嚴重塑性變形對納米碳化物強化型肥粒鐵鋼之顯微結構與機械性能影響

Abstract

經由界面析出物強化的肥粒鐵鋼已經被發展且應用超過了十年，但僅有少數的文章已發表關於其強變形後的行為現象。本論文主要研究具有奈米等級的析出物之肥粒鐵經過兩種不同處理方式後的微結構變化，其中主要分成兩部分的製程方式：(1)冷壓退火處理 (2)高壓扭轉 (1) 冷壓退火處理： 本研究引入經冷壓退火處理後細化晶粒的理論，並透過光學顯微鏡，電子背向繞射(EBSD) ，機械性能則以硬度與拉伸試驗的方式來觀測結果。結果顯示，硬度以及拉伸強度皆隨著退火溫度升高而下降且比原材性質劣化。此外，退火後材料呈現比例低的再結晶度。經由（掃描）穿透式電子顯微鏡的幫助，本研究確認了其析出物的粗化效應造成了冷壓退火後的性質劣化。再者，經由與IF鋼對比，同時也確認析出物延緩了再結晶的發生，導致再結晶比例下降。 (2) 高壓扭轉： 本研究採用3120B材料經高壓扭轉後，透過硬度量測及JeoL JSM-7800F Prime的電子背向繞射(EBSD)來研究其微結構變化。經本研究觀測，此材料需要一定大小的應變量，其硬度才始之劇烈上升。換句話說，當應變量超過某值後，材料才開始改變其微結構造成硬度的上升。根據先前的研究，由於析出物能有效抑制差排的蜂巢狀結構產生，本研究進一步指出，析出物的角色因而延緩了高壓扭轉時的第一步驟（差排的蜂巢狀結構產生），造成材料在第一階段硬度的上升被延後。而當壓延量超過此極值，金相才開始發生晶粒形貌變化，接著伴隨其他步驟發生。 總結來說，與其他鋼鐵與合金相比，具有緻密且細小之碳化物的肥粒鐵鋼有相當多不同的行為結果，而這些不同可以透過其碳化物與差排或晶界的的交互作用所解釋。因此，透過研究缺陷及其動態反應，像壓延與再結晶現象，將讓我們更透測了解材料的性質與微結構。透過本研究的結果則可應用於冷軋帶鋼或是嚴重鍛造成分的發展。

Very strong ferritic steels hardened by interphase-precipitated carbides has been developed and applied for over 10 years. Only few reports discussed its stability under thermo-mechanical process or severe deformation. The current work investigates microstructural evolutions of nanometer-sized carbide-strengthened ferritic steels subjected two different processes: (1) cold-rolling followed by annealing and (2) high pressure torsion. In cold-rolling and annealing process, it was found that dense and tiny carbides greatly retard recrystallization in annealing. Moreover, significant loss of strength was observed after annealing even for carbides with excellent thermal stability. The current work demonstrated that the loss of strength results from coarsening of carbides. Two mechanisms were proposed based on the investigations by transmission electron microscopy. In summary, deformation-and-annealing process breaks thermal stability of (Ti, V)C or (Ti, V, Mo)C nanometer-sized carbides. In high pressure torsion, hardness of strong ferritic steel can greatly increase from 313Hv to 568 Hv. Such a huge work hardening has never seen in any ferritic steel. Besides, an abnormal hardening evolution with shear strain was observed. Although the detailed mechanism is not realized, it might be explained by the interaction between dislocation and carbide. In conclusion, the ferritic steels with dense and tiny carbides revels extremely different behaviors when they are subjected to both deformation-and-annealing and server deformation. These differences result from the micro-mechanisms between carbide with dislocation or grain boundary. Hence, playing with defects and dynamic responses, such as deformation and recrystallization, enables materials novel properties with microstructural complexity.