伺服沖床運動曲線對超高強度鋼板沖壓成形特性影響之研究

Abstract

近年來，伺服沖床因具備高效能與高靈活度，已逐漸應用於板金成形領域。相較於傳統機械式沖床，伺服沖床無飛輪機構，能有效降低成形所需能耗，並透過伺服馬達實現可程式化之沖頭運動控制。其運動曲線具有高度可調性，能於空行程階段提升速度以縮短循環時間(Cycle Time)，進而提升整體產能效率。除此之外，相關研究文獻指出，透過調整沖頭運動曲線，可提升材料成形極限與改善回彈現象。因此本論文針對常見之伺服運動曲線(如脈衝運動曲線、Stepwise運動曲線)進行實驗分析與有限元素分析。 在實驗方面，使用三種超高強度鋼(MS 1300、MS 1500及MS 1700)針對常見伺服運動曲線進行相對應之實驗。由單軸拉伸-釋放-再拉伸實驗結果顯示，三種超高強度鋼靠近材料均勻伸長率(UEL)進行應力釋放後，再次拉伸會使材料總伸長率(TEL)下降；另外，應力鬆弛實驗中加入雙相鋼(DP 600)材料進行實驗比較。由實驗結果顯示，雙相鋼其肥粒鐵組織具有良好差排重組能力，能有效降低差排密度，提升材料伸長率；相較之下，超高強度鋼因全麻田散鐵組織，差排滑移能力受限，導致無法有效增加材料伸長率。 針對超高強度鋼(MS 1500)材料建立U型帽狀引伸模型，採用Yoshida-Uemori材料模型分析脈衝運動曲線參數(抬升位置、抬升量及抬升次數)對成形性之影響。模擬結果顯示， 抬升位置為主要影響成形性關鍵參數，且抬升量達總沖程5%時回彈改善已趨於穩定。此外，根據應力鬆弛實驗數據擬合 Time Power Law 蠕變模型，結合彈塑性與蠕變行為建立材料模型，用於模擬 Stepwise 運動曲線中之持壓階段。模擬結果顯示，持壓行為對於超高強度鋼回彈改善無明顯效益。最後，進行先弱後強階梯式變壓料力分析，以取得改善側壁捲曲之變壓料力範圍。 本研究整合實驗與模擬方法，探討伺服沖床運動曲線對超高強度鋼成形行為之影響，相關成果可提供未來運動曲線參數設計與成形製程優化之參考。

In recent years, servo presses have been increasingly applied in the field of sheet metal forming due to their high efficiency and flexibility. Compared to conventional mechanical presses, servo presses eliminate the flywheel mechanism, thereby reducing energy consumption during forming operations, and achieve programmable ram motion control through servo motors. The motion curves of servo presses are highly adjustable, allowing an increase in speed during the idle stroke to shorten cycle time and improve overall production efficiency. Moreover, relevant studies have indicated that appropriate design of ram motion curves can enhance the forming limit of materials and reduce springback. Therefore, this study focuses on two commonly used servo motion curves—pulse motion and stepwise motion—through experimental investigation and finite element analysis (FEA). In the experimental part, three types of ultra high strength steels (MS 1300, MS 1500, and MS 1700) were tested under corresponding servo motion conditions. Results from the uniaxial tension–unloading–reloading experiments show that stress release near the material’s uniform elongation (UEL) leads to a reduction in total elongation (TEL) upon subsequent reloading. Additionally, dual-phase steel (DP 600) was introduced in the stress relaxation experiments for comparison. The results reveal that the ferrite structure in DP 600 facilitates effective dislocation rearrangement, lowering dislocation density and enhancing elongation. In contrast, the fully martensitic microstructure of ultra-high strength steels restricts dislocation mobility, limiting elongation improvement. In the numerical analysis, a U hat drawing model was established for MS 1500, and the Yoshida-Uemori material model was implemented in the FEA framework to investigate the influence of pulse motion parameters, including lift position, lift height, and lift count, on formability. Simulation results indicate that the lift position is the most critical parameter affecting formability, and that increasing the lift height beyond 5% of the total stroke yields diminishing improvement in springback. Furthermore, the Time Power Law creep model was fitted using stress relaxation data and integrated with elastic–plastic behavior to construct a coupled material model, enabling simulation of the pressure holding stage under the stepwise motion curve. The results suggest that pressure holding offers no significant benefit for springback reduction in ultra high strength steels. Finally, a variable blank holder force strategy featuring a weak to strong sequence was analyzed to identify an effective force range for reducing sidewall curl defects. This study integrates experimental observations with finite element analysis to comprehensively investigate the effects of servo press motion curves on the forming behavior of ultra high strength steels. The findings provide practical guidance for the design of motion curve parameters and optimization of sheet metal forming processes.