigue test results of many welded joints. Specifically, we changed the conditions of the welded joint part, produced welded joint structural specimens that reflected each condition, and conducted a fatigue test. It is.
そこで、特許文献１では、実際に溶接を行わずに鋼材の溶接熱影響部における疲労破壊感受性を簡便・迅速に評価する試験方法が開示されている。ここでは、疲労き裂は応力集中の最も激しい溶接止端部から発生し、伝播するが、疲労き裂が最も発生し易い位置は、溶接熱影響部（ＨＡＺ）である点に注目し、所定の熱履歴と切欠加工を賦与した試験片を用いてＨＡＺの疲労破壊感受性を評価している。しかしながら、上記特許文献１の方法は、溶接継手部の材料特性から検討されたものではないため、溶接継手部の疲労特性評価に有用な材料設計指針の提供が望まれている。 Therefore, Patent Document 1 discloses a test method for simply and quickly evaluating the fatigue fracture susceptibility of a welded heat-affected zone of a steel material without actually performing welding. Here, the fatigue crack is generated and propagated from the weld toe portion where the stress concentration is the most intense, but the position where the fatigue crack is most likely to occur is the weld heat affected zone (HAZ). The fatigue fracture susceptibility of HAZ is evaluated using a test piece imparted with heat history and notch processing. However, since the method of Patent Document 1 is not studied from the material characteristics of the welded joint, it is desired to provide a material design guideline useful for evaluating the fatigue characteristics of the welded joint.
本発明は上記事情に鑑みてなされたものであり、その目的は、Ｔ型溶接継手構造体におけるＴ継手部の疲労特性を、煩雑な疲労試験を行なうことなく、簡便かつ迅速に評価（予測・推定）し得る方法を提供することにある。 The present invention has been made in view of the above circumstances, and its object is to easily and quickly evaluate the fatigue characteristics of a T joint portion in a T-type welded joint structure without conducting a complicated fatigue test (prediction / It is to provide a method that can be estimated.
上記課題を解決することができた本発明の評価方法は、引張強度５００〜６５０ＭＰａのベイナイト組織鋼板を用いたＴ型溶接継手構造体におけるＴ継手部の疲労特性を評価する方法であって、前記Ｔ継手部の溶接熱影響部において、下記式（１）で表される繰り返し軟化パラメータ（１）を用いてＴ型溶接継手構造体の疲労特性を評価するところに要旨を有するものである。
繰り返し軟化パラメータ（１）＝１／√（Ａ×ＫＡＭ） ・・・ （１）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。
The evaluation method of the present invention capable of solving the above-mentioned problems is a method for evaluating the fatigue characteristics of a T joint part in a T-type welded joint structure using a bainite structure steel plate having a tensile strength of 500 to 650 MPa, In the welding heat-affected zone of the T joint portion, the summary is obtained by evaluating the fatigue characteristics of the T-type welded joint structure using the repeated softening parameter (1) represented by the following formula (1).
Repeat softening parameter (1) = 1 / √ (A × KAM) (1)
However, A × KAM> 10
Where
A is an average circle equivalent diameter (μm) of the crystal grains when a region surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more is defined as a crystal grain;
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
また、上記課題を解決することができた本発明の他の評価方法は、引張強度５００〜６５０ＭＰａのベイナイト組織鋼板を用いたＴ型溶接継手構造体におけるＴ継手部の疲労特性を、切欠き試験片を用いて予測する方法であって、前記切欠き試験片を用いて１０万回の疲労試験を行ったとき、下記式（２）で表される繰り返し軟化パラメータ（２）を用いて、負荷応力範囲Δσ（ＭＰａ）を切欠き試験片の引張強度ＴＳ（ＭＰａ）で除した値（Δσ／ＴＳ）で表わされる疲労特性を予測するところに要旨を有するものである。
繰り返し軟化パラメータ（２）
＝｛０．４８／√（Ａ×ＫＡＭ）｝＋０．４０ ・・・ （２）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、切欠き試験片において、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。
In addition, another evaluation method of the present invention that has solved the above-described problem is that a fatigue test is performed on the fatigue characteristics of a T-joint portion in a T-type welded joint structure using a bainite structure steel plate having a tensile strength of 500 to 650 MPa. This is a prediction method using a piece, and when a fatigue test is performed 100,000 times using the notched test piece, a load is calculated using a repeated softening parameter (2) represented by the following formula (2). It has a gist in predicting fatigue characteristics represented by a value (Δσ / TS) obtained by dividing the stress range Δσ (MPa) by the tensile strength TS (MPa) of the notched specimen.
Repeat softening parameter (2)
= {0.48 / √ (A × KAM)} + 0.40 (2)
However, A × KAM> 10
Where
A is an average equivalent circle diameter (μm) of a crystal grain when a region surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more in the notched specimen is used as a crystal grain. ,
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
本発明で規定する式（１）および式（２）のパラメータは、ベイナイト組織鋼板を用いて得られるＴ型溶接継手構造体におけるＴ継手部の溶接止端部疲労特性の代替評価パラメータとして有用であり、実際に溶接継手を作製して疲労試験を行なわなくても、Ｔ継手部の疲労特性を評価（予測、推定）することができる。 The parameters of the formulas (1) and (2) defined in the present invention are useful as an alternative evaluation parameter for the weld toe fatigue characteristics of the T joint in a T-type welded joint structure obtained using a bainite structure steel plate. Yes, it is possible to evaluate (predict and estimate) the fatigue characteristics of the T joint portion without actually producing a welded joint and conducting a fatigue test.
本発明に係るＴ継手部の疲労特性評価方法は、溶接止端部に局部的な塑性歪みが繰返し負荷される場合に非常に有用であり、式（１）に示すように、２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域で構成される結晶粒の平均円相当直径と、ＫＡＭ（結晶粒内の平均方位差）とで表される繰り返し軟化パラメータ（１）を用いることにより、Ｔ継手部の疲労特性を評価することができる。 The method for evaluating the fatigue characteristics of a T joint according to the present invention is very useful when local plastic strain is repeatedly applied to the weld toe. As shown in Equation (1), The repetitive softening parameter (1) expressed by the average equivalent circle diameter of the crystal grains composed of the region surrounded by the large-angle grain boundary with an orientation difference of 15 ° or more and the KAM (average orientation difference in the crystal grains) By using it, the fatigue characteristics of the T joint can be evaluated.
具体的には、式（２）で表される繰り返し軟化パラメータ（２）は、１０万回（１０5回）の繰り返し疲労試験に耐えられる疲労き裂寿命（疲労き裂寿命が１０万回に対応する負荷応力範囲Δσを、引張強度ＴＳで除した値；Δσ／ＴＳ）と密接な相関関係を有しており、当該パラメータ（２）を用いれば、Ｔ継手部の疲労特性を精度良く評価することができる。 Specifically, the repeated softening parameter (2) represented by the formula (2) is a fatigue crack life (with a fatigue crack life of 100,000 times) that can withstand a repeated fatigue test of 100,000 times (10 5 times). The value obtained by dividing the corresponding load stress range Δσ by the tensile strength TS; Δσ / TS) has a close correlation, and if this parameter (2) is used, the fatigue characteristics of the T joint can be accurately evaluated. can do.
本発明者らは、Ｔ型溶接継手構造体におけるＴ継手部の疲労特性を、煩雑な疲労試験を行なうことなく、材料設計学的観点（特に組織形態的観点）から評価（予測、推定）し得る方法を提供するため、検討してきた。その結果、下記式（１）または下記式（２）で表わされる、繰り返し軟化パラメータ（１）または（２）を用いれば、繰り返し応力が負荷されたＴ継手部の疲労特性を簡便且つ迅速に評価できることを見出し、本発明を完成した。本明細書において、「√（Ａ×ＫＡＭ）」とは（Ａ×ＫＡＭ）1/2を意味する。 The present inventors evaluated (predicted and estimated) the fatigue characteristics of the T-joint portion in the T-type welded joint structure from the viewpoint of material design (particularly from the viewpoint of structure morphology) without conducting a complicated fatigue test. Has been studied to provide a way to get. As a result, if the repeated softening parameter (1) or (2) represented by the following formula (1) or the following formula (2) is used, the fatigue characteristics of the T joint portion subjected to repeated stress can be evaluated easily and quickly. The present invention has been completed by finding out what can be done. In this specification, “√ (A × KAM)” means (A × KAM) 1/2 .
