| 摘要 |
先進高強度鋼板目前已廣泛應用於汽車結構件中易受撞擊的部位，以吸收、緩衝及分散第一時間的撞擊能量。因成形塑流應力較高，其成形問題較傳統低強度鋼更為複雜且嚴重。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，目前相關研究開始從建構新的材料模型來提高回彈模擬預測的準確性。
在先進高強度鋼板材料模型之加工硬化特性中，當板材受拉、壓反覆應力時會顯現出材料包辛格效應，而目前建構含包辛格效應之材料模型的方式多為進行單軸拉伸-壓縮實驗以取得材料參數。但在實際沖壓成形製程中，板材之變形模式與反覆彎曲實驗更為接近。因此本論文將針對先進高強度鋼板590Y、780Y、980Y及1180Y在三點式反覆彎曲及四點式反覆彎曲下之材料包辛格特性與變形機制進行探討。CAE模擬及實驗結果証實經由單軸拉伸-壓縮試驗取得之材料參數應用於反覆彎曲，可達85%以上之回彈預測準確性。接著以受力模式與反覆彎曲實驗相似但受力更為複雜之U形帽狀基礎載具進行實驗驗證，確認經由單軸拉伸-壓縮試驗取得之材料參數可達80%以上之回彈預測準確性。
本論文透過沖頭受力及量測之板材表面應變值計算四點式反覆彎曲時材料之應力-應變曲線，由此曲線可看出明顯之材料包辛格效應。同時與單軸拉伸-壓縮之曲線相比較可以觀察到反覆彎曲下材料應力值於正彎曲及反彎曲時皆來得小，且材料受反覆彎曲變形時，材料永久軟化特性更明顯。
本論文最後討論反覆彎曲實驗與單軸拉伸-壓縮實驗之優缺點並說明反覆彎曲實驗之必要性。因反覆彎曲實驗欲取得板材不同應變量之方式較為直覺且較基礎載具試驗方便，且可發現先進高強度鋼板採用含Yoshida-Uemori
model之材料模型於不同應變量之變形下準確度有所不同，因此可將反覆彎曲試驗作為材料於真實沖壓受力情況下不同應變範圍之驗證，並可以此驗證結果來針對CAE分析結構件中不同應變量之成形部位進行改善，以增加CAE模擬分析之準確性。
關鍵字：先進高強度鋼板材料模型、反覆彎曲成形、回彈、有限元素法分析、反覆彎曲應力-應變曲線
Advanced high strength steel has been widely used in automotive
structures, especially in parts where prone to collide, to
absorb and buffer impact energy at the first time. It’s suffered
more complex and serious forming problems than traditional steel
due to its higher flow stress. Besides cracking and wrinkling,
the major defects needed to be solved are springback, side-wall
curl and distortion. To overcome these problems, recent studies
began to develop material models to better understand the
forming characteristics of advanced high strength steel sheets
and to improve the accuracy of the simulations.
Advanced high strength steel would appear the Bauschinger effect
while under cyclic tension-compression load. Cyclic tension –
compression tests are usually conducted to obtain the
stress-stain relationship of steel sheets with Bauschinger
effect. However, steel sheets are subjected to cyclic reversed
bending in real stamping processes. Therefore, stress-strain
curves obtained by uniaxial tension – compression tests may be
not able to accuratedly predict the material behavior in CAE
simulations for stamping process. Therefore, this thesis focuses
on investigating the characteristics of advanced high strength
steel DP590, DP780, DP980 and DP1180 under three-point reversed
bending and four-point reversed bending. Compared with
experimental results, CAE simulation results with material
parameters obtained by uniaxial tension – compression tests show
a discrepancy of less than 15% in predicting the springback of
advanced high strength steels in cyclic reversed bending tests.
For U-hat drawing, whose loading history is similar to that of
cyclic reversed bending but more complicated, the discrepancy in
the springback prediction based on CAE simulations is less than
20% compared with the experimental data.
The stress – strain curve of advanced high strength steels is
also established based on the data of the load and the strain on
the sheet surfaces in four-point bending tests. Compared to the
stress – strain curve obtained by uniaxial tension – compression
tests, it can be observed that stress values are lower in both
bending and reversed bending stages.In addition, characteristics
of secondary softening and springback are more remarkable as
observed in the stress-strain curves..
Advantages and disadvantages between cyclic reversed bending
tests and the uniaxial tension – compression test are discussed
in this study. The stress-strain relationships of advanced high
strength steel sheets are easier to be established in the
uniaxial tension – compression tests than those in cyclic
reversed bending tests. On the other hand, AHSS sheets are
subjected to cyclic reversed bending in real stamping processes.
Because cyclic reversed bending tests can be easily perfomed at
different strain ranges with the fixtures presented in this
study, they can be used to verify the adequacy of the
stress-strain curves obtained in the uniaxial tension –
compression tests. The results of this study can support future
researches on the development of material models of advanced
high strength steel sheets in CAE analysis for stamping
processes.
Key words：Advanced high strength steel, Bauschinger effect,
Cyclic reversed bending, Springback, Finite element analysis
|
