先進高強度鋼板反覆彎曲與雙軸拉伸材料模型之研究

Abstract

為因應日益嚴苛的環保要求以及燃油效率提升的壓力，輕量化正是國際間各大汽車廠努力的目標。因此為了兼顧生產成本與達成車輛輕量化的目標，使用先進高強度鋼板於汽車結構件之生產已成為國際車廠的共同趨勢。 但隨著鋼板強度的提高，其沖壓成形所面臨的問題相較於傳統低強度鋼更加困難，且側壁外開、側壁捲曲等缺陷亦更為嚴重。針對高強度鋼板之成形問題，目前國際間產學研各界已紛紛投入高強度鋼之材料模型研究，包括考慮包辛格效應(Bauschinger effect)之加工硬化準則與考慮雙軸向受力行為之降伏準則，希望藉由瞭解材料的塑性變形特性，提升對於高強度鋼板沖壓成形特性分析與回彈預測之能力。因此，本研究針對先進高強度鋼板反覆彎曲成形與雙軸拉伸塑性變形特性進行研究，藉此建立完整的先進高強度鋼板沖壓成形材料模型。 在加工硬化準則研究方面，對於包辛格效應之研究方式大多利用單一試片進行拉伸-壓縮實驗，以觀察其反覆受力模式。而實際板材在沖壓成形製程中，較偏向反覆彎曲之受力模式，因此本研究針對高強度鋼板在反覆彎曲成形下之材料包辛格特性與變形機制進行探討，並與板材受拉伸-壓縮之受力狀況與變形特性比較，同時設計三點式反覆彎曲夾治具，進行反覆彎曲實驗，並與有限元素軟體分析之結果比較，驗證Yoshida-Uemori材料模型之必要性。 而在降伏準則研究方面，本研究為探討適用於先進高強度鋼板雙軸受力下之降伏準則。因此，針對雙軸拉伸塑性變形特性進行研究。為建立板材雙軸向拉伸試驗之平台，本研究改良現有之雙軸夾治具機構，使其進行雙軸拉伸試驗時，更為穩定且架設方便。而在雙軸試片 方面，由於目前並無雙軸拉伸試片形狀與尺寸之規範，因此，本研究利用有限元素軟體分析雙軸十字形試片之拉伸狀況，探討不同形狀與幾何造型之試片對量測數據之影響性，並設計出最符合實驗量測之十字形溝槽試片。 在完成雙軸夾治具機構平台之改良與雙軸試片之設計後，即進行雙軸向拉伸實驗與夾治具機構摩擦力實驗，根據實驗數據探討適合描述先進高強度鋼板在雙軸受力下之塑性變形行為的降伏準則，並建立有限元素分析所需之各降伏準則材料模型參數。最後，結合考慮包辛格效應之Yoshida-Uemori材料模型搭配不同降伏準則，應用於基礎載具與業界汽車載具沖壓成形模擬分析，比較模擬與實驗之厚度與回彈結果，其結果顯示Barlar 91降伏準則結合Yoshida-Uemori材料模型，可有效提升先進高強度鋼板於沖壓成形模擬之回彈預測準確性。

As increasing attention and concern are drawn to environmental preservation and sustainability, the lightweight design of automobiles becomes a major goal for vehicle manufacturers all around the globe. Lightweight vehicles also provide a solution relieving fuel inflation in recent years. In order to achieve vehicle lightweight design with cost minimization considered, the use of advanced high strength steels in automobile structural parts manufacture becomes a trend internationally. However, the increase in steel strength leads to greater obstacles concerning its formation, particularly when comparing to traditional low strength steel. Flaws in springback and side-wall curl also become more severe. Researches and studies have been carried out internationally on the development of advanced high strength steel material models, taking Bauschinger effect's work hardening criteria and yield criteria under biaxial stress into consideration. By gaining understanding of material properties during plastic deformation, hopefully better analysis on the stamping of advanced high strength steel can be achieved with springback prediction becoming more reliable. This research thus aims to explore the biaxial stretching plastic deformation of advanced high strength steel, and establish complete advanced high strength steel material models. Concerning work hardening criteria, research in Bauschinger’s effect often applies uniaxial specimens to carry out tension-compression tests to obtain its reversed stress status. However, in the actual stamping process, the sheet metal is usually subjected to bending and reversed bending deformation such as it passes through the draw bead or the die corner. This research therefore aims to explore the material properties of Bauschinger’s effect and deformation mechanism of advanced high strength steel under cyclic reversed bending deformation, and compare the findings with sheet metal tension-compression tests results. Three-point cyclic reversed bending clamping apparatus was in the same time designed and used to perform cyclic reversed bending experiments. The test data were compared with the finite element simulation results as a way to prove the necessity of the use of Yoshida-Uemori material model to describe the Bauschinger’s effect occurred in the stamping of advanced high strength steel sheets. This research also aims to explore the suitable yield criteria for advanced high strength steel subjected to biaxial stress states. Therefore, aspects on the characteristics of biaxial stretching plastic deformation are focused, in which the mechanisms of biaxial stretching on its equipment are targeted for improvement. The goal is to achieve better stability and ease installation during biaxial stretching tests. As for the biaxial specimen, specifications and restrictions for specimen shape and dimension are not available to date. The finite element analysis was then selected to determine the performance of biaxial stretching crossed-shaped specimen. The impact of specimen shape and geometric modeling on the measurement data was analyzed and used to design the most ideal cross-shaped specimen with slots for experimental purposes. Once achieving improvement on biaxial stretching mechanism and biaxial specimen, biaxial stretching test and clamping apparatus friction test were carried out. The experimental data obtained were used to explore the most suitable yield criteria for defining the plastic deformation behavior of advanced high strength steel under biaxial stress states. The yield criteria parameters for finite element analysis are established as the outcome. Finally, the Yoshida-Uemori model is taken into consideration for the selective combination with various yielding criteria, as means of simulating the stamping of basic and industrial vehicle carriers. The comparison of simulated and experimental results on material thickness and springback shows that the combination of Barlar 91 yield criterion with Yoshida-Uemori model is very effective in springback prediction of advanced high strength steel stamping.