繰り返し軟化パラメータ（１）＝１／√（Ａ×ＫＡＭ） ・・・ （１）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。
Repeat softening parameter (1) = 1 / √ (A × KAM) (1)
However, A × KAM> 10
Where
A is an average circle equivalent diameter (μm) of the crystal grains when a region surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more is defined as a crystal grain;
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
繰り返し軟化パラメータ（２）
＝｛０．４８／√（Ａ×ＫＡＭ）｝＋０．４０ ・・・ （２）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、切欠き試験片において、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。
Repeat softening parameter (2)
= {0.48 / √ (A × KAM)} + 0.40 (2)
However, A × KAM> 10
Where
A is an average equivalent circle diameter (μm) of a crystal grain when a region surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more in the notched specimen is used as a crystal grain. ,
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
なお、疲労特性向上のメカニズムとしては、（ア）応力集中部でのき裂の発生防止と、（イ）一旦発生したき裂の進展を遅くすること、の二つが挙げられるが、本発明は、前者（ア）に関連する技術であり、本発明で提起する上記（１）または（２）のパラメータは、疲労き裂発生防止の指標として極めて有用である。詳細には後者（イ）では、き裂が発生した後のき裂先端の歪み集中を対象としているのに対し、本発明［前者（ア）］では、き裂がない場合の溶接止端部での歪み集中を対象としており、両者は、歪みが発生する領域（広さ）歪み量のレベルが全く相違している。後者（イ）では歪みが集中する領域が極めて狭いため（ミクロン単位）、繰り返し軟化によってその歪集中領域が大きくなり、最大の歪み量が低減するのに対し、本発明［前者（ア）］では歪みが集中する領域がもともと広いため（ミリ単位）、繰り返し軟化しても歪み集中領域はほとんど変わらず、逆に最大の歪み量が大きくなってしまう。 There are two mechanisms for improving fatigue properties: (a) prevention of cracks at stress-concentrated parts, and (b) slowing the growth of cracks once generated. The parameter (1) or (2) proposed in the present invention is a technique related to the former (a), and is extremely useful as an index for preventing the occurrence of fatigue cracks. In detail, the latter (A) targets strain concentration at the crack tip after the crack has occurred, whereas the present invention [former (A)] has a weld toe when there is no crack. The two are completely different from each other in the level of the distortion amount (area) distortion amount. In the latter (b), the strain concentration area is very narrow (in micron units), so that the strain concentration area becomes larger by repeated softening, and the maximum distortion amount is reduced. In the present invention [the former (a)] Since the region where the strain concentrates is originally wide (in millimeters), the strain concentration region is hardly changed even when repeatedly softened, and the maximum strain amount is increased.
これらのうち、繰り返し軟化パラメータ（１）は、溶接止端部に局部的な塑性歪みが繰返し負荷されたときの疲労特性の評価に有用な、本願発明の評価方法において基本となるパラメータである。一方、繰り返し軟化パラメータ（２）は、後記する実施例に記載の切欠き試験片を用いて１０万回の疲労試験を行ったときに耐えられる応力（１０万回の疲労寿命に対する疲労強度）を精度良く評価（予測、推定）する有用なパラメータである。なお、「１０万回の疲労寿命に対する疲労強度」の測定に当たっては、本発明では引張強度の影響を除くため、負荷応力範囲Δσ（ＭＰａ）を切欠き試験片の引張強度ＴＳ（ＭＰａ）で除した値（Δσ／ＴＳ）で算出することにした。 Among these, the repeated softening parameter (1) is a basic parameter in the evaluation method of the present invention, which is useful for evaluating fatigue characteristics when local plastic strain is repeatedly applied to the weld toe. On the other hand, the repetitive softening parameter (2) is a stress (a fatigue strength with respect to a fatigue life of 100,000 times) that can be endured when a fatigue test is performed 100,000 times using a notch test piece described in an example described later. It is a useful parameter that is evaluated (predicted and estimated) with high accuracy. In the measurement of “fatigue strength with respect to fatigue life of 100,000 times”, in the present invention, in order to eliminate the influence of tensile strength, the load stress range Δσ (MPa) is divided by the tensile strength TS (MPa) of the notched specimen. It was decided to calculate with the value (Δσ / TS).
本明細書では、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域で構成される結晶粒を「大角結晶粒」と呼び、当該結晶粒の平均円相当直径を「大角結晶粒の粒径」、または単に「大角粒径」と呼ぶ場合がある。本発明では、後記するとおりベイナイト組織鋼板を対象としているため、大角粒径はいわば、ベイナイトブロック径に相当する。 In this specification, a crystal grain composed of a region surrounded by a large-angle grain boundary whose orientation difference between two adjacent crystals is 15 ° or more is called a “large-angle crystal grain”, and the average equivalent circle diameter of the crystal grain is defined as Sometimes referred to as “large-angle grain size” or simply “large-angle grain size”. In the present invention, as will be described later, a bainite structure steel plate is used, so that the large-angle particle diameter corresponds to the bainite block diameter.
以下、本発明に到達した経緯について、本願出願人による先願発明（特願２０１０−２５８２１２に記載の発明）、およびその後の基礎実験を交えて説明する。 Hereinafter, the background of the present invention will be described together with the prior invention by the applicant of the present application (the invention described in Japanese Patent Application No. 2010-258212) and the subsequent basic experiment.
まず、先願発明の到達経緯について説明する。 First, the background of the invention of the prior application will be described.
本発明者らは、上記目的を達成するため、溶接継手形状の中でも溶接構造物において汎用されているＴ継手形状（Ｔ継手部）を対象とし、高強度鋼板を突き合わせ溶接した垂直部材と、高強度鋼板を突き合わせ溶接した水平部材を溶接によって接合してなるＴ型溶接継手構造体を用いて鋭意検討を行なった。 In order to achieve the above-mentioned object, the present inventors target a T-joint shape (T-joint portion) widely used in welded structures among weld joint shapes, and a vertical member obtained by butt welding a high-strength steel plate, The present inventors conducted intensive studies using a T-type welded joint structure in which horizontal members butt-welded to high-strength steel plates were joined by welding.
検討に当たっては、疲労特性との密接な関係が知られている溶接止端部曲率半径ρ（ｍｍ）だけでなく、材料特性因子として、特に均一伸び（ＵＥ）および降伏応力（ＹＰ）に着目した。これらに着目したのは、溶接止端部に発生する局部塑性歪み量が疲労特性の支配因子であり（図２を参照）、ＵＥおよびＹＰは、塑性歪みに関連するからである。また、疲労き裂は、歪み集中の最も激しい溶接止端部から発生・伝播し、この止端部は溶接熱影響部（ＨＡＺ）であることから、特に、ＨＡＺ部のＵＥおよび降伏応力に着目した。 In the study, we focused not only on the weld toe curvature radius ρ (mm), which has a close relationship with fatigue properties, but also on the uniform elongation (UE) and yield stress (YP) as material property factors. . The reason for focusing on these is that the amount of local plastic strain generated at the weld toe is a governing factor of fatigue characteristics (see FIG. 2), and UE and YP are related to plastic strain. In addition, fatigue cracks are generated and propagated from the weld toe where the strain concentration is the most intense, and this toe is a weld heat affected zone (HAZ), so pay particular attention to the UE and yield stress in the HAZ part. did.
その結果、Ｔ継手部の疲労強度は、溶接熱影響部の特性Ｓ（応力）−Ｓ（ひずみ）特性（引張変形特性）が影響を及ぼしていることが判明した。詳細には、Ｔ継手部の溶接止端部曲率半径ρ（ｍｍ）、Ｔ継手部の溶接熱影響部の均一伸びＵＥHAZ（％）、およびＴ継手部の溶接熱影響部の降伏応力ＹＰHAZ（ＭＰａ）の三要件を用い、上記要件以外の条件（例えば、水平部材および垂直部材の板厚、溶接方法など）は変化させずに同一条件とすることを前提にした場合、下記式（１）で表わされる溶接止端部歪み評価パラメータ（１）が、溶接止端部の疲労特性と良好な相関関係を有することを見出した。下記式（１）で算出されるパラメータの値が小さいほど（すなわち、Ｔ継手部の溶接熱影響部の降伏応力ＹＰHAZが大きいほど）、溶接止端部の局部塑性歪み量は大きくなり、疲労特性に優れることを示している。
溶接止端部歪み評価パラメータ（１）
＝（１．１３×１０-2×ρ-0.59）×（１．０５×１０-4×ＵＥHAZ＋１．６４×１０-2）×（５．１５×ＹＰHAZ -0.92） ・・・ （１）
As a result, it has been found that the fatigue strength of the T joint is affected by the characteristic S (stress) -S (strain) characteristic (tensile deformation characteristic) of the weld heat affected zone. Specifically, the weld toe curvature radius ρ (mm) of the T joint, the uniform elongation UE HAZ (%) of the weld heat affected zone of the T joint, and the yield stress YP HAZ of the weld heat affected zone of the T joint. When the three requirements (MPa) are used and the conditions other than the above requirements (for example, the thickness of the horizontal member and the vertical member, the welding method, etc.) are assumed to be the same without changing, the following formula (1 It was found that the weld toe distortion evaluation parameter (1) represented by (2) has a good correlation with the fatigue characteristics of the weld toe. The smaller the value of the parameter calculated by the following equation (1) (that is, the greater the yield stress YP HAZ of the weld heat affected zone of the T joint), the greater the amount of local plastic strain at the weld toe. It shows excellent characteristics.
