含釩鋼界面析出之顯微結構及模型之研究

Abstract

本論文針對含釩中碳鋼的肥粒鐵基地中的兩種碳化物: 界面析出碳化物和纖維狀碳化物進行探討。論文內容可以分為三部分:顯微結構、理論模型、以及機械性質。 在論文第一部分，我們主要利用電子顯微鏡 (光學顯微鏡、穿透式電子顯微鏡、掃描式電子顯微鏡)去了解不同碳化物的形貌差異以及顯微結構的表徵，並就觀察結果討論碳化物在進行形貌轉換時的可能機制。其結果指出形貌轉換時機取決於相變態過程中的界面移動速度，沃斯田鐵相變態形成肥粒鐵初期，肥粒鐵加上界面析出物為首要共析反應，在相變態進行過程，由於驅動力下降導致介面移動速度變慢，纖維狀析出物即開始形成。 由於僅藉由電子顯微鏡的觀察並無法去了解並預測界面析出物和相變態溫度間的相依性，因此我們進而發展了一套新的理論模型來描述界面析出之現象，而在論文第一部份藉由電子顯微鏡的觀察結果和量測數據，則可用來確認我們所發展的模型的可行性。理論模型的發展主要是和法國 Yves Brechet 教授和 Mohamed Goune 兩位教授一起合作之結果，所發展的模型稱為 “superledge”模型。有別於先前已發表的模型，我們所發展的模型同時考慮了兩個界面析出的特性: (1) 沃斯田鐵至肥粒鐵的相變態由ledge 機制完成; (2) 析出過程伴隨相變態進行。在給定材料組成及相變態溫度下，superledge模型可以預測三點特性: (1) 界面析出物的層間間距 (sheet spacing); (2) 界面析出物的層內間距 (particle spacing)，以及 (3) 相界移動速度 (interface velocity)。其計算結果和已發表的參考文獻所量測的數據比較下，我們所發展的superledge模型呈現良好的預測能力。 最後，奈米硬度壓痕技術被用以檢測界面析出物對肥粒鐵的強化效果。有別於傳統的拉伸試驗所得到的巨觀機械性質，藉由奈米硬度機微小壓痕的優點，單一肥粒鐵的機械性質可以被獨立分析，且析出強化值的大小可以被明確地定量。我們藉由改變相變態的溫度來取得界面析出物於肥粒鐵基地內的分布情況，並依其奈米硬度所量測的結果來討論碳化物大小、層間間距、以及層內間距對Orowan 強化機制的影響。其所量測結果和理論值比較下有良好的一致性，其結果指出奈米硬度壓痕對析出強化機制的研究相當有幫助。 總體而言，於本研究論文所呈現的結果有助於提供中碳合金鋼中的析出物(界面析出、纖維狀析出)之顯微特徵以及發展機制。並發展出一新的模型去探討相變態溫度和合金組成對碳化物發展的影響，最後再利用新的奈米硬度壓痕技術去了解析出物對肥粒鐵基地內的強化效果。本論文的研究結果，已發表至以下之國際期刊: Scripta Materialia (已發表) Acta Materialia (已投稿) Advanced Materials Research (已發表) ISIJ International (已投稿) 關鍵字: 超高強度鋼、碳化物、界面析出、模型、肥粒鐵、相變態、奈米硬度壓痕

The present thesis aims to examine the features of interphase precipitation and carbide fibers in a medium-carbon vanadium alloyed steel. It covers microstructure, modeling, and mechanical properties aspects of interphase precipitation and carbide fiber. The different features of interphase precipitation and carbide fiber are initially examined by TEM. The transition of the interphase-precipitated carbides to the fibrous carbides is discussed. It shows that the fibrous carbide would form at the later stage of the austenite-to-ferrite transformation, indicating that the occurrence of the fibrous carbide depends on the interface velocity. The conditions for the developments of the interphase precipitation and fibrous carbide are then clarified. The obtained TEM results are then analyze to validate to a new model to elucidate the carbide precipitation with the growing ferrite phase. This new model is based on superledge mechanism of austenite-to-ferrite transformation. It deals with the ferrite and carbide nucleation rates and the driving force for austenite-to-ferrite transformation at the same time. The model gives good results at low transformation temperatures (< 700 oC) and reveals the evolutions of characteristic features of interphase precipitation with the progressive of austenite-to-ferrite transformation. The proposed superledge model is then extended to develop a model for carbide fiber growth and to understand the effect of carbon and solute contents on interphase precipitation in alloyed steels. . Finally, local mechanical properties were characterized by the use of nanoindentation after the samples were isothermally transformed at different temperatures. It gives the elasto-plastic mechanical behaviors of the ferrite strengthened by interphase-precipitated carbides. The characteristic features of interphase-precipitated carbide predicted by the model are used to be incorporated with tested mechanical properties. For the steel strengthened by the interphase-precipitated carbides, the contributed yield strength by these carbides is measured and the link between precipitation state and resulting mechanical properties is discussed. Finally, from a theoretical analysis, we show that precipitation state topology plays a key role in mechanical properties. The results presented in this thesis are expected to provide some original information related to the interphase precipitation in medium carbon alloyed steels. The following research results have been published in the international journals: Advanced materials research (EI), published Scripta Materialia (SCI), published Institute of Iron and Steel of Japan (ISIJ), submitted Acta Materialia (SCI), submitted Keywords: ultra-high strength steels, carbide, interphase precipitation, modeling, superledge, ferrite, transformation, nano-indentation