先進高強度鋼沖壓成形邊緣破裂現象之研究

Abstract

先進高強度鋼由於具有高強度及良好的塑性，目前已經被廣泛應用於汽車結構件，然而先進高強度鋼在沖壓成形上容易產生邊緣破裂。邊緣破裂難以透過CAE模擬進行預測，且目前預測沖壓板材破裂最準確之成形極限曲線(FLC曲線)也無法有效預測邊緣破裂發生，因此為了解決邊緣破裂現象，本論文將建立一些準則與方法預測及避免此現象。 本論文首先針對邊緣破裂之成因進行文獻收集與歸納，進而了解先進高強度鋼相較於一般傳統鋼容易產生邊緣破裂之原因，其中影響邊緣破裂時機之原因非常複雜，除了先進高強度鋼在微觀結構上容易產生微孔破壞之外，板材邊緣在不同沖切製程及不同軋延方向下，將有不同的邊緣破裂時機，因此為了有效預測板材在不同邊緣條件下邊緣破裂之時機，本論文將針對如何定義邊緣條件之方法進行深入探討，並以簡易之邊緣破裂實驗建立破壞準則，包括擴孔實驗、圓孔拉伸實驗、HSDT實驗。 在簡易之邊緣破裂實驗中，本論文完成了不同材料以及不同邊緣條件之實驗，而透過研究結果發現，當兩個不同邊緣破裂實驗之邊緣條件相同時，邊緣破裂時機將會非常接近，因此本論文將透過此實驗特性，建立預測邊緣破裂之破壞準則，進而預測實際載具邊緣破裂之時機。 為了建立預測實際載具邊緣破裂之CAE模擬，本論文透過PAM-STAMP軟體建立基礎載具之CAE模擬模型，並架設基礎載具之實驗平台，而將建立完成之破壞準則預測基礎載具之邊緣破裂，並比較實驗與模擬之結果後，發現預測之結果具有良好的準確性，模擬除了可以區分不同材料之邊緣成形性外，也可以區分相同材料在不同邊緣條件下之破壞時機，因此已成功建立預測邊緣破裂之方法。未來將可透過本論文建立之方法避免邊緣破裂，並提供模具設計之參考。

Advanced high-strength steel (AHSS) has been widely used in automobile structural parts due to its high strength and good plasticity. However, AHSS is prone to edge cracking during stamping process. Edge cracking is difficult to predict by CAE simulation, Even the most accurate method for predicting fracture, Forming Limit Curve (FLC curve), cannot predict edge cracking. Therefore, in order to solve the edge cracking phenomenon, this paper will establish criterion and methods to predict and avoid edge cracking. This paper first collects the causes of edge cracking, and then summarizes why AHSS is more prone to edge cracking than conventional steel. The reasons that affect the timing of edge cracking are very complicated. In addition to the micro-structures that are prone to generate micro-void in AHSS, the edge of the sheet will have different timings of edge cracking under different punching processes and different rolling directions. Therefore, in order to predict the timing of edge cracking under different edge conditions, this paper will discuss how to define edge conditions and establish failure criterion with simple edge cracking experiments, including hole expansion test, hole tensile test, Half Specimen Dome Test (HSDT). In the simple edge cracking experiment, this paper has completed experiments with different materials and different edge conditions. Through the research results, it is found that when the edge conditions of two different edge cracking experiments are the same, the timing of edge cracking will be very close. Therefore, this paper will establish the damage criterion through this experimental feature, and then predict the edge cracking timing of the actual stamped part. In order to establish a CAE simulation for predicting the edge cracking of actual stamped part, this paper uses the PAM-STAMP software to create CAE simulation model and set up simple stamping experiment. After predicting the edge cracking of the simple stamping experiment through the failure criterion, and comparing the results of the experiment and the simulation, it is found that the predicted results have good accuracy. In addition to distinguishing the edge formability of different materials, the simulation can also distinguish the edge formability of the same material under different edge conditions. Therefore, methods for predicting edge cracking have been successfully established. In the future, the method established in this paper can avoid edge cracking and provide a reference for mold design.