Weld toe distortion evaluation parameters (1)
= (1.13 × 10 −2 × ρ −0.59 ) × (1.05 × 10 −4 × UE HAZ + 1.64 × 10 −2 ) × (5.15 × YP HAZ −0.92 ) (1 )
更に上記パラメータ（１）は、溶接止端部に局部的な塑性歪みが繰返し負荷されたときの疲労特性の評価にも有用であり、この場合は、下記式（２）の溶接止端部歪み評価パラメータを用いれば良いことが分かった。すなわち、鋼種などによっては、降伏応力を超える応力が繰返し負荷されることによって降伏応力および均一伸びが変化する場合があり、その場合、溶接止端部においても局部的な塑性歪みが繰返し負荷されることから、ＨＡＺ部の降伏応力および均一伸びが変化し、溶接止端部の疲労寿命に大きな影響を及ぼすようになるが、下記式（２）を用いれば、繰返し負荷によって降伏応力および均一伸びが変化する場合の疲労寿命を高い精度で評価することができる。ここで下記式（２）は、上記式（１）において、ＵＥHAZ（Ｔ継手部の溶接熱影響部の均一伸び）をＵＥHAZ,cycle（Ｔ継手部の溶接熱影響部の１０００００回（１０万回）塑性歪負荷後の均一伸び）に、ＹＰHAZ（Ｔ継手部の溶接熱影響部の降伏応力）をＹＰHAZ,cycle（Ｔ継手部の溶接熱影響部の１０００００回塑性歪負荷後の降伏応力）にそれぞれ置き換えたこと以外は、上記式（１）と全く同じである。
溶接止端部歪み評価パラメータ（２）
＝（１．１３×１０-2×ρ-0.59）×（１．０５×１０-4×ＵＥHAZ,cycle＋１．６４×１０-2）×（５．１５×ＹＰHAZ,cycle -0.92） ・・・ （２）
式中、ρはＴ継手部の溶接止端部曲率半径（ｍｍ）、ＵＥHAZ,cycleはＴ継手部の溶接熱影響部の１００００回塑性歪負荷後の均一伸び（％）、ＹＰHAZ,cycleはＴ継手部の溶接熱影響部の１００００回塑性歪負荷後の降伏応力（ＭＰａ）を意味する。
Furthermore, the parameter (1) is also useful for evaluating fatigue characteristics when local plastic strain is repeatedly applied to the weld toe. In this case, the weld toe strain of the following formula (2) is used. It was found that evaluation parameters should be used. That is, depending on the steel type, the yield stress and uniform elongation may change due to repeated loading of stress exceeding the yield stress. In this case, local plastic strain is repeatedly loaded even at the weld toe. As a result, the yield stress and uniform elongation of the HAZ part changes, and the fatigue life of the weld toe part is greatly affected. However, if the following equation (2) is used, the yield stress and uniform elongation are increased by repeated loading. The fatigue life when changing can be evaluated with high accuracy. Here, the following formula (2) is obtained by changing UE HAZ (uniform elongation of the welded heat affected zone of the T joint portion) to UE HAZ, cycle (100 thousand times of the welded heat affected zone of the T joined portion (10 10000 HAZ (uniform elongation after plastic strain loading) and YP HAZ (yield stress of welded heat-affected zone of T joint) after YP HAZ, cycle (welding heat-affected zone of T-jointed zone ) Except for replacing each with yield stress), it is exactly the same as equation (1) above.
Weld toe distortion evaluation parameters (2)
= (1.13 × 10 −2 × ρ −0.59 ) × (1.05 × 10 −4 × UE HAZ, cycle + 1.64 × 10 −2 ) × (5.15 × YP HAZ, cycle −0.92 ) (2)
Where ρ is the radius of curvature of the weld toe at the T joint (mm), UE HAZ, cycle is the uniform elongation (%) of the welded heat-affected zone of the T joint after 10,000 plastic strains, YP HAZ, cycle Means the yield stress (MPa) after 10,000 times plastic strain loading of the weld heat affected zone of the T joint.
このように先願発明は、上記式（１）または上記式（２）で表わされる溶接止端部歪み評価パラメータが、Ｔ型溶接継手構造体におけるＴ継手部の疲労特性の代替評価パラメータとして有用であることを見出した点に特徴があり、上記パラメータを用いれば、従来のように溶接継手を作成して疲労試験を現実に行なわなくても、Ｔ継手部の疲労特性を簡易且つ精度良く評価することができる。 Thus, in the prior invention, the weld toe distortion evaluation parameter represented by the above formula (1) or (2) is useful as an alternative evaluation parameter for the fatigue characteristics of the T joint in the T-type welded joint structure. It is characterized by the fact that it has been found, and if the above parameters are used, the fatigue characteristics of the T joint can be evaluated easily and accurately without creating a welded joint and conducting a fatigue test as in the past. can do.
更に本発明者らは、上記の先願発明を開示した後も、特に組織形態的観点から、Ｔ継手部の疲労特性を簡易且つ精度良く評価することができる方法を提供するため、検討を重ねてきた。本発明では特に、結晶粒自体が硬く、成分やプロセスなどの調整により結晶粒を均一分散させ易いために高強度と高靱性を両立できるなどの観点から、溶接構造物の素材として好適に用いられるベイナイト組織鋼板（ベイナイト組織を主体として含む鋼板であり、詳細は後述する。）を用いてＴ型溶接継手構造体を作製する場合について検討を行なった。 Furthermore, even after disclosing the above-mentioned invention of the prior application, the present inventors have repeatedly studied in order to provide a method capable of easily and accurately evaluating the fatigue characteristics of the T joint part from the viewpoint of the structure morphology. I came. Particularly in the present invention, the crystal grains themselves are hard and can be suitably used as a material for a welded structure from the viewpoint that both high strength and high toughness can be achieved because the crystal grains can be uniformly dispersed by adjusting components and processes. The case where a T-type welded joint structure was produced using a bainite structure steel sheet (a steel sheet mainly containing a bainite structure, details will be described later) was studied.
上記のようにベイナイト組織鋼板をＴ型溶接継手構造体の作製に用いると、溶接熱影響部にベイナイト組織が含まれることになるが、ベイナイト組織は、疲労き裂進展試験のような繰り返し変形を受けると加工軟化し、降伏応力が減少する傾向にあることが知られており、この現象は繰り返し軟化とも呼ばれている。すなわち、ベイナイト組織は通常、結晶粒内に多くの転位が蓄積されており、複雑に密集されているために転位の成長が阻害され、結果的に降伏応力が向上するが、このようなベイナイト組織を含む鋼材に対し、降伏応力を超える繰り返し歪を加えると、繰り返し歪によって蓄積されていた転位が合体・消滅し、転位の密集度（転位密度）が低下する。この状態では、転位は成長し易くなり、また新たな転位が発生し易くなるため、降伏応力は低減する。 As described above, when a bainite structure steel sheet is used for the production of a T-type welded joint structure, the bainite structure is included in the weld heat affected zone. However, the bainite structure undergoes repeated deformation as in a fatigue crack growth test. It is known that when subjected to work softening, the yield stress tends to decrease, and this phenomenon is also called repeated softening. In other words, the bainite structure usually has many dislocations accumulated in the crystal grains, and the density of the dislocations is hindered due to the complex concentration, resulting in an increase in yield stress. When a repeated strain exceeding the yield stress is applied to a steel material containing, dislocations accumulated by the repeated strain coalesce and disappear, and the density of dislocations (dislocation density) decreases. In this state, the dislocations are easy to grow and new dislocations are easily generated, so that the yield stress is reduced.
一方、前述した先願発明の知見によれば、き裂が発生する溶接熱影響部の降伏応力が小さいと、溶接止端部の局部塑性歪み量は小さくなり、疲労特性は低下する。すなわち、ベイナイト組織鋼板を用いた場合、繰り返し軟化による降伏応力の低下が大きいほど、溶接熱影響部における歪は増大し、疲労き裂は発生し易くなる。 On the other hand, according to the knowledge of the prior invention described above, if the yield stress of the weld heat affected zone where cracks occur is small, the amount of local plastic strain at the weld toe portion becomes small, and the fatigue characteristics deteriorate. That is, when a bainite-structure steel plate is used, the greater the decrease in yield stress due to repeated softening, the greater the strain at the weld heat affected zone and the more likely fatigue cracks are generated.
よって、ベイナイト組織鋼板を用いた場合における、Ｔ継手部の疲労特性（疲労強度）を向上させるためには、溶接熱影響部の繰り返し軟化を抑制する必要がある。本発明者らは、上記観点に基づき、繰り返し軟化挙動に及ぼす組織形態の影響について更に検討を行なった。その結果、繰り返し軟化挙動に対しては、大角粒径［２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域で構成される結晶粒（大角結晶粒）の平均円相当直径］、および初期の転位密度［更には、ＫＡＭ（結晶粒内の平均方位差）］が際めて大きな影響を及ぼしており、これらの要件を適切に制御しさえすれば、繰り返し軟化挙動が抑制され、Ｔ継手部の疲労強度を予測できることが判明し、本発明に到達した。 Therefore, in order to improve the fatigue characteristics (fatigue strength) of the T joint portion when using a bainite structure steel plate, it is necessary to suppress repeated softening of the weld heat affected zone. Based on the above viewpoint, the present inventors have further studied the influence of the structure morphology on the repeated softening behavior. As a result, for repetitive softening behavior, the large-angle grain diameter [average equivalent circle diameter of crystal grains (large-angle crystal grains) composed of a region surrounded by a large-angle grain boundary where the orientation difference between two crystals is 15 ° or more. , And the initial dislocation density [and KAM (average orientation difference within the grain)] has a significant influence, and repeated softening behavior can be suppressed by appropriately controlling these requirements. As a result, it was found that the fatigue strength of the T joint can be predicted, and the present invention has been achieved.
すなわち本願発明は、これまで不明であった、ベイナイト組織鋼板をＴ継手部に用いたときにおける、繰り返し軟化に対する組織形態の影響を、先願発明の知見をベースにして明らかにし、溶接熱影響部の組織と疲労強度との関係を、所定のパラメータ［具体的には、後記する繰り返し軟化パラメータ（１）、および（２）］で規定した点に技術的意義を有するものである。本願発明によれば、特に、グラインダーなどの溶接止端部処理を行なわないために局所塑性変形による繰り返し軟化が顕著に見られたＴ継手部の疲労特性を簡便且つ精度良く評価できる点で、極めて有用である。 That is, the present invention clarifies the influence of the structure morphology on repeated softening based on the knowledge of the prior invention, when the bainite structure steel plate is used for the T joint part, which has been unknown until now, The relationship between the microstructure and the fatigue strength is technically significant in that it is defined by predetermined parameters [specifically, the repeated softening parameters (1) and (2) described later]. According to the present invention, in particular, the fatigue characteristics of the T joint portion where repeated softening due to local plastic deformation was noticeable because the welding toe processing such as a grinder is not performed can be easily and accurately evaluated. Useful.
