高強度含銅雙相鋼銅顆粒及奈米碳化物雙析出行為之穿透式電子顯微鏡分析研究

TEM Investigation on the Dual Precipitation Behavior of Nanometer-sized Carbides and Copper Precipitates in High Strength Copper-Bearing Dual-Phase Steels

李承翰(Cheng-Han Li)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201601140

摘要

近年來為了降低溫室氣體二氧化碳的排放，在同時改善次世代汽車用鋼的能耗需求，以及考量良好的穩定性及安全性的條件下，輕量化先進高強度鋼的開發在汽車工業上扮演極為重要角色。而其中雙相鋼由於良好的強度和成形性，早在二十世紀末就已經商業化的大量生產。因此，現今雙相鋼已大量用於汽車工業上。除此之外，近年來在環保的意識抬頭下，銅元素在鋼鐵中的影響逐漸受到重視。在鋼鐵的生產過程中，廢鋼的添加是現今鋼材重要的一環，由於銅元素為廢鋼中主要的微合金元素之一，因此，在經濟及環保的考量下，高強度鋼含銅鋼的開發是極具有吸引力的。
本研究引入界面析出合金碳化物以及奈米銅顆粒析出的概念，來同時強化雙相鋼中肥粒鐵與麻田散鐵，並藉此降低兩相之間的強度差距以提升雙相鋼的成形性。並且利用穿透式電子顯微鏡來分析碳化物及奈米銅顆粒在低碳雙相鋼中雙析出的現象以及分析高溫時效處理對其硬度的影響。
本研究除了對於雙相鋼硬度的提升外，也利用掃描穿隧電子顯微鏡的環形暗場技術進行顯微結構的觀察，並搭配能譜分析儀(EDS)分析合金元素的分布，進而探討在雙相中二次強化的機制。此外，雙相鋼整體的機械性能也透過拉伸試驗來分析時效前後的性質變化。特別的是，除了雙相鋼的降伏強度及抗拉強度有大幅的提升外，延性也能透過時效處理的方式而增加。然而，由於兩階段的淬回火熱處理製程會大幅提升生產成本，因此若能透過在盤捲製程中鋼帶中的殘留溫度來達成如同時效處理的強化效果，便能使含銅雙相鋼的生產更節能且更有效率。因此本研究亦利用一系列的模擬盤捲製程來比較其與時效處理的差異。
此外，本研究發現，奈米界面析出碳化物能作為奈米銅顆粒析出的有效成核位置，因此能使最佳的時效時間提前。本研究嘗試透過發展新的材料設計以達到優異的機械性能，並期許能引起工業應用及科學研究的興趣。

關鍵字

雙相鋼 ； 含銅鋼 ； 界面析出物 ； 雙析出 ； 時效硬化 ； 穿透式電子顯微鏡

並列摘要

For the sake of reducing the CO2 emission and improving for the efficiency of fuel consumption in the next generation of vehicles, light-weight design for advanced high-strength steels (AHSSs) as well as requirements for durability and safety play an important role in automotive industries in recent years. Dual-phase (DP) steels as one type of AHSSs commercialized by the end of 20th demonstrate special mechanical properties with combination of good strength and formability. Therefore, DP steels have been widely implemented in automotive industries. In addition, with increasing environmental consideration, copper has called lots of attentions as a part of residual elements contained in recycling steel scraps for producing considerable amounts of commercial steels. Therefore, the development of Cu bearing high-strength steels with better mechanical properties is very attractive from both the economic and environmental points of view.
In the thesis, the concept of interphase precipitation of alloyed carbide with copper precipitation is introduced in ferrite and martensite to reduce the strength mismatch and improve the strength and formability in DP steels. The objective of the present work is to clarify the effect of dual precipitation of interphase precipitation and nanometer-sized copper particles in a low carbon Cu-Ti-bearing dual-phase steel and to investigate hardness evolution with the effect of aging treatment at elevated temperature.
Besides the enhancement in hardness of both phases, the dual precipitation effects of interphase-precipitated carbide and copper particle with microstructure evolution are identified with the aid of TEM. With imaging techniques of STEM ADF with EDS mapping methods, the mechanisms for secondary hardening in both phases are elucidated. Furthermore, tensile test is carried out to investigate the overall mechanical properties of the DP steels with or without aging treatment. Surprisingly, not only the yield strength and the tensile strength are enhanced, the ductility is also improved by the treatment of aging. However, owing to the two-step heat treatment of quench-tempering resulting in higher cost of production, it would be more effective and energy-saving if the aging treatment has been finished during the coiling process with the residual temperature of the coil. As a result, a series of simulated coiling process is conducted to compare with the results of aging treatment.
From the present work, the nano-sized interphase-precipitated carbides have been found to serve as effective nucleation site for copper particles and thus shift the peak aging ahead. This research attempts to design a new material with superior mechanical properties with great interest both for industrial application and scientific research.

並列關鍵字

Dual-phase steel ； Copper-bearing Steel ； Interphase precipitation ； Dual precipitation ； Aging hardening ； Transmission electron microscope

參考文獻

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