臺灣博碩士論文加值系統

本實驗研究中錳鋼經冷軋及不同熱處理後，鋼材擴孔率與顯微組織及拉伸性質之關係，實驗結果顯示，熱處理溫度上升或熱處理時間增加，會使衝孔試片的剪切影響區深度增加，擴孔率也提升。改變熱處理參數對擴孔率的影響，比改變開孔方式對擴孔率的影響還要小。實驗結果顯示，機械衝孔對試片造成剪切影響區，並且在該區內部觀察到許多碳化物周圍生成的微裂縫，是造成衝孔試片擴孔率不佳的原因之一。放電線切割開孔試片內部幾乎沒有觀察到裂縫生成，且沒有剪切影響區，因此擴孔率較高。本研究之中錳鋼試片擴孔率與抗拉強度呈反向關係，與降伏強度與抗拉強度之比值、總伸長率、真實破壞應變及截面積縮減率呈正相關，與降伏強度則沒有明顯關係。
本研究利用三離子槍拋光機取代傳統機械拋光或電解拋光，觀察機械衝孔及放電線切割開孔後擴孔試片顯微組織。結果顯示，擴孔裂縫附近的一些微裂縫在碳化物及介在物周圍生長，很大比例裂縫在肥粒鐵及麻田散鐵或沃斯田鐵的相界上生長。擴孔過程中，裂縫經過前在試片內部產生一個塑性變形區，並使周圍的沃斯田鐵相變化為麻田散鐵，相變化後較細小的硬相麻田散鐵與周圍大顆的軟相肥粒鐵形成強度差異，可能因此促進裂縫之成長，導致擴孔率的下降。

The relationship between hole expansion behavior, microstructure and tensile properties of a medium-Mn steel has been studied. The results show that the increase of the heat treatment temperature and time can increase the depth of the shear affected zone (SAZ) of the test pieces, and the hole expansion ratio. Heat treatment has less effect on hole expansion ratio than hole opening method. Mechanical punching caused an SAZ on the specimens, and some microcracks were generated beside carbides within the SAZ, which is one of the reasons for the poor hole expansion ratio. For specimens with the hole of test pieces formed by wire electrical discharge machining, almost no cracks were found in the specimens, therefore, the hole expansion ratio obtained is higher. In this study, the higher the tensile strength obtained, the lower the hole expansion ratio of the specimens, and the higher the yield strength / ultimate tensile strength ratio, total elongation, true fracture strain and reduction of cross sectional area, the higher the hole expansion ratio obtained.
An ion polishing machine was used to prepare specimens for microstructural characterization. The results show that some microcracks near the main crack of tested specimens grow beside carbides and inclusions, and a large number of cracks grow at the phase boundaries of ferrite and martensite or austenite. During hole expansion test, a plastic zone is generated by the crack, and deformation induced martensite is formed. The strength difference between martensite and ferrite may promote the growth of cracks so that leads to a low hole expansion ratio.

論文審定書 i
摘要 ii
Abstract iii
目錄 iv
圖目錄 vi
表目錄 xvi
一、前言 1
二、文獻回顧 2
2-1 中錳鋼 2
2-2 TRIP效應 2
2-3 剪切邊緣成型性 3
2-3-1 剪切表面和剪切影響區 3
2-3-2 剪切間隙 6
2-3-3 剪切邊緣成形性的預測 7
2-4 擴孔試驗 7
2-4-1 先進高強度鋼的擴孔性質 10
2-5 放電加工 14
三、研究目的 16
四、實驗方法 17
4-1 實驗材料 17
4-2 試片製備 17
4-3 拉伸試驗 17
4-4 擴孔試驗 17
4-5 試片前處理 18
4-6 顯微組織分析 18
4-7 開孔試片孔緣橫截面分析 18
4-8 顯微硬度分析 19
4-9 擴孔後裂縫顯微組織分析 19
五、實驗結果 20
5-1 母材顯微組織 20
5-2 機械衝孔孔緣組織 20
5-3 EDM開孔孔緣組織 22
5-4 擴孔後之孔緣組織 23
5-5 擴孔後裂縫之觀察 24
5-6 擴孔試驗結果 26
5-7 拉伸試驗結果 26
5-8 拉伸試驗與擴孔率之關聯性 26
六、討論 27
七、結論 30
八、參考文獻 31

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