以下、本発明で規定する各パラメータについて説明する。 Hereinafter, each parameter prescribed | regulated by this invention is demonstrated.
（Ｉ）式（１）で表される繰り返し軟化パラメータ（１）について
はじめに、本発明の評価方法に関する基本骨格をなす、下記式（１）で表される繰り返し軟化パラメータ（１）を構成する各要件について説明する。
繰り返し軟化パラメータ（１）＝１／√（Ａ×ＫＡＭ） ・・・ （１）
但し、Ａ×ＫＡＭ＞１０
(I) Repetitive softening parameter (1) represented by formula (1) First, each of the constituents of the repeated softening parameter (1) represented by the following formula (1), which forms the basic skeleton related to the evaluation method of the present invention. Explain the requirements.
Repeat softening parameter (1) = 1 / √ (A × KAM) (1)
However, A × KAM> 10
（ア）Ａ：大角粒径（２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域で構成される結晶）について
本発明において大角粒界に着目したのは、ベイナイト組織鋼板の降伏応力を向上させるためである。すなわち、ベイナイト組織では、粒界が疲労き裂発生の抵抗となるが、粒界を形成する両端の方位差が１５°以上の大角粒界（大傾斜境界）を対象とすれば、粒界エネルギーが大きくなって疲労き裂発生抵抗効果が大きくなる。更に後記するように、当該大角粒界の平均円相当直径を小さくすれば、降伏応力が大きくなる。
(A) A: A bainite-structured steel sheet that focused on the large-angle grain boundary in the present invention for a large-angle grain size (a crystal composed of a region surrounded by a large-angle grain boundary whose orientation difference between two crystals is 15 ° or more). This is to improve the yield stress. That is, in the bainite structure, the grain boundary serves as a resistance to fatigue crack initiation, but if the grain boundary is a large-angle grain boundary (large slant boundary) where the orientation difference between both ends forming the grain boundary is 15 ° or more, the grain boundary energy Increases the fatigue crack initiation resistance effect. Furthermore, as will be described later, the yield stress increases as the average equivalent circle diameter of the large-angle grain boundary is reduced.
一方、ホール・ペッチの法則によれば、結晶粒径が小さい程、降伏応力は高くなることが知られている。すなわち、ベイナイト組織に繰り返し負荷が加わり、結晶粒内の転位密度が低下したとしても（その結果、前述したように転位は成長し易くなり、また新たな転位が発生し易くなるため、降伏応力が低減するようになるとしても）、大角粒界の平均円相当直径（大角粒径）を小さくすれば、繰り返し軟化後の降伏応力は高く保たれるようになり、疲労き裂は発生し難くなるのである。 On the other hand, according to Hall-Petch's law, it is known that the yield stress increases as the crystal grain size decreases. That is, even when a repeated load is applied to the bainite structure and the dislocation density in the crystal grains decreases (as a result, dislocations are likely to grow and new dislocations are likely to occur as described above. If the average equivalent circle diameter (large-angle particle diameter) of the large-angle grain boundary is reduced, the yield stress after repeated softening will be kept high, and fatigue cracks will not easily occur. It is.
よって本発明では、ベイナイト組織鋼板における繰り返し軟化抑制のための第１の要件として、まず大角粒径Ａを規定することにした。大角粒径Ａが小さい程、Ｔ継手部の疲労強度は向上するようになる。この大角粒径Ａは、ＥＢＳＤ（電子後方散乱回折：Ｅｌｅｃｔｒｏｎ Ｂａｃｋ Ｓｃａｔｔｅｒ ｄｉｆｆｒａｃｔｉｏｎ）によって測定することができる。 Therefore, in the present invention, as a first requirement for suppressing repeated softening in a bainite-structure steel plate, the large-angle particle size A is first defined. As the large angle particle size A is smaller, the fatigue strength of the T joint portion is improved. The large-angle particle diameter A can be measured by EBSD (Electron Back Scattering Diffraction).
上記大角粒径Ａの測定方法は、例えば特開２００９−６８０７８号公報などに詳述しており、当該内容を参照して測定することができる。 The method for measuring the large-angle particle size A is described in detail in, for example, Japanese Patent Application Laid-Open No. 2009-68078, and can be measured with reference to the contents.
詳細には、Ｔ継手部の溶接熱影響部（ＨＡＺ部）を測定対象とし（具体的には、図１中、溶接金属と鋼材熱影響部の境界のＨＡＺ側において表層を含む２００μｍ×２００μｍの領域部分）、圧延方向に平行な断面において、ＦＥ−ＳＥＭ−ＥＢＳＤ（電子放出型走査電子顕微鏡を用いた電子後方散乱回折像法）によって測定した。具体的には、Ｔｅｘ ＳＥＭ Ｌａｂｏｒａｔｒｉｅｓ社のＥＢＳＤ装置（商品名：「ＯＩＭ」）を、ＦＥ−ＳＥＭと組み合わせて用い、傾角（結晶方位差）が１５°以上の境界を結晶粒界として、結晶粒径（平均円相当直径）を測定した。このときの測定条件は、測定領域：２００μｍ、測定ステップ：１．０μｍ間隔とし、測定方位の信頼性を示すコンフィデンス・インデックス（Ｃｏｎｆｉｄｅｎｃｅ Ｉｎｄｅｘ）が０．１よりも小さい測定点は解析対象から除外した。このようにして求められる結晶粒径の平均値を算出して、本発明における平均結晶粒径とした。尚、結晶粒径が２．０μｍ以下のものについては、測定ノイズと判断し、結晶粒径の平均値計算の対象から除外した。 Specifically, the welding heat-affected zone (HAZ portion) of the T-joint portion is measured (specifically, in FIG. 1, 200 μm × 200 μm including the surface layer on the HAZ side of the boundary between the weld metal and the steel material heat-affected zone). In the section parallel to the rolling direction, measurement was performed by FE-SEM-EBSD (electron backscatter diffraction image method using an electron emission scanning electron microscope). Specifically, an EBSD device (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with FE-SEM, and a grain having a tilt angle (crystal orientation difference) of 15 ° or more is used as a crystal grain boundary. The diameter (average equivalent circle diameter) was measured. The measurement conditions at this time were a measurement region: 200 μm, a measurement step: 1.0 μm interval, and measurement points having a confidence index (Confidence Index) indicating reliability of the measurement direction smaller than 0.1 were excluded from the analysis target. . The average value of the crystal grain sizes thus obtained was calculated and used as the average crystal grain size in the present invention. Incidentally, those having a crystal grain size of 2.0 μm or less were judged as measurement noise and excluded from the target of calculating the average value of the crystal grain size.
（イ）初期の転位密度（特に、ＫＡＭ：結晶粒内の平均方位差）について
また本発明において初期の転位密度に着目したのは、前述したように初期の転位密度によって溶接熱影響部の降伏応力が大きく変化し、疲労強度に大きな影響を及ぼすためである。すなわち、初期の転位密度が大きい場合、繰り返し負荷による転位の合体・消滅の影響により、転位が大きく低下し、降伏応力も大きく低下するのに対し；初期の転位密度が小さい場合には、合体・消滅する転位自体が少ないため、繰り返し軟化の影響は小さくなり、降伏応力は初期の降伏応力から大きく低下せず、高い降伏応力が保たれる。
(A) Regarding the initial dislocation density (in particular, KAM: average orientation difference in the crystal grains) and in the present invention, the initial dislocation density was focused on, as described above, the yield of the heat affected zone due to the initial dislocation density. This is because the stress changes greatly and greatly affects the fatigue strength. That is, when the initial dislocation density is large, the dislocation is greatly reduced and the yield stress is also greatly reduced due to the effect of dislocation coalescence / annihilation caused by repeated loading; when the initial dislocation density is small, Since there are few dislocations to disappear, the effect of repeated softening is reduced, and the yield stress is not greatly reduced from the initial yield stress, and a high yield stress is maintained.
よって本発明では、ベイナイト組織鋼板における繰り返し軟化抑制のための第２の要件として、初期の転位密度を規定した。初期の転位密度が小さい程、Ｔ継手部の疲労強度は向上するようになる。初期の転位密度は、ＸＲＤ（Ｘ線回折：Ｘ−ｒａｙ ｄｉｆｆｒａｃｔｉｏｎ）によって測定することができる。 Therefore, in this invention, the initial dislocation density was prescribed | regulated as the 2nd requirement for the repetition softening suppression in a bainite structure steel plate. The smaller the initial dislocation density, the better the fatigue strength of the T joint. The initial dislocation density can be measured by XRD (X-ray diffraction).
詳細には、前述した大角粒径の測定対象と同様に、Ｔ継手部の溶接熱影響部（ＨＡＺ部）を測定対象とし（具体的には、図１中、溶接金属と鋼材熱影響部の境界のＨＡＺ側において表層を含む２００μｍ×２００μｍの領域部分）、Ｘ線回折ピークを測定し、そのピーク半価幅（ピークの広がり）から転位密度を推定することができる。 Specifically, the welding heat-affected zone (HAZ portion) of the T joint portion is measured as in the measurement target of the large-angle particle size described above (specifically, in FIG. A 200 μm × 200 μm region portion including the surface layer on the HAZ side of the boundary) and an X-ray diffraction peak can be measured, and the dislocation density can be estimated from the peak half width (peak broadening).
