先進高強度鋼板之沖壓成形回彈改善研究

A Study on Reducing Springback in the Stamping of Advanced High Strength Steel sheets

姚順偉(Shun-Wei Yao)

指導教授 ： 陳復國

國立臺灣大學/工學院/機械工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201703633

摘要

在全球暖化日益嚴重下，節能減碳已是一個主要的議題，因此國際各大汽車產業均以輕量化為目標，且汽車安全法規越來越嚴格，促使先進高強度鋼板之開發，因其具有強度高之特性。但隨著鋼板強度的提高，其沖壓成形所面臨之破裂、回彈等成形缺陷相較於傳統低強度鋼更為嚴重，尤以回彈現象更不易克服。
因高強度鋼成形困難，在回彈補償的試模階段需耗費大量的金錢及時間成本，因此導入Computer Aided Engineering(CAE)分析技術，希望藉由CAE之準確分析，以達更精確的模具設計，可以大幅減少試模時間。而材料模型於CAE分析中是相當重要的一環，直接決定破裂、回彈等預測的準確性，其中先進高強度鋼之包辛格效應(Bauschinger effect)較傳統軟鋼更為明顯，因此材料模型需考慮使用包含包辛格效應之加工硬化準則。此外，對於複雜的板金成形，為多軸受力之狀態，因此需以雙軸拉伸試驗決定較適當之降伏準則，期望藉由瞭解材料的塑性變形特性，提升對於高強度鋼板沖壓成形特性分析與回彈預測之能力。
本論文首先針對目前常用之Hill48、Hill90、Barlat89、Barlat91和Yld2000-2d等降伏準則進行探討，此外，由文獻可知，包辛格效應對於提升回彈預測準確性有很大的幫助，因此本論文亦探討可以描述包辛格效應之Yoshida-Uemori材料模型，以及該材料模型所需之參數。
在確認材料模型之準確性後，接著討論板金成形之回彈改善方法。在目前許多使用高強度鋼之汽車結構件皆包含U形帽狀造型，而高強度鋼之U形帽狀引伸成形除了有側壁外開問題，還有嚴重的側壁捲曲。文獻顯示，以後拉伸(Post-Stretch)方式可以有效改善回彈，在實際沖壓過程中，主要以阻料條及變壓料力兩種方式可達到「後拉伸」之效果。因此本論文以1180級先進高強度鋼針對U形帽狀引伸成形進行變壓料力與可調式阻料條對於回彈改善之研究。研究顯示，變壓料力設計較容易且改善佳，因此以變壓料力之阻料力設計作為阻料條造型設計之參考。本論文分別建立兩種方式之參數設計流程，結果顯示，兩種方式皆可以有效改善回彈，並消除側壁捲曲。
本論文亦探討三種U形帽狀之兩道次彎曲方式。第一種方式可避免捲曲之產生，但仍有外開的問題。第二種為文獻提出的兩道次彎曲設計，可以有效改善回彈、捲曲問題，但側壁會產生S形現象，本論文針對其設計加以改善，設計出第三種兩道次彎曲方式，以CAM模改變沖壓方向，可以有更大的補償空間，以消除其S形現象。
本論文之變壓料力與阻料條之設計流程及優化後的兩道次彎曲設計，皆可能用於實際汽車結構件之沖壓設計上，期望以此對業界能有所貢獻。

關鍵字

先進高強度鋼板 ； 側壁捲曲 ； 後拉伸 ； 變壓料力 ； 阻料條 ； 有限元素分析

並列摘要

Global warming is increasingly serious so Energy saving and carbon reduction is a main issue. Therefore many automotive industries aim for lightweight. Automotive safety regulations become more stringent, thus the development of advanced high-strength steels (AHSS) has been promoted. AHSS are high strength. With the increasing sheet metal strength, the stamping process of sheet metal faces some defects, such as rupture, springback, etc which are more serious than mild steel.
The stage of mold trial needs to consume a lot of money, manpower and time because of the forming difficult of AHSS. So the Computer Aided Engineering (CAE) has been introduced. Accurate analysis with CAE can reach accurately die design and reduce time of mold trial significantly. Material modal is a very important part in CAE analysis that deside accuracy of prediction of rupture and springback. Bauschinger effect in AHSS is more serious than mild steels so material modal need to use hardening rule including Bauschinger effect. Furthermore, multiaxial force in the complex metal forming need to use yield criterion including biaxial force. By understanding the plastic deformation characteristics of materials, to enhance abilities of the analysis of AHSS in the stamping and the prediction of springback.
The applicability of yield criteria, such as Hill48, Hill90, Barlat89, Barlat91 and Yld2000-2d for describing the plastic deformation of the advanced high strength steel were investigated first in this thesis. Also according to the literature survey conducted in this thesis, it reveals that the Yoshida-Uemori model is suitable to describe the Bauschinger’s effect and its model would improve the springback prediction.
After confirming the accuracy of the material model, the springback improvement method of sheet metal forming has been discussed. At present, many car structural parts that use AHSS contain U-hat shape section. The U-hat shape drawing that uses AHSS has serious defects of springback and side-wall curl. The literature shows that the “post-stretch” approach can effectively improve the springback. The use of drawbead and variable blank holder force can achieve the “post-stretch” effect in the actual stamping process. Drawbead and variable blank holder force used in U-hat shape drawing with 1180 grade AHSS have been study in this thesis. Research shows that the design of variable blank holder force is easier and better to improve springback, so the design of blank restraining force of variable blank holder force can be used as the reference of drawbead geomtry design. The parameter design flow of two methods have been established in this thesis. The result shows that both methods can effectively improve springback and eliminate side-wall curl.
Three ways of two-step bending process have been study in this thesis. The first way can avoid the occurrence of side-wall curl, but the side-wall springback remains unresolved. The two-step bending design in the literature is the second way that can effectively improve the springback, side-wall curl, but the side-wall will produce S-shaped defect. So the third two-step bending process has been designed to improve S-shaped defect in this thesis. The use of cam die can change stamping direction, thus the allowable compensation range is greater and S-shaped defect can be eliminated.
The design flow of the variable blank holder force and drawbead and optimized two-step bending design in this thesis may be used for the actual stamping design of car structural parts, hoping to contribute to the industry.

並列關鍵字

advanced high strength steels ； side-wall curl ； post stretch ； variable blank holder force ； drawbead ； finite element analysis

參考文獻

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