裁縫式焊接鈑金熱沖壓汽車B柱沖擊強度之有限元素分析

Abstract

隨著環保意識的抬頭，節能減碳已成為工業發展的趨勢與目標，而汽車產業在符合安全標準下，亦積極追求車身的輕量化以降低油耗。為減輕車身重量，各大車廠均致力於高強度鋼板的開發，然而高強度鋼板在常溫下易產生直線回彈、側壁捲曲以及扭曲等成形缺陷，因此熱作的熱沖壓製程因應而生，特殊鋼種透過模內淬火方式，可使工件金相組織轉變為高強度之麻田散鐵，抗拉強度可達1500MPa以上，且高溫成形可減少回彈以及扭曲之問題，是未來汽車結構件重要之成形技術。隨著高強度鈑件的開發，各個國家訂立的車輛安全標準不斷提升，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。因此在單一零件上採用兩種不同機械性質的設計，部分為高強度抵抗變形，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LS-DYNA建立沖擊試驗模型，並針對具有B柱特徵造型的帽形基礎載具，利用裁縫式焊接鈑金的方式，強區、弱區分別採用15B22與 JSC 440W，分析不同比例的沖擊試驗結果，並將結果與傳統汽車用鋼比較。結果顯示，裁縫式焊接設計與傳統780級、980級鋼材強度相同時，分別可減少13.80 %和7.45 %的重量。

Owing to the rising awareness of environment protection, 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 disadvantages, the hot stamping process has been developed. 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 and 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 the demands of the two key properties. The research uses the finite element analysis software LS-DYNA to establish the impact model for the B-pillar and the hat-shape part which has B-pillar’s characteristics. Applying the method of tailor welded blank (TWB), the tailored part possesses two materials in a single sheet. The part in the local regions with softer mechanical strength is made of alloy steel JSC 440W to improve the energy absorption characteristics while other regions are made of the quenchable boron steel 15B22 to achieve high strength. The analysis of the distribution of JSC 440W and 15B22 would compare with the traditional steels to quantify the benefits of tailor properties. As the result of the research, compared to the traditional steels with tensile strength 780 MPa and 980 MPa, the design of tailor welded blanks can save 13.80 % and 7.45 % in weight.