そして本発明者らが、Ｔ継手部の溶接熱影響部（止端部直近の鋼材側）において、ＥＢＳＤにより測定される上記（ア）の大角粒径と、ＸＲＤにより測定される初期の転位密度（初期転位密度）とが、繰り返し軟化後の疲労特性に及ぼす影響を調べた結果、下記式（１Ａ）で表される繰り返し軟化パラメータ（１Ａ）が、Ｔ継手部の疲労特性と極めて密接な相関関係を有することが判明した。
繰り返し軟化パラメータ（１Ａ）＝１／√（Ａ×初期転位密度）・・・（１Ａ）
And, the present inventors, at the welded heat affected zone of the T joint part (the steel material side closest to the toe), the large angle particle diameter of (a) measured by EBSD and the initial dislocation density measured by XRD. As a result of investigating the influence of (initial dislocation density) on the fatigue properties after repeated softening, the repeated softening parameter (1A) represented by the following formula (1A) has a very close correlation with the fatigue properties of the T joint. It turns out to have a relationship.
Repeat softening parameter (1A) = 1 / √ (A × initial dislocation density) (1A)
一方、近年の研究によれば、上記の初期転位密度に相当するパラメータは、ＥＢＳＤによって測定される結晶粒内の平均方位差（ＫＡＭ：Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ）によっても評価できることが報告されている（例えば、佐々木ら、日本金属学会誌、第７４巻、第７号（２０１０）、４６７−４７４頁の「緒言」の欄を参照）。ここでＫＡＭは、結晶粒内の局部的な方位変化量の面積平均を表しており、ＫＡＭの測定により、転位が発生することによって生じた局部的な方位のズレの多さを評価できていると考えられる。従って、ＥＢＳＤにより算出されるＫＡＭ値は、初期転位密度の代替パラメータとしても有用である。 On the other hand, according to recent studies, it has been reported that the parameter corresponding to the above-mentioned initial dislocation density can also be evaluated by an average orientation difference (KAM) in the crystal grains measured by EBSD (for example, Sasaki et al., Journal of the Japan Institute of Metals, Vol. 74, No. 7 (2010), pp. 467-474 (see “Introduction” column). Here, KAM represents the area average of local azimuth change amounts in crystal grains, and by measuring KAM, it is possible to evaluate the amount of local misalignment caused by the occurrence of dislocations. it is conceivable that. Therefore, the KAM value calculated by EBSD is useful as an alternative parameter for the initial dislocation density.
実際のところ、本発明者らが、Ｔ継手部の溶接熱影響部（止端部直近の鋼材側）において、ＥＢＳＤにより測定される上記（ア）の大角粒径と、同じＥＢＳＤにより測定されるＫＡＭ（結晶粒内の平均方位差）とが、繰り返し軟化後の疲労特性に及ぼす影響を調べた結果、下記式（１）で表される繰り返し軟化パラメータ（１）が、Ｔ継手部の疲労特性と極めて密接な相関関係を有することが判明した。
繰り返し軟化パラメータ（１）＝１／√（Ａ×ＫＡＭ） ・・・ （１）
As a matter of fact, the present inventors measure the above-mentioned (A) large-angle particle diameter and the same EBSD at the welding heat-affected zone of the T joint portion (the steel material side closest to the toe portion). As a result of investigating the influence of KAM (average orientation difference in crystal grains) on the fatigue properties after repeated softening, the repeated softening parameter (1) expressed by the following formula (1) shows the fatigue properties of the T joint. And was found to have a very close correlation.
Repeat softening parameter (1) = 1 / √ (A × KAM) (1)
上記繰り返し軟化パラメータ（１）は、前述した繰り返し軟化パラメータ（１Ａ）において、初期の転位密度を、結晶粒内の平均方位差（ＫＡＭ）に置き換えたものである。上記の繰り返し軟化パラメータ（１）を用いれば、Ｔ継手部の溶接熱影響部（止端部直近の鋼材側）においてＥＢＳＤによる測定を行なうことにより、上記（ア）の大角粒径のみならず、上記のＫＡＭ（結晶粒内の平均方位差）も同時に測定することができる点で、測定上、極めて効率的である。 The repeated softening parameter (1) is obtained by replacing the initial dislocation density with the average orientation difference (KAM) in the crystal grains in the above-described repeated softening parameter (1A). If the above-mentioned repeated softening parameter (1) is used, not only the large-angle particle size of (a) above, but also by measuring by EBSD in the weld heat affected zone of the T joint (the steel material side closest to the toe), The above KAM (average orientation difference in crystal grains) can be measured at the same time, which is extremely efficient in measurement.
このような測定上の便宜も考慮したうえで、本発明では、繰り返し軟化抑制のための第２の要件として、初期転位密度の代わりに結晶粒内の平均方位差（ＫＡＭ）を規定した。結晶粒内の平均方位差（ＫＡＭ）が小さい程、Ｔ継手部の疲労強度は向上するようになる。結晶粒内の平均方位差（ＫＡＭ）は、上記のとおり、ＥＢＳＤによって測定することができる。 In consideration of such convenience in measurement, in the present invention, as a second requirement for suppressing repeated softening, the average misorientation (KAM) in crystal grains is defined instead of the initial dislocation density. The smaller the average orientation difference (KAM) in the crystal grains, the better the fatigue strength of the T joint. The average orientation difference (KAM) in the crystal grains can be measured by EBSD as described above.
詳細には、前述したとおりＴ継手部の溶接熱影響部（ＨＡＺ部）を測定対象とし、圧延方向に平行な断面において、ＦＥ−ＳＥＭ−ＥＢＳＤ（電子放出型走査電子顕微鏡を用いた電子後方散乱回折像法）によって測定した。具体的には、ＴＳＬ社のＥＢＳＤ装置（商品名：「ＯＩＭ」）を用い、結晶粒（大角結晶粒に限らず測定対象中に観察される結晶粒すべて）を含む領域を画像観察したとき、画素間隔の方位差（隣接方位差）を各格子点において求め、測定領域（２００μｍ×２００μｍ）内における平均値を求めた［単位はｄｅｇ（角度、°）］。画素間隔は１μｍとした。上記測定方法に基づいてＫＡＭを厳格に定義すると、組織を画像化したときの、隣接する画素間の方位差の平均（画像内での平均）となる。 Specifically, as described above, the welding heat-affected zone (HAZ zone) of the T joint is measured, and the cross section parallel to the rolling direction is FE-SEM-EBSD (electron backscattering using an electron emission scanning electron microscope). (Diffraction image method). Specifically, using an EBSD device (trade name: “OIM”) of TSL, when observing an image of a region including crystal grains (all crystal grains observed in a measurement object as well as large-angle crystal grains), An azimuth difference (adjacent azimuth difference) between pixel intervals was obtained at each lattice point, and an average value within a measurement region (200 μm × 200 μm) was obtained [unit is deg (angle, °)]. The pixel interval was 1 μm. If KAM is strictly defined based on the above measurement method, it becomes the average of the azimuth differences between adjacent pixels (average in the image) when the tissue is imaged.
更に、本発明者らが、上記式（１）で表される繰り返し軟化パラメータ（１）が適用される範囲（前提条件）について検討したところ、式（１）の分母を構成する「Ａ×ＫＡＭ」については、下記式（３Ａ）を満足することが必要であることも判明した。
Ａ×ＫＡＭ＞１０ ・・・ （３Ａ）
Furthermore, when the present inventors examined the range (precondition) to which the repeated softening parameter (1) represented by the above formula (1) is applied, “A × KAM constituting the denominator of the formula (1)” It was also found that it is necessary to satisfy the following formula (3A).
A × KAM> 10 (3A)
上記式（３Ａ）の意味するところは、本発明で規定する繰り返し軟化パラメータ（１）を用いた疲労強度評価方法では、大角粒径Ａ、および平均隣接方位差ＫＡＭを小さくしすぎなくする必要があることを意味する。実際のところ、大角粒径Ａが小さくなりすぎると繰り返し軟化が生じ易くなって、上記パラメータ（１）ではＴ継手部の疲労強度を精度良く評価できないことが判明した（後記する実施例を参照）。 The above formula (3A) means that in the fatigue strength evaluation method using the repeated softening parameter (1) defined in the present invention, it is necessary not to make the large-angle particle size A and the average adjacent orientation difference KAM too small. It means that there is. Actually, if the large-angle particle size A becomes too small, it becomes easy to repeatedly soften, and it has been found that the fatigue strength of the T-joint portion cannot be accurately evaluated with the above parameter (1) (see Examples described later). .
