高強度汽車結構件沖壓成形側壁捲曲現象之研究

Abstract

由於能源短缺以及地球暖化的危機，環保節能已成為國際間的趨勢，為了降低油耗，目前國際間各大車廠均朝向輕量化、強度化發展，故具有高強度且低成本特性的高強度鋼板逐漸被廣泛地應用於汽車工業上。 相較於傳統軟鋼，高強度鋼板在沖壓成形過程中更容易產生破裂與回彈缺陷，為有效解決高強度鋼板之沖壓成形問題，目前各大車廠與板金模具廠均使用有限元素分析輔助模具設計，以改善成形缺陷。然而有限元素法目前在回彈分析之準確性仍有所不足，雖然在過去研究當中，已經針對模擬參數進行收斂性分析，但為進一步提升回彈分析之準確性，本研究特針對材料模型中的降伏準則進行研究，並進而探討降伏準則對高強度複雜造型汽車結構件成形側壁捲曲之影響。 本研究首先針對目前常用之Hill 48、Hill 90、Barlat 89及Barlat 91等降伏準則進行探討，並利用本實驗室設計之試驗夾治具進行雙軸拉伸試驗以驗證各降伏準則於鋼材承受雙軸受力時之適用性。由於目前並無雙軸拉伸試片形狀與尺寸之規範，本研究為確認所量測區域範圍為均勻受力且皆為主應力，採用有限元素法設計較佳之試片形狀與尺寸，並以雙軸拉伸實驗驗證有限元素模擬之正確性與該試片之適用性。 在複雜造型汽車結構件方面，本研究選擇汽車結構件後大樑進行分析，此載具包含高斷面比、斜坡造型及高引伸量等特徵造型，在成形時更容易產生破裂、皺褶與回彈之缺陷。為縮短開發時間，亦利用有限元素軟體進行分析，進行較佳之餘肉造型設計。此外，也歸納後大樑之特徵參數，包括引伸量、上平面寬度比與側壁曲率半徑等，同時探討其對側壁捲曲之影響性，並分析其捲曲機制，以及各餘肉造型與成形工法對捲曲之影響，藉以改善高強度鋼板之成形捲曲現象。最後在驗證方面，本研究利用超音波測厚儀與逆向掃描取得實際成品之厚度分佈與外型尺寸，並與採用不同降伏準則之分析結果進行比較，結果顯示使用Barlat 91降伏準則較有助於回彈準確率提升。

To cope with the energy crisis problem and the more stringent collision regulations, the goals in achieving weight reduction and structure strength enhancement are now the priority issues for the automobile industry. Therefore, the advanced high strength steel, which has the characteristics of high strength and low cost, are being widely applied to the automobile industry. However, in comparison to the stamping process of traditional steel, the use of high strength steel is apt to raise drawbacks, such as cracks and springback. In order to resolve these problems efficiently, the finite element analysis is introduced into the automobile industry and die design factories, to better improve the stamping process. Though the simulation parameters have been examined using convergence analysis approach in previous studies, in order to increase the accuracy of springback prediction, this study aims to focus on discussing the yield criteria used in the material model. In addition, this study further investigates the influences of yield criteria on the side-wall curls occurred in the stamping of high strength automotive structural parts with complex shapes. This study first takes the most frequently used yield criteria, Hill 48, Hill 90, Barlat 89, and Barlat 91 into consideration, and applies the novel apparatus designed by this lab to conduct biaxial tensile experiments for validating the applicability of each yield criterion to the steel. Since there are no standard principles to follow in designing the biaxial tensile test specimens, this study also takes the finite element analysis to examine the geometry and dimension of the specimen to ensure the stress distribution at the center area of the specimen is equally expressed as principle stresses. Additionally, the biaxial tensile experiments are conducted to verify the accuracy of the finite element simulation results and the applicability of the designed biaxial tensile test specimens. As for the complex automobile structural parts, this study chooses to examine the rear frame element of a car body structure. The rear frame contains a lot of featured shapes, including high cross-section ratio, slope shapes, high drawing rates and so on, which would be apt to trigger drawbacks of cracks, wrinkles and springback. In order to save the processing time, the finite element software is also employed to design better die face addendums. In addition, the feature parameters of rear frame are also rendered which includes drawings, upper surface width ratio, radius of side wall curvature, etc. and further examined its effects on the side-wall curl. The discussions of each addendum shape and formatting methods are also brought in for the purpose of studying and improving side-wall curl effects. Last, this study employs the ultrasonic thickness gauge and reverse scanning technique to measure the thickness distribution and appearance size of the actual production part to compare with the finite element simulation results. The comparison results not only validate the finite element analysis and the proposed die face addendums, but also indicate that the Barlat 91 yield criterion renders more accurate springback predictions than other yield criteria do.