裁縫式模具淬火熱沖壓製程之有限元素分析與模具設計

Abstract

節能減碳已成為各國工業積極追求的目標，而汽車產業亦致力投入低油耗之輕量化車身結構設計，為達到減輕車身重量的目的，各大車廠開始採用先進高強度鋼，但高強度鋼種在常溫下沖壓成型容易導致工件回彈、扭曲、捲曲等缺陷，熱沖壓成形製程即因此產生。藉由高溫下金屬軟化的特性，使上述問題大幅改善，且特殊鋼種透過模內淬火的方式快速冷卻，可使工件的抗拉強度達1500MPa以上，汽車結構件可改用厚度較低的鋼板藉此達到車體輕量化的效果。但是隨著高強度鈑件的開發，各個國家訂立的車輛安全標準亦不斷提升，車身設計不只是要求高強度，亦越來越重視車體吸收撞擊能量的能力，避免能量直接傳至人體造成傷害。因此在單一鈑件(例如最常選用汽車之B柱)上採用兩種不同機械性質的裁縫式設計，部分區域為高強度抵抗變形，而部分強度較低用於吸收撞擊能量。 本研究藉由有限元素軟體DEFORM建立裁縫式模內淬火CAE分析模型，並針對具有B柱特徵的帽型基礎載具，探討製程參數對成品強弱區強度之影響，並探討模具加熱系統設計參數對成品強弱區強度分布之影響。 接著將分析結果導入U型帽狀載具中進行裁縫式模具淬火沖壓實驗，並將沖壓結果與模溫恆定之理想模具沖壓模擬進行比較，結果顯示，強弱區強度穩定值十分接近，且成品弱區強度分布亦十分均勻，僅過渡區寬度有所增加，顯示本研究中所採用之模具加熱系統設計參數對改善模面均溫性與成品弱區強度之分布有所幫助，可作為模具設計時的初步參考。

Energy saving and carbon reduction are the main goals of industries all over the world. Automotive industry aims at cutting carbon dioxide emissions under the premise of the vehicle safety standards by utilizing the ultra high strength steels. However, the forming of the ultra high strength steels in room temperature will induce some problems such as the tendency to springback and fracture. In order to overcome these defects, the hot stamping process has been developed. High temperature in hot stamping process are able to soften the steel and solve the defects. The tensile strength of hot stamped steels can exceed 1500MPa by transforming the ferritic-pearlitic microstructure to the high strength martensitic microstructure. With the development of high-strength steels, each country establishes higher vehicle safety standards. The design of vehicle parts pursues not only high strength but also the ability to absorb impact energy. Thus, the tailored blanks are applied to meet those demands. The research uses the finite element analysis software DEFORM to establish the tailor die quenching process model for the hat-shape part which has B-pillar’s characteristics, and the influence of the process parameters on the strength of the hot stmap product was discussed. The design parameters of the die heating system were also discussed. The previous results are introduced into the U-shaped model for tailor die quenching process, and compare the result with constant die temperature model. The results show that the hot stamp product of die heating system got uniform hardness distribution, which is similar to the hot stamp product of constant die temperature model. Only the width of transition zone is increased. The result shows that the die heating system design parameters in this study are able to improve the strength distribution of tailor die quenching product and can be used as a initial die design reference.