この点についてもう少し詳しく説明すると、例えば、初期転位密度の殆どないフェライト単相組織では、結晶粒径が小さい場合、新たな転位の発生により結晶粒がセル化され、結晶粒が分断されて細かくなるが、夫々の粒界は結合力が極めて低いため、分断された細かい結晶粒が自由に動くため、強度が低下し、繰り返し軟化が起き易くなることが報告されている（変形初期の軟化に関し、幡中ら、日本機械学会論文集（Ａ編）、４７巻４１４号（１９８１）、第１２３頁を参照）。そして本願発明者らの実験結果によれば、上記の現象は、本発明のようにベイナイト組織を対象とする場合でも同様に見られ、転位密度を少なくして結晶粒を小さくしすぎると、前述したフェライト単相組織の場合と同様、繰り返し軟化が起き易くなり、継手疲労強度を精度良く評価できなくなることが判明したため、前提となる条件として、上記式を規定した次第である。 This point will be explained in more detail. For example, in a ferrite single phase structure having almost no initial dislocation density, when the crystal grain size is small, the crystal grains are formed into cells by the generation of new dislocations, and the crystal grains are divided and become finer. However, since each grain boundary has a very low bonding force, since the finely divided fine grains move freely, it has been reported that the strength decreases and repetitive softening easily occurs ( (See Hatanaka et al., Transactions of the Japan Society of Mechanical Engineers (A), 47, 414 (1981), p. 123). And according to the experiment results of the inventors of the present application, the above phenomenon is similarly seen even when the bainite structure is targeted as in the present invention. As in the case of the ferrite single-phase structure, it has been found that repeated softening is likely to occur, and the joint fatigue strength cannot be evaluated with high accuracy. Therefore, as a precondition, the above formula is defined.
上記式（３Ａ）において、「Ａ×ＫＡＭ」で表される値は、１１以上であることが好ましく、１２以上であることがより好ましい。なお、これらの上限は、歪み集中や疲労強度への影響などの観点からは特に限定されないが、靱性や強度などを考慮すると、おおむね、５０以下であることが好ましく、４０以下であることがより好ましい。 In the above formula (3A), the value represented by “A × KAM” is preferably 11 or more, and more preferably 12 or more. These upper limits are not particularly limited from the viewpoints of strain concentration and influence on fatigue strength, but considering toughness and strength, it is generally preferably 50 or less, and more preferably 40 or less. preferable.
より詳細には、上記式（３Ａ）を満足するための各要件の好ましい範囲は以下のとおりである。 In more detail, the preferable range of each requirement for satisfying the above formula (3A) is as follows.
大角粒径Ａは、１０〜３０μｍであることが好ましく、１０〜２５μｍであることがより好ましい。大角粒径Ａの下限が上記範囲を外れると繰り返し軟化が増大するようになり、一方、大角粒径Ａの上限が上記範囲を外れると靱性などが劣化するようになる。 The large angle particle size A is preferably 10 to 30 μm, and more preferably 10 to 25 μm. When the lower limit of the large-angle particle size A is out of the above range, the softening repeatedly increases. On the other hand, when the upper limit of the large-angle particle size A is out of the above range, toughness and the like are deteriorated.
また、結晶粒内の平均方位差ＫＡＭは、０．５〜２．０°であることが好ましく、０．６〜１．５°であることがより好ましい。ＫＡＭの下限が上記範囲を外れると繰り返し軟化が増大するようになり、一方、ＫＡＭの上限が上記範囲を外れると靱性などが劣化するようになる。 Further, the average orientation difference KAM in the crystal grains is preferably 0.5 to 2.0 °, more preferably 0.6 to 1.5 °. When the lower limit of KAM is out of the above range, the softening repeatedly increases. On the other hand, when the upper limit of KAM is out of the above range, toughness and the like are deteriorated.
上記以外の前提条件としては、以下の要件が挙げられる。 Other prerequisites include the following requirements.
まず本発明は、ベイナイト組織鋼板を用いたＴ型溶接継手構造体におけるＴ継手部の疲労特性（厳密には、Ｔ継手部の溶接熱影響部の疲労特性）を評価する方法であるため、当該Ｔ継手部の溶接熱影響部が、少なくともベイナイト組織で構成されていることが必要である。 First, the present invention is a method for evaluating the fatigue characteristics of a T joint portion in a T-type welded joint structure using a bainite structure steel plate (strictly, the fatigue characteristics of a weld heat affected zone of a T joint portion). It is necessary that the weld heat affected zone of the T joint is composed of at least a bainite structure.
本明細書において「少なくともベイナイト組織で構成されている」または「ベイナイト組織鋼板」とは、ベイナイト組織を主体として含むことを意味する。詳細には、溶接熱影響部において、全組織に対するベイナイト組織の比率がおおむね、５０面積％以上（好ましくは７５面積％以上）であり、最も好ましくは１００面積％、すなわち、ベイナイト組織単相である。ベイナイト以外の残部組織としては、例えばフェライト、炭化物等の介在物などが挙げられ、当該残部組織との混合組織で構成されていても良い。具体的には例えば、ベイナイト−フェライトの混合組織、炭化物・窒化物などの介在物などが例示される。 In the present specification, “consisting of at least a bainite structure” or “bainite structure steel sheet” means containing a bainite structure as a main component. Specifically, in the weld heat affected zone, the ratio of the bainite structure to the entire structure is generally 50 area% or more (preferably 75 area% or more), and most preferably 100 area%, that is, a bainite structure single phase. . Examples of the remaining structure other than bainite include inclusions such as ferrite and carbide, and may be composed of a mixed structure with the remaining structure. Specifically, for example, a mixed structure of bainite-ferrite, inclusions such as carbide and nitride, and the like are exemplified.
上記のＴ型溶接継手構造体としては、代表的には、高強度鋼板を突き合わせ溶接した垂直部材と、高強度鋼板を突き合わせ溶接した水平部材を溶接によって接合してなるＴ型溶接継手構造体が挙げられる。本発明では、水平部材に発生する疲労き裂の発生を防止する技術であるため、少なくとも水平部材（母材）を構成する高強度鋼板が、上述した「ベイナイト組織鋼板」で構成されていることが必要であり、更には、垂直部材を構成する高強度鋼板も、上述した「ベイナイト組織鋼板」で構成されていることが好ましい。後者の場合、水平部材も垂直部材も、全く同じ組織で構成されている必要は必ずしもなく、例えば水平部材がベイナイト組織単相で構成されており、垂直部材が上記要件を満足するベイナイト−フェライトの混合組織で構成されていても良い。勿論、両者が共に、同じ組織で構成されていても良い。 As the T-type welded joint structure, typically, a T-type welded joint structure is formed by welding a vertical member butt-welded to a high-strength steel plate and a horizontal member butt-welded to a high-strength steel plate. Can be mentioned. In the present invention, since it is a technique for preventing the occurrence of fatigue cracks generated in the horizontal member, at least the high-strength steel plate constituting the horizontal member (base material) is composed of the above-described “bainite structure steel plate”. Further, it is preferable that the high-strength steel plate constituting the vertical member is also made of the above-described “bainite structure steel plate”. In the latter case, it is not always necessary that the horizontal member and the vertical member have the same structure. For example, the horizontal member has a single phase of bainite structure, and the vertical member satisfies the above requirements. It may be composed of a mixed tissue. Of course, both may be composed of the same organization.
また、上記ベイナイト組織鋼板（特に水平部材）の引張強度は、５００〜６５０ＭＰａ（好ましくは５００〜６２０ＭＰａ）である。本発明のようにベイナイトを主体するベイナイト組織鋼板を用いたときの引張強度は、おおむね、５００ＭＰａ以上になるが、引張強度が高過ぎると靱性が劣化するため、本発明では、引張強度の範囲を上記のように制御した。同様に、垂直部材および溶接熱影響部（ＨＡＺ）の好ましい引張強度は、おおむね、５００〜６５０ＭＰａである。 Moreover, the tensile strength of the said bainite structure steel plate (especially horizontal member) is 500-650 MPa (preferably 500-620 MPa). The tensile strength when using a bainite-structured steel sheet mainly composed of bainite as in the present invention is generally 500 MPa or more. However, if the tensile strength is too high, the toughness deteriorates. Control as above. Similarly, the preferable tensile strength of the vertical member and the weld heat affected zone (HAZ) is approximately 500 to 650 MPa.
なお、上記では引張強度を規定しているが、特にＨＡＺ部の引張強度は熱影響部が狭いために引張試験片の採取が難しいため、引張強度の代わりに、簡便に測定可能な硬さ（ビッカース硬さ）で評価することもできる。 Although the tensile strength is defined in the above, since the tensile strength of the HAZ part is difficult to collect a tensile test piece because the heat-affected zone is narrow, a hardness that can be easily measured instead of the tensile strength ( (Vickers hardness) can also be evaluated.
また、水平部材の板厚は、溶接止端部の応力集中係数への影響を考慮すると、概ね、１０〜８０ｍｍであることが好ましい。垂直部材についても、上記と同様、概ね、１０〜８０ｍｍであることが好ましい。 Moreover, it is preferable that the plate | board thickness of a horizontal member is 10-80 mm in general when the influence on the stress concentration factor of a weld toe part is considered. Also about a vertical member, it is preferable that it is generally 10-80 mm like the above.
なお、上述した垂直部材と水平部材を接合するための溶接方法は特に限定されず、例えばサブマージアーク溶接法や炭酸ガスアーク溶接法が挙げられる。 In addition, the welding method for joining the vertical member and horizontal member mentioned above is not specifically limited, For example, a submerged arc welding method and a carbon dioxide arc welding method are mentioned.
また、垂直部材および水平部材を構成する鋼板の種類についても、組織および引張強度が上記要件を満足する限り、溶接構造体に通常用いられる鋼板を適用することができる。垂直部材および水平部材を構成する鋼板の種類は、同一であることが好ましい。 Moreover, the steel plate normally used for a welded structure can also be applied to the types of steel plates constituting the vertical member and the horizontal member as long as the structure and tensile strength satisfy the above requirements. The types of the steel plates constituting the vertical member and the horizontal member are preferably the same.
なお、前述したとおり、本発明は、先願発明の知見をベースにして完成された発明であるため、先願発明と同様、以下の要件を満足することが好ましい。 As described above, since the present invention is an invention completed based on the knowledge of the prior application invention, it is preferable that the following requirements are satisfied as in the prior application invention.
