熱沖壓介面熱傳係數之量測方法與實驗平台建立

Experimental Platform Design for the Determination of the Heat Transfer Coefficient in the Hot Stamping Process

蔡沛吾(Pei-Wu Tsai)

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(2014年)

https://doi.org/10.6342/NTU.2014.00505

摘要

近年來隨著環保意識提升，節能減碳成為全球產業界需面臨的議題，汽車界亦積極投入製造輕量化之車體結構。然而現今車廠常用之先進高強度鋼種，隨著鋼種強度提高，其成形性、回彈量以及扭曲變形，造成產品開發不易，以及生產成本無法降低，故熱沖壓成形即因應而生。透過高溫成形並模內淬火之方式，可使得板材金相組織轉變成為高強度之麻田散鐵，其抗拉強度可達到1400MPa以上，並且在較薄的板材上能夠達到此強度，名副其實達到輕量化與高強度之目標。
熱沖壓成形能藉由有限元素分析，於產品開發前有效預測產品的性能以及製程分析與設計，而介面熱傳係數即為其關鍵製程參數之一。為了提升模擬準確性，本研究係於建立介面熱傳係數之實驗平台以及分析方法，利用熱沖壓成形之板材與模具材料，進行實驗並量測溫度場，進而推導介面熱傳係數，以作為後續產品開發時熱沖壓成形分析之運用。
本研究首先藉由文獻收集，瞭解熱沖壓製程對於介面熱傳係數之研究以及發展現況，並且透過基礎介面熱傳特性、微觀模型以及理論數學式探討，整理與歸納出影響因子；透過U形帽狀基礎載具之模擬分析，探討介面熱傳係數對熱沖壓成形的冷卻速度、減薄率以及金相組織之影響性，並確認介面熱傳係數於熱沖壓成形之重要性以及研究之必要性。
利用CAE模擬評估實驗平台適用性，包含板材與模具量測位置設計、板材形狀以及冷卻系統之設計，最後以軸對稱之圓形模具作為實驗平台，實驗參數包含不同接觸壓力、不同間隙以及不同初始溫度進行實驗。分析方法以牛頓冷卻定律與逆向熱傳技巧為基礎，利用實驗量測之板材與模具溫度歷程圖，計算出介面熱傳係數，其結果隨著接觸壓力的增加而提高；隨著間隙減小，介面熱傳係數亦提高，但是相對於接觸壓力其影響性相對少；而不同初始溫度亦會有不同介面熱傳係數。
透過本研究所建立之實驗平台，獲得之介面熱傳係數，應用於熱沖壓防撞樑上；成形工法以及模具間隙之探討，分析對於產品減薄率以及冷卻速率之影響，將分析模擬所獲得成果應用於模具設計上；利用熱影像儀所拍攝之溫度圖與模擬做對照，並且量測成品厚度與模擬做比對。結果顯示熱沖壓成形模擬與成品之符合性相當高，表示本研究之介面熱傳係數有其應用性。
本研究已經建立介面熱傳係數之實驗平台以及分析手法，並且實際應用於熱沖壓成形結構件上，針對未來熱沖壓成形所可能面臨之介面熱傳係數問題，此實驗平台能夠針對其他影響因子進行更加深入研究與探討。

關鍵字

熱沖壓成形 ； 介面熱傳係數 ； 接觸壓力 ； 牛頓冷卻定律 ； 有限元素分析 ； 防撞樑

並列摘要

As the raise of environmental awareness and demand of carbon reduction, the automobile companies have dedicated to reducing the weight of automobile structure to achieve higher fuel efficiency. Due to the low formability, springback and twisting defects by raising the strength of steels, there still exists difficulties in manufacturing and cost reduction. Hot stamping has become the solution for these problems instead. Through a series of hot stamping stages leads the microstructure to Martensite transformation. The tensile stress can reach over 1400MPa with thinner blank. Therefore, hot stamping can achieve both objects of lightweight and high strength.
Through finite element analysis, performance and process designs of products can be predicted effectively before production. And the heat transfer coefficient (HTC) is one of the most critical parameter for numerical simulation. In order to improve simulation accuracy, the experimental platform and the method of calculating HTC are established. By measuring the temperature history both in die and blank, the HTC can be computed and used in CAE analysis of hot stamping components.
Influential factors associated with HTC are listed by reviewing recent development of hot stamping and research of HTC. In order to confirm the importance and necessity of HTC, the effects of cooling rate, thinning percentage and transformation of microstructure through U-shape simulation with different HTC from references are discussed.
The experimental platform is evaluated and designed by CAE analysis which includes the positions of measurement, shape of the specimen and design of cooling system. Finally, the axial symmetric circular die is employed with the experimental parameters containing different contact pressure, gaps and the various initial temperatures. Based on the history of temperature measurement, Newton’s cooling law and inverse technique are utilized to compute HTC. The HTC increases with higher contact pressure, lower gap and higher initial temperature.
Finally, different forming processes and various gaps between die and blank are discussed in door beam to investigate the effects on thinning percentage and cooling rate. The proper parameters and HTC from previous experiment are employed in the door beam analysis. By comparison of the temperature and the thickness between simulation and experiment, the result shows that there exists high consistence. Therefore, it reveals that the HTC has actual application and the hot stamping analysis of door beam can be employed in evaluation of die design.
An experimental method and computing techniques of heat transfer coefficient have been set up in this research. And it also applies in analysis of automobile components. In the future, more factors on HTC can be further investigated and discussed.

並列關鍵字

Hot stamping ； heat transfer coefficient ； contact pressure ； Newton’s cooling law ； Finite element analysis ； door beam

參考文獻

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