熱沖壓成形模具冷卻系統設計之研究

A Study on Cooling System Design for Hot Stamping Tools

劉正國(Cheng-Kuo Liu)

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(2015年)

https://doi.org/10.6342/NTU.2015.10390

摘要

近年來，由於環保意識的提升，各大車廠皆致力於開發低耗能之汽車，並以車體輕量化為目的，因此普遍採用高強度鋼板作為車體結構。然而鋼種強度等級的提高，在一般沖壓成形過程中容易產生回彈及扭曲變形等缺陷，使得複雜造型成形不易。為了解決這些生產問題，熱沖壓成形技術應用於汽車結構件之開發逐漸受到重視，利用高溫成形及模內淬火的方式，使板材微觀組織轉變為強度較高的麻田散鐵組織，其抗拉強度可達1400 MPa以上，並改善一般沖壓成形之缺陷問題，故熱沖壓成形技術已是目前汽車產業發展的重要指標。
熱沖壓製程中最關鍵的步驟為板材模內淬火階段，模內冷卻品質的好壞會直接影響板材成形後的機械性質。然而在大量生產過程中，模具不斷從高溫板材上吸收熱能，若沒有一套設計良好的模具冷卻系統使模具降溫，將會影響板材的冷卻速率。本論文以CAE (Computer Aided Engineering)數值模擬分析方法探討管路配置參數、熱傳係數及板材厚度對冷卻系統之影響性，並以U形載具為模型，提出一套針對不同特徵部位的管路配置方法，最後應用於改善實際開發模具之冷卻系統中，並以實驗驗證模擬分析之準確性。
在參數探討方面，本論文研究成果顯示，在管路配置參數中，管路數量主要影響板材冷卻均勻性，而管緣與模面距離與管徑大小主要影響板材冷卻速率；在熱傳係數分析中，模具材料之熱傳導係數較低時，管壁之熱對流係數對板材冷卻速率的影響較顯著，且當熱傳係數越大，對板材冷卻速率的敏感度越低；在板材厚度分析中，薄板材冷卻速率明顯比厚板材高，且熱傳係數對於薄板材冷卻速率之敏感度明顯降低。
在管路配置方法方面，本論文提出針對頂面及側壁之管路配置模型，並發現該特徵長度與該位置之冷卻速率呈線性關係，可以用三個資料點作線性回歸直線近似，找出不同管路配置模型下所適用之臨界特徵長度，藉以建立管路配置之資料庫。後續則以ANSYS-Fluent軟體對一實際開發之U形載具進行流體分析，並將邊界條件代入PAM-STAMP軟體進行成形與循環冷卻分析，探討及改善其冷卻不均勻之問題，然後以此論文之方法重新建立該模具之冷卻系統，由模擬分析中可知，本論文所建立之冷卻系統其冷卻效率優於原先設計，且可以用線性回歸方程式推估板材冷卻速率。最後利用實際開發之U形載具進行實驗驗證，該實驗所得結果與模擬分析相近，可證明本論文所提的管路設計方法有一定的準確性。

關鍵字

熱沖壓成形 ； 模具冷卻系統設計 ； 管路配置參數 ； 熱對流係數 ； 熱傳導係數 ； 板材厚度 ； U形模具 ； 平板模具

並列摘要

With the raise of environmental consciousness, automobile companies have made efforts in reducing the weight of automotive structures in order to enhance the fuel efficiency. However, increasing the strength of steels leads to the defects of low formability, springback and distortion in sheet metal forming. While in hot stamping process the above-mentioned forming defects could be much improved. In addition, the transformation of austenite into martensite during the die-quenching process could increase the tensile strength of the production part up to 1400 MPa. Therefore, the hot stamping technology has been widely adopted in the automotive industry to manufacture the light-weight structural parts.
In the hot stamping process, die-quenching is the most critical step to ensure the completion of martensitic transformation. The die cooling system thus plays an important role in the hot stamping tool design. The cooling efficiency depends on the heat transfer coefficient at the water-tool interface and the conductivity of the tools. In this thesis, The finite element simulations were first performed to investigate the influences of cooling channel configurations on the interface heat transfer coefficient. The hot stamping of a U-hat shaped part with various cooling channel configurations is adopted to construct the finite element model. It is found that the cooling uniformity of the sheet blank is affected mostly by the number of cooling channels. While the cooling rate of the sheet blank mainly depends on the distance between cooling channel and die surface as well as the diameter of the circular channel. The finite element simulation results also reveal that the heat transfer coefficient between the water channel and die surface dominates the cooling rate of sheet blank as the heat conductivity coefficient of dies is low, vice versa. It also noted that the heat conductivity coefficient of dies affects the cooling rate significantly if the sheet blank is thicker. The effect of the set-up of cooling channels, including the number and location of channels on the top flat surface and side wall, on the cooling rate was also examined. The cooling rates of sheet blank at top surface and side walls are found to be linear with its length and can be approximated by linear regression. A database then can be established efficiently by the design rule proposed in this thesis.
With the developed design rule for the cooling system, the cooling efficiency of an existing hot stamping die for manufacturing a U-hat part was examined with the use of ANSYS-Fluent and PAM-STAMP software to carry out the flow analysis, forming analysis, and thermal analysis. The finite element analysis identified the inefficient design of the existing die cooling system and a modified cooling channel configuration was established using the developed design rule. The simulation results indicate that the cooling efficiency of sheet blank with the modified die cooling system is much better than that generated by the original design.
In order to validate the proposed design rule for die cooling system, a U-hat hot stamping experiment was implemented. The finite element simulation results are found quite consistent with the experimental data and the efficiency of the proposed design rule is thus confirmed.

並列關鍵字

hot stamping ； die cooling system ； cooling channel configuration ； convection heat transfer coefficient ； thermal conductivity ； U-shape model ； flat-shape model

參考文獻

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