まず、溶接止端部に局所的な塑性変形を発生させるためには、水平部材（母材）の公称応力の範囲は、概ね１５０〜３５０ＭＰａである。水平部材の公称応力が１５０ＭＰａ未満では、局所的に塑性変形しない可能性があり、一方、３５０ＭＰａを超えると、鋼板全面が塑性変形する可能性がある。 First, in order to cause local plastic deformation at the weld toe, the nominal stress range of the horizontal member (base material) is approximately 150 to 350 MPa. If the nominal stress of the horizontal member is less than 150 MPa, local plastic deformation may not occur. On the other hand, if it exceeds 350 MPa, the entire surface of the steel sheet may be plastically deformed.
また、Ｔ継手部の溶接止端部曲率半径ρ（ｍｍ）、Ｔ継手部の溶接熱影響部の均一伸びＵＥHAZ（％）、およびＴ継手部の溶接熱影響部の降伏応力ＹＰHAZ（ＭＰａ）の許容範囲については特に限定されないが、溶接止端部に局所的な塑性変形を発生させるという観点からすれば、概ね、ρ：０．１ｍｍ以上１．０ｍｍ以下、ＵＥHAZ：５％以上２０％以下、ＹＰHAZ：３００ＭＰａ以上６５０ＭＰａ以下の範囲内であることが好ましい。 Also, the weld toe curvature radius ρ (mm) of the T joint, the uniform elongation UE HAZ (%) of the weld heat affected zone of the T joint, and the yield stress YP HAZ (MPa of the weld heat affected zone of the T joint. ) Is not particularly limited, but from the viewpoint of causing local plastic deformation at the weld toe, ρ is generally from 0.1 mm to 1.0 mm, and UE HAZ is from 5% to 20%. %, YP HAZ : preferably in the range of 300 MPa to 650 MPa.
以上、本発明の評価方法に関する基本骨格をなす、上記式（１）で表される繰り返し軟化パラメータ（１）について説明した。 The repeated softening parameter (1) represented by the above formula (1), which forms the basic skeleton related to the evaluation method of the present invention, has been described above.
（ＩＩ）式（２）で表される繰り返し軟化パラメータ（２）について
次に、上記式（２）で表される繰り返し軟化パラメータ（２）を構成する各要件について説明する。前述したとおり、このパラメータ（２）は、１０万回（１０5回）の疲労寿命に対する疲労強度（疲労き裂寿命が１０万回に対応する負荷応力範囲Δσを、引張強度ＴＳで除した値；Δσ／ＴＳ）と密接な相関関係を有するパラメータとして、以下の実験結果に基づいて導出したものである。
繰り返し軟化パラメータ（２）
＝｛０．４８／√（Ａ×ＫＡＭ）｝＋０．４０ ・・・ （２）
但し、Ａ×ＫＡＭ＞１０
(II) Repetitive Softening Parameter (2) Represented by Formula (2) Next, each requirement constituting the repeated softening parameter (2) represented by the above formula (2) will be described. As described above, this parameter (2) is a value obtained by dividing the fatigue strength for a fatigue life of 100,000 times (10 5 times) (the load stress range Δσ corresponding to a fatigue crack life of 100,000 times) by the tensile strength TS. As a parameter having a close correlation with Δσ / TS), which is derived based on the following experimental results.
Repeat softening parameter (2)
= {0.48 / √ (A × KAM)} + 0.40 (2)
However, A × KAM> 10
（試験片の作製）
溶接継手を模擬する試験片として、図３に示す平滑板状の微小切欠き試験片（溶接部などの不連続形状がない試験片）を作製した。この試験片を用いると、切欠き部に歪みが集中して局部的な塑性歪みが発生することから、Ｔ継手の溶接止端部の歪み状態を再現することができる。具体的には、縦６５ｍｍ×横１６ｍｍ×厚さ４ｍｍの平滑試験片を用いた。
(Preparation of test piece)
As a test piece for simulating a welded joint, a smooth plate-shaped micro-notch test piece (a test piece having no discontinuous shape such as a welded portion) shown in FIG. 3 was produced. When this test piece is used, strain concentrates in the notch portion and local plastic strain is generated, so that the strain state of the weld toe portion of the T joint can be reproduced. Specifically, a smooth test piece of 65 mm length × 16 mm width × 4 mm thickness was used.
具体的には上記試験片として、均一伸びＵＥ（約１５％）、および切欠き曲率Ｒ（０．５）が同じであり、降伏応力、引張強度、大角粒径Ａ、およびＫＡＭが異なる表１の試験片１〜９を用いた。ここで、大角粒径およびＫＡＭの測定方法は、前述したとおりであり、この結果に基づき、各試験片の繰り返し軟化パラメータ（１）を算出した。また、各試験片の降伏応力および引張強度は、以下のようにして測定した。 Specifically, as the above test pieces, the uniform elongation UE (about 15%) and the notch curvature R (0.5) are the same, and the yield stress, tensile strength, large-angle particle size A, and KAM are different. Test pieces 1 to 9 were used. Here, the measuring method of the large-angle particle size and KAM is as described above, and the repeated softening parameter (1) of each test piece was calculated based on this result. Moreover, the yield stress and tensile strength of each test piece were measured as follows.
（試験片の降伏応力、引張強度、繰り返し軟化パラメータ（１）の測定）
上記試験片の降伏応力および引張強度は、各試験片の板厚１／４部位からＪＩＳ Ｚ２２０１で規定されている１４号試験片（平行部径は１０ｍｍ）を用い、ＪＩＳ Ｚ２２４１で規定されている「金属材料引張試験方法」に基づいて測定した。引張試験時の試験速度は０．５ｍｍ／秒とした。
(Measurement of yield stress, tensile strength, and repeated softening parameter (1) of the specimen)
The yield stress and tensile strength of the above test pieces are specified in JIS Z2241, using No. 14 test pieces (parallel part diameter is 10 mm) specified in JIS Z2201 from the 1/4 thickness portion of each test piece. The measurement was performed based on the “metal material tensile test method”. The test speed during the tensile test was 0.5 mm / second.
（疲労試験）
上記の各試験片について、以下のようにして１０万回に対する疲労き裂寿命を測定した。ここで、１０万回に対する疲労き裂寿命は、疲労き裂発生寿命が１０万回に対応する負荷応力範囲Δσ（疲労試験において作用する繰返し最大応力Ｓ1と繰返し最小応力Ｓ2の差）を試験片の引張強度ＴＳで除した値（Δσ／ＴＳ）により評価した。
(Fatigue test)
About each said test piece, the fatigue crack life with respect to 100,000 times was measured as follows. Here, the fatigue crack life for 100,000 times is the load stress range Δσ corresponding to the fatigue crack initiation life of 100,000 times (difference between the repeated maximum stress S 1 and the repeated minimum stress S 2 acting in the fatigue test). Evaluation was made by the value (Δσ / TS) divided by the tensile strength TS of the test piece.
具体的には、上記の各試験片に対し、軸方向（図３の矢印方向）に引張荷重が加わるように試験片を油圧式疲労試験機に取り付け、Ｓ1とＳ2が一定となる条件で繰返し荷重を加えた。Ｓ1とＳ2は、試験片の切欠きから十分に離れた位置に貼付した歪みケージで測定した。また、同一の試験片を５本用意し（ｎ＝５）、疲労亀裂発生寿命が約１０4〜１０6の範囲に入る条件で応力負荷条件を変えて試験を行い、応力範囲と疲労亀裂発生寿命の関係を求め、応力範囲を疲労亀裂発生寿命の関数として定式化し、疲労亀裂発生寿命＝１０5としたときの応力範囲を算出した。 Specifically, the test piece is attached to a hydraulic fatigue testing machine so that a tensile load is applied in the axial direction (the arrow direction in FIG. 3) to each of the above test pieces, and S 1 and S 2 are constant. The load was repeatedly applied. S 1 and S 2 were measured with a strain cage attached at a position sufficiently away from the notch of the test piece. Also, five identical specimens were prepared (n = 5), and the test was conducted under different stress loading conditions under conditions where the fatigue crack initiation life was in the range of about 10 4 to 10 6. The relationship of life was obtained, the stress range was formulated as a function of fatigue crack initiation life, and the stress range was calculated when fatigue crack initiation life = 10 5 .
これらの結果を表１に併記する。更に、図４に繰り返し軟化パラメータ（１）とΔσ／ＴＳとの関係をグラフ化して示す。図４中、○は、大角粒径Ａ×ＫＡＭの値が１０超の要件を満足するもの（表１のＮｏ．１〜８）であり、×は、大角粒径Ａ×ＫＡＭの値が１０以下のもの（表１のＮｏ．９）である。 These results are also shown in Table 1. FIG. 4 is a graph showing the relationship between the repeated softening parameter (1) and Δσ / TS. In FIG. 4, ○ indicates that the value of the large-angle particle size A × KAM satisfies the requirement of more than 10 (No. 1 to 8 in Table 1), and X indicates that the value of the large-angle particle size A × KAM is 10 It is the following (No. 9 in Table 1).
図４に示すように、大角粒径Ａ×ＫＡＭの値が１０超の要件を満足する試験片では、本発明で規定する繰り返し軟化パラメータ（１）と、Δσ／ＴＳで表される１０万回に対する疲労強度（実測値）とは、極めて良好な相関関係を有すること分かる。 As shown in FIG. 4, in the test piece satisfying the requirement that the value of the large-angle particle size A × KAM exceeds 10, the repeated softening parameter (1) defined in the present invention and 100,000 times represented by Δσ / TS It can be seen that there is a very good correlation with the fatigue strength (measured value) for.
