冷沖壓超高強度汽車保險桿內鐵之成形性研究

Abstract

在現代汽車車身結構中，超高強度鋼的應用使鈑金件輕量化與高強度設計得以兼顧，然而此類鋼材延伸率較低，於冷沖壓成形過程中容易產生皺褶與破裂等缺陷，且成形後之回彈量顯著，嚴重影響零件尺寸精度。本研究以MS1500超高強度保險桿內鐵為研究載具，採用Hill 48降伏準則與Yoshida-Uemori混合加工硬化材料模型，建立具高精度回彈預測能力之有限元素模擬，以此模型探討此保險桿內鐵於冷沖壓成形中之缺陷成因、改善對策及回彈抑制機制。 比較引伸成形與有背壓板之彎曲成形兩種工法，透過材料流速、厚度分佈與成形極限曲線（FLC）分析，發現兩種工法皆於特徵造型處產生皺褶與破裂缺陷。針對前述缺陷，本研究進行系統性改善，建立一無明顯成形缺陷之基本造型研究載具，作為後續回彈研究之基礎。 針對此研究載具之回彈行為進行機制分析，並透過內外表面厚度方向應力差之量化指標，驗證回彈量與應力差分佈之對應關係。在此基礎上，本研究探討伺服沖床脈衝運動曲線之抬升位置、抬升量及抬升次數對回彈及減薄率之影響，側壁捲曲曲率比起直接沖壓改善幅度達16%。進一步導入先弱後強階梯式變壓料力和可調式阻料條於沖程末端施加後拉伸之策略，並藉由將阻料條作用時機與強壓料力作用時機分離，成功避開沖程末端之局部破裂風險，曲率改善幅度達54%。 本研究成果顯示，透過伺服沖床脈衝運動曲線、變壓料力與分段式可調式阻料條之組合應用，可在避免破裂之前提下有效抑制超高強度保險桿內鐵之側壁捲曲回彈，為此類高強度鋼結構件之冷沖壓製程開發提供可行之參數化改善方法論。

In modern automotive body structures, the application of ultra high strength steel (UHSS) enables both lightweight design and high structural strength in sheet metal components. However, this class of steel exhibits relatively low elongation, making it prone to defects such as wrinkling and fracture during cold stamping, while also exhibiting significant springback after forming that severely compromises dimensional accuracy. This study uses an MS1500 ultra-high strength steel automotive bumper beam as the research specimen. A finite element model with high-precision springback prediction capability was established using the Hill 48 yield criterion and the Yoshida-Uemori (Y-U) mixed hardening material model, in order to investigate the causes of forming defects, corresponding improvement strategies, and the springback suppression mechanisms of this component during cold stamping. Two forming methods—drawing and bending with a back-pressure plate—were compared. Through analysis of material flow velocity, thickness distribution, and the forming limit curve (FLC), both methods were found to produce wrinkling and fracture defects at the characteristic feature regions of the part. To address these defects, systematic improvements were carried out, resulting in a defect-free basic-geometry research specimen that served as the foundation for the subsequent springback study. The springback behavior of this research specimen was then analyzed mechanistically, and a quantitative indicator based on the through-thickness stress difference between the inner and outer surfaces was used to verify the correspondence between the magnitude of springback and the stress-difference distribution. Building on this foundation, the influence of the servo press pulse motion curve—specifically the lift position, lift amount, and number of lifts—on springback and thinning rate was investigated; the sidewall curl curvature was improved by up to 16% compared to conventional direct stamping. A weak-to-strong stepped variable blank holder force strategy and an adjustable drawbead applying post-stretching near the end of the stroke were further introduced. By separating the actuation timing of the drawbead from that of the strong blank holder force, the risk of localized fracture near the end of the stroke was successfully avoided, and the sidewall curl curvature was ultimately improved by 54% compared to conventional direct stamping—the best result achieved among all strategies examined in this study. The results of this study demonstrate that the combined application of the servo press pulse motion curve, variable blank holder force, and a staged adjustable drawbead can effectively suppress sidewall curl springback in the UHSS bumper beam while avoiding fracture, thereby providing a feasible and parameterized improvement methodology for the cold stamping process development of such high-strength steel structural components.