- 學位論文
先進高強度鋼板沖壓成形包辛格效應之研究
A Study of Bauschinger Effect in Stamping of Advanced High Strength Steel Sheets
摘要
先進高強度鋼板已經被廣泛應用於汽車結構件，由於流應力較高，其成形問題較傳統低強度鋼複雜且嚴重。除了沖壓成形過程中，一般傳統鋼板也會面臨到的破裂及皺褶缺陷外，其最主要必需解決的是回彈、側壁捲曲及扭曲等缺陷。為了克服這些問題，故使用有限元素法分析進行模具設計，減少板金成形過程中之回彈量。在過去研究當中，雖然已經努力對模擬參數進行最佳化之收斂性分析，包括網格尺寸大小、積分點數目、沖壓速度，但對於提升預測回彈準確度仍不足。因此，對於模擬先進高強度鋼板之沖壓成形過程，包含降伏準則以及硬化準則的材料模型，被考量為可能是影響有限元素法模擬準確度的因素之一。 在本論文當中，透過實驗以及有限元素法分析，進行包辛格效應特性之研究。首先使用本實驗室蔡恆光學長所設計之夾具，進行270軟鋼以及590Y級鋼板拉伸與壓縮試驗，在試驗結果中，可清楚觀察到590Y級鋼板之包辛格效應效應較270級軟鋼明顯。接著將試驗結果運用在描述材料的動態硬化特性，供有限元素法模擬使用。本論文使用Hill48降伏準則搭配等向硬化準則以及Hill48降伏準則搭配混和硬化準則(Yoshida-Uemori model)兩種材料模型，進行590Y級鋼板之V型彎曲、U型彎曲及U形帽狀引伸成形模擬分析，並與本實驗室之實驗值進行比較。由比較結果得知，不論是側壁外開或是側壁捲曲，有考慮包辛格效應的模擬結果都較接近於實驗值。此外，590Y級鋼板於汽車結構件之回彈情形，在本論文當中透過有限元素法分析以及實際部品開發進行研究。其中注意到，當汽車部品採用先進高強度鋼板，其變形機制明顯受到包辛格現象的影響。在進行590Y級鋼板實際部品之模擬驗證時也顯示出，有考慮包辛格效應之Yoshida-Uemori材料模型，其回彈模擬結果準確度較高。因此，本論文結論認為，在先進高強度鋼板成形過程中包辛格效應扮演著重要角色，以及在有限元素法模擬中必需考慮動態硬化準則。
並列摘要
The advanced high strength steel has been widely applied to the car body structural components. However, the high flow stress makes the sheet metal forming process even more difficult. In addition to the defects of fracture and wrinkling, which are commonly present in the stamping of conventional steel sheet, the occurrence of springback, side-wall curl and distortion is the main issue to be solved. In order to cope with this dilemma, the finite element analysis was employed to help the tooling design aiming to eliminate the springback in the sheet forming process. The efforts were endeavored to establish the optimum simulation parameters, such as element size, number of integration point, and punch velocity. However, the accuracy in the prediction of springback is yet to be improved even with the optimum simulation parameters adopted. Hence, the material model including the yield criteria and the work hardening rules has been considered as the other possible reasons that may cause the inaccuracy of the finite element simulations in the sheet forming of advanced high strength steel. In the present study, the Bauschinger effect that induces anisotropic work hardening properties was examined by both the experimental approach and the finite element analysis. The tension-compression tests were first conducted for the specimens made of conventional 270 and DP590Y steel sheets with the special experimental apparatus designed by Heng-Kuang Tsai in our lab. The Bauschinger effect was clearly observed in the test results of DP590Y steel sheets rather than those of 270 steel sheets. The test results were then used in the finite element simulations as part of the material properties characterizing the kinematic hardening behavior of the sheet metal. Both the Hill 48 yield criterion with isotropic hardening rule and Yoshida-Uemori model (Y-U model) were adopted to simulate the V-bend, U-bend, and U-hat bend of DP590Y steel sheets. The simulation results were compared with the experimental data obtained from the tests conducted in our lab. It is found that the simulation results with the Bauschinger effect considered are more consistent with the experimental data either in springback or in side wall curl than those of simulations using an isotropic hardening rule. The springback occurs in the stamping of an automotive structural part made of DP590Y steel sheets were also examined in the present study by both the finite element analysis and the actual production process. It is noted that the deformation mechanism of these automotive parts is significantly affected by the Bauschinger phenomenon as the advanced high strength steel is adopted. The comparison of the finite element simulation results and the actual production parts also reveals that the simulation results with the use of the Yoshida-Uemori model renders to a more accurate prediction in springback in the stamping of the DP590Y steel sheets. It thus concludes that the Bauschinger effect plays an important role in the stamping of advanced high strength steel sheets and the employment of the kinematic hardening rule in the finite element simulations is necessary.