図４の実験結果に基づき、１０万回に対する疲労強度（Δσ／ＴＳ）を評価（予測、推定）することができるパラメータとして、下式で表される繰り返し軟化パラメータ（２）を導出した（図４の式を参照）。参考のため、上記表１に、上記繰り返し軟化パラメータ（２）の算出結果を併記した。
１０万回に対する疲労強度（Δσ／ＴＳ）
＝｛０．４８／√（Ａ×ＫＡＭ）｝＋０．４０ ・・・ （２）
Based on the experimental results of FIG. 4, the repeated softening parameter (2) represented by the following equation was derived as a parameter that can evaluate (predict and estimate) the fatigue strength (Δσ / TS) for 100,000 times (see FIG. 4). (See equation 4). For reference, the calculation results of the repeated softening parameter (2) are also shown in Table 1 above.
Fatigue strength for 100,000 times (Δσ / TS)
= {0.48 / √ (A × KAM)} + 0.40 (2)
なお、大角粒径Ａ×ＫＡＭが１０以下のもの（図４中、×であり、表１のＮｏ．９）については、本発明で規定する繰り返し軟化パラメータを用いても精度良く評価できなかったが、これは、大角粒径Ａが本発明の好ましい範囲を下回って小さくなり、結晶粒のセル化が発生し、繰り返し軟化による降伏応力の低下が起きたためである。 In addition, the large-angle particle size A × KAM of 10 or less (in FIG. 4, “×” in Table 1, No. 9 in Table 1) could not be evaluated with high accuracy even using the repeated softening parameters defined in the present invention. However, this is because the large-angle grain size A is smaller than the preferred range of the present invention, and the crystal grains become cellular, and the yield stress is lowered due to repeated softening.
以上、本発明で規定する上記式（２）で表される繰り返し軟化パラメータ（２）の導出方法について説明した。 The method for deriving the repeated softening parameter (2) represented by the above formula (2) defined in the present invention has been described above.
本発明で提示された上記繰り返し軟化パラメータの代表的な適用例としては、例えば、以下の態様が挙げられるが、本発明はこれに限定する趣旨ではない。 As typical application examples of the above-described repeated softening parameters presented in the present invention, for example, the following modes may be mentioned, but the present invention is not limited to this.
疲労特性が不明な供試材（Ｔ型溶接継手）について、上記のようにしてＥＢＳＤ法により大角粒径Ａ、およびＫＡＭを測定し、繰り返し軟化パラメータ（１）を算出することにより、実際に煩雑な疲労試験を行わなくても、疲労特性をおおよそ予測することができる。数値が大きいものほど、疲労特性に優れている。その際、これらの評価パラメータと必要な疲労特性との関係を、事前にデータベース化しておけば、上記供試材の疲労特性を、精度良く評価することができる。 For specimens with unknown fatigue properties (T-type welded joints), the large-angle particle diameter A and KAM are measured by the EBSD method as described above, and the softening parameter (1) is calculated repeatedly. The fatigue characteristics can be roughly predicted without performing a simple fatigue test. The larger the value, the better the fatigue characteristics. In that case, if the relationship between these evaluation parameters and necessary fatigue characteristics is created in advance in a database, the fatigue characteristics of the specimen can be evaluated with high accuracy.
特に、上記供試材について、繰り返し軟化パラメータ（２）を算出すれば、１０万回に対する疲労強度（実測値）を、精度良く評価（推測）することができる。 In particular, if the softening parameter (2) is repeatedly calculated for the test material, the fatigue strength (measured value) with respect to 100,000 times can be evaluated (estimated) with high accuracy.
あるいは、水平部材および垂直部材の板厚の組み合わせが同じであるＴ型溶接継手を幾つか試作したとき、各継手に対して煩雑な疲労試験を行わなくても、ＥＢＳＤ法によって大角粒径ＡおよびＫＡＭを測定し、上記式（１）または上記式（２）で表わされる評価パラメータの値を算出してこれらの値を比較することにより、疲労特性に優れた溶接継手を決定することが挙げられる。数値が大きいものほど、疲労特性に優れているため、疲労特性に最も優れたＴ継手を選定することができる。 Alternatively, when several T-type welded joints having the same combination of plate thicknesses of the horizontal member and the vertical member are produced as prototypes, the large-angle particle size A and E can be obtained by the EBSD method without performing a complicated fatigue test on each joint. By measuring KAM, calculating the value of the evaluation parameter represented by the above formula (1) or (2) and comparing these values, it is possible to determine a welded joint having excellent fatigue characteristics. . The larger the numerical value, the better the fatigue characteristics. Therefore, the T joint having the best fatigue characteristics can be selected.
更に、上記式で表わされる評価パラメータと必要な疲労特性との関係を、事前にデータベース化しておけば、疲労特性に優れたＴ継手部の材料設計指針や溶接条件などを決定することもできる。ここで、大角粒径Ａは溶接条件・鋼材の化学成分や圧延条件などを変えることによって、一方、ＫＡＭや初期転位密度は鋼材の化学成分や圧延条件などを変化させることによって、いずれも変化させることが可能であるため、溶接条件と材料特性（組織形態）との関係をデータベース化することができる。 Furthermore, if the relationship between the evaluation parameter represented by the above formula and the necessary fatigue characteristics is created in advance in a database, the material design guidelines, welding conditions, and the like of the T joint portion having excellent fatigue characteristics can be determined. Here, the large-angle particle size A is changed by changing the welding conditions, the chemical composition of the steel material, the rolling conditions, and the like, while the KAM and the initial dislocation density are changed by changing the chemical composition of the steel material, the rolling conditions, etc. Therefore, it is possible to create a database of the relationship between welding conditions and material properties (structure morphology).
Claims (2)
前記Ｔ継手部の溶接熱影響部において、
下記式（１）で表される繰り返し軟化パラメータ（１）を用いてＴ型溶接継手構造体の疲労特性を評価することを特徴とするＴ継手部の疲労特性評価方法。
繰り返し軟化パラメータ（１）＝１／√（Ａ×ＫＡＭ） ・・・ （１）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。 A method for evaluating fatigue characteristics of a T joint part in a T-type welded joint structure using a bainite structure steel plate having a tensile strength of 500 to 650 MPa,
In the weld heat affected zone of the T joint,
A fatigue characteristic evaluation method for a T joint part, wherein the fatigue characteristic of a T-type welded joint structure is evaluated using a repeated softening parameter (1) represented by the following formula (1).
Repeat softening parameter (1) = 1 / √ (A × KAM) (1)
However, A × KAM> 10
Where
A is an average circle equivalent diameter (μm) of the crystal grains when a region surrounded by a large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more is defined as a crystal grain;
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
前記切欠き試験片を用いて１０万回の疲労試験を行ったとき、
下記式（２）で表される繰り返し軟化パラメータ（２）を用いて、負荷応力範囲Δσ（ＭＰａ）を切欠き試験片の引張強度ＴＳ（ＭＰａ）で除した値（Δσ／ＴＳ）で表わされる疲労特性を予測することを特徴とするＴ継手部の疲労特性評価方法。
繰り返し軟化パラメータ（２）
＝｛０．４８／√（Ａ×ＫＡＭ）｝＋０．４０ ・・・ （２）
但し、Ａ×ＫＡＭ＞１０
式中、
Ａは、切欠き試験片において、隣接する２つの結晶の方位差が１５°以上の大角粒界で囲まれた領域を結晶粒としたとき、前記結晶粒の平均円相当直径（μｍ）であり、
ＫＡＭは、結晶粒内の平均方位差（Ｋｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ、°）である。 A method for predicting fatigue characteristics of a T joint portion in a T-type welded joint structure using a bainite structure steel plate having a tensile strength of 500 to 650 MPa using a notch test piece,
When the fatigue test was performed 100,000 times using the notch test piece,
Using the repeated softening parameter (2) represented by the following formula (2), the load stress range Δσ (MPa) is divided by the tensile strength TS (MPa) of the notched specimen (Δσ / TS). A method for evaluating fatigue characteristics of a T-joint, characterized by predicting fatigue characteristics.
Repeat softening parameter (2)
= {0.48 / √ (A × KAM)} + 0.40 (2)
However, A × KAM> 10
Where
A is an average equivalent circle diameter (μm) of a crystal grain when a region surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more in the notched specimen is used as a crystal grain. ,
KAM is an average misorientation (Kerrel Average Misorientation, °) within crystal grains.
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| WO2019035098A1 (en) * | 2017-08-18 | 2019-02-21 | The Regents Of The University Of Michigan | Unified fatigue life evaluation method for welded structures |
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| CN114813416A (en) * | 2021-06-02 | 2022-07-29 | 天津大学 | Evaluation method for softening and usability of girth weld joint in service stage of pipeline steel pipe |
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| CN114813332A (en) * | 2021-06-02 | 2022-07-29 | 天津大学 | Flexible joint usability evaluation method for pipeline steel pipe girth welding joint based on bending strain |
| CN114813332B (en) * | 2021-06-02 | 2024-03-19 | 天津大学 | Pipeline steel pipe ring-welded joint softening fit usability evaluation method based on bending strain |
| CN114813416B (en) * | 2021-06-02 | 2024-03-22 | 天津大学 | Method for evaluating soft chemical combination and usability of girth welded joint in service stage of pipeline steel pipe |
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