臺灣博碩士論文加值系統

摘要
隨著能源議題日益受到重視，汽車之結構件亦趨複雜。汽車產業致力發展之材料必須不只擁有輕且強的優良性質，在加工方面還要能夠兼具優良之成形性及焊接性等；於成本考量下不僅要便宜，也要方便透過熱處理等方式控制材料顯微結構及機械性能以因應不同部件之不同強度、延展性等需求。雙相鋼於1970年代初入產業，其發展從未停滯，至今，透過工程師及科學家的努力，其性能仍然不斷提升。雙相鋼雖具有優良之機械性能與高加工硬化率，但因兩相間之強度差異使得應變分布不均勻，使之在承受應力時，容易在相界處產生裂孔。倘若能強化肥粒鐵減少兩相強度差異，便得以提高降伏強度、擴孔性與非均勻變形區段之延展性。透過添加鈦(Ti)鈮(Nb)釩(V)等利於形成碳化物之合金元素，以形成奈米析出物來強化肥粒鐵減少兩相間強度差異乃雙相鋼發展之一大趨勢。
在熱軋鋼板之產線上，熱機處理(TMCP)可謂最重要之製程，其控制軋延的目的不單單只為了減薄材料，透過良好的設計還能夠同時達到晶粒細化提高材料性質之優點，但在控制軋延的過程中包含了許多甚為複雜的物理及冶金反應將影響材料的微結構發展與完軋後之性能。
本研究的核心概念是透過添加鈦(Ti)來形成奈米析出物強化肥粒鐵，並透過模擬熱軋之參數過程以及高矽低矽材料來了解最終形成雙相組織之肥粒鐵強度暨顯微結構之關係
實驗第一部分為探討沃斯田鐵化溫度之影響，沃斯田鐵化溫度對於鈦形成奈米析出物之行為有決定性的影響。降低沃斯田鐵化溫度雖然會形成較小的沃斯田鐵母相晶粒，最終可以形成小至2-4 um大小的肥粒鐵晶粒，但會使得肥粒鐵之強度較弱，有兩個原因會造成此結果，首先，較低沃斯田鐵化溫度會容易在沃斯田鐵化過程中生成較為粗大化的析出物；其次，低沃斯田鐵化溫度無法將原材之鈦碳化析出物重新固溶，這樣的情況會使得原本預計在兩相區持溫析出的較小析出物受到抑制，藉由TEM及SEM的觀察能夠解析出肥粒鐵內不同沃斯田鐵化溫度下之奈米析出物的大小及分布，此外在高沃斯田鐵化溫度的條件下才有界面析出物的發現。
實驗第二部分為探討不同熱軋溫度對於析出物之影響，在實際的產線上並不會只在某一特定的溫度軋延，但是對於奈米析出物強化雙相鋼來說，這是重要的，在高溫軋延的過程中會產生較大型的高溫析出物，此析出物的成長與在兩相區溫度中所形成的界面析出物或是過飽和析出物之間為競爭關係，簡而言之高溫鈦碳化物在熱軋階段的析出行為對於鋼材最終性質具有決定性的影響。必須透過微硬度測試以及TEM的觀察來分析析出物在不同軋延條件下之情形，研究發現軋延過程不宜在高溫停留過久而且熱軋溫度應較高，否則肥粒鐵之微硬度會下降。
實驗第三部分為矽含量的差異，矽能夠提高Ar3、固溶強化、排碳加速肥粒鐵相變態等有相當多元的效果，再混合熱機製程與雙相鋼之熱處理，有許多有趣的現象出現。提高Ar3會使得我們將材料加熱至沃斯田鐵化溫度後晶粒較小，所以高矽材料在生成肥粒鐵後仍擁有晶粒細化的效果；固溶強化方面，根據過往經驗及文獻，1wt%的矽能為300HV左右的肥粒鐵帶來30HV左右的強度提升；而作為與碳互相排斥的元素，它能夠加速肥粒鐵的相變態，也會促進碳化物之析出，在本研究中透過微硬度測試以及兩階段熱壓之軟化綠實驗針對矽的影響做綜合性的討論，實驗發現高矽導致了鈦碳化物在軋延階段的粗大化，而不利於奈米析出物的強化作用。

ABSTRACT
As the issues of energy become more important and the structural members in automobile become more complicated. The materials developed in the automobile industry have to be not only light and strong but also need great formability and weldability. Based on cost consideration and the convenience of controlling the microstructure and mechanical behavior of materials by heat treatment to adapt to the different strength, ductility demands in different part in automobile. The development of DP steels was never stopped since it was showed up in industrial application in 1970s. The performance and properties have been improved continually by the effort of scientists and engineers until now. Although dual phase steel has great mechanical properties and high work hardenability, there is a large strength difference between two phases which makes strain not uniformly distributed. This problem results in the crack formed at the interface of two phases when the steel suffered stress. Strengthening ferrite can reduce the strength difference between two phases and enhance yield strength, hole expansion ratio and the elongation of non-uniform plasticity deformation region. One of the trend in the DP development is that adding Nb,Ti,V to form nano-carbides to enhance ferrite strength and reduce the strength difference between two phases.
Thermo-Mechanical control process (TMCP) is the most important process in the production line of hot-rolled strip. The purpose of well-designed controlled rolling process is not only for the thickness reason but also can refine the grain size and control microstructure to reach a better mechanical properties of materials. However, there is still a lot of complicated physical and metallurgical phenomenon to influence the properties and performance of materials after TMCP.
Adding titanium to form nano-precipitates to strengthen ferrite is the core concept in this research. In addition, the parameters of s controlled rolling simulation and the silicon content effect on the microstructure and the behavior of nano-precipitates in ferrite will be discuss in this research.
In the first section, the effect of austenitizing temperature will be discussed. Austenitizing temperature has a decisive influence on the behavior of precipitates of titanium carbides. Although lower austenitizing temperature can make smaller prior austenite grain and formed smaller ferrite grain about 2-4μm, it caused softer ferrite matrix. There are two possible reasons, first, coarser precipitates formed in the austenitizing process when austenitizing temperature is low; and second, the titanium carbide which existed in the material as received didn’t dissolve in the austenite matrix. These reasons inhibited the precipitation behavior of titanium carbides which is formed in the temperature of two phase region. Analyzing the size and distribution of carbides in ferrite by the observation of TEM and SEM. In addition, there is no interphase precipitation was observed in low austenitizing temperature condition.
In the second section, the effect of temperature and time of hot rolling on the precipitation behavior will be discussed. The temperature is not controlled at specific numeric value in the actual rolling process, but the rolling temperature is important to the nano-precipitates strengthened dual phase steel. Larger precipitates formed in the high temperature rolling process has competitive relationship with smaller precipitates which is formed in the temperature of two phase region. In brief, the precipitation behavior of titanium carbides in the high temperature rolling process has a decisive influence on the final property of steels. The behavior of nano-precipitates under different rolling condition was analyzed by the microhardness test and TEM observation. This research found that the time during rolling process should be short and the rolling temperature should be high, or the hardness of ferrite will be low.

In the third section, the effect of silicon content will be discussed. Silicon has a lot of interesting effect on the TMCP and heat treatment of dual phase