超高強度鋼冷沖壓成形特性分析之研究

Abstract

隨著全球對節能減碳的重視與汽車輕量化趨勢的推進，超高強度鋼(Ultra High Strength Steel, UHSS)因具備高強度、良好的耐碰撞性能與相對低成本的優勢，已廣泛應用於車體結構件的製造。然而，UHSS在冷沖壓成形過程中，常因伸長率不足而產生破裂問題，同時亦因高強度特性導致回彈顯著，進一步增加模具設計與成品精度控制之難度。為解決上述問題，學術界與產業界普遍導入有限元素分析(Finite Element Analysis, FEA)技術，藉由模擬預測成形缺陷，進行模具幾何與製程參數之優化，以提升成形品質與尺寸穩定性，並有效降低試模次數與生產成本。 為深入瞭解超高強度鋼冷沖壓成形行為，本論文首先透過材料性質實驗，比較MS 1300、MS 1500_A、MS 1500_B及MS 1700四種超高強度鋼之基本成形特性。此外，為提升超高強度鋼於冷沖壓成形之回彈預測能力，建立固定式治具以進行反覆拉伸與壓縮實驗，進而建構考慮包辛格效應(Bauschinger Effect)之Yoshida-Uemori材料模型，並發現對於降伏比值較高之鋼材，適度調整模型參數Y，可更準確描述材料之實際變形行為，並提升回彈預測準確度。 應用上述所建立之材料模型，本論文建立V-shape、U-shape 及 U-hat-shape三種具代表性之基礎載具模型，分析四種鋼材於不同成形模式下之減薄與回彈行為，探討其成形性差異。此外，亦進一步分析材料參數對成形性的影響，發現r值≥ 1、0.6≤降伏比≤0.7使得材料有較佳之抗減薄能力。同時針對幾何參數變化進行模擬分析，其中板厚1.6mm之MS 1500_B於 U-hat 引伸成形中，當母模圓角為 3 mm 時易產生破裂，因此未來於模具幾何參數設計建議母模圓角尺寸不小於 4 mm。 本論文所建立之材料模型、模擬分析與實驗驗證結果，不僅可作為超高強度鋼冷沖壓製程開發之依據，亦具備應用於新鋼種評估與模具設計優化之參考價值。

With the global emphasis on energy conservation and carbon reduction, along with the ongoing trend toward vehicle lightweighting, ultra high strength steels (UHSS) have been widely adopted in the manufacturing of automotive structural components due to their high strength, excellent crash resistance, and relatively low cost. However, in cold stamping processes, UHSS often suffers from poor elongation, leading to fracture issues. Additionally, their high strength characteristics result in significant springback, increasing the difficulty of die design and dimensional control. To address these challenges, both academia and industry have widely adopted finite element analysis (FEA) to predict forming defects, optimize die geometry and process parameters, enhance forming quality and dimensional stability, and reduce tryout iterations and production costs. To better understand the cold stamping behavior of UHSS, this study first conducted material characterization tests on four UHSS grades: MS 1300, MS 1500_A, MS 1500_B, and MS 1700, to compare their fundamental forming properties. In order to improve the prediction accuracy of springback behavior, a fixed fixture was developed to perform repeated tension-compression tests. Based on the results, a Yoshida–Uemori (Y-U) material model capable of capturing the Bauschinger effect was constructed. The study also found that, for steels with a high yield ratio, appropriate adjustment of the Y parameter in the model enables better prediction of the actual deformation behavior and improves springback accuracy. Using the developed material models, this study further established three representative benchmark forming geometries—V-shape, U-shape, and U-hat-shape—to analyze the thinning and springback behavior of the four UHSS grades under different forming modes. The influence of material parameters on formability was also investigated, showing that materials with an r-value ≥ 1 and a yield ratio between 0.6 and 0.7 exhibited better resistance to thinning. Additionally, geometric sensitivity analysis revealed that MS 1500_B with a sheet thickness of 1.6 mm is prone to fracture during U-hat stretch forming when the die radius is 3 mm. Therefore, it is recommended that the die radius be no less than 4 mm in future die designs. The developed material models, simulation results, and experimental validations presented in this study not only serve as a basis for UHSS cold stamping process development but also provide valuable references for the evaluation of new steel grades and the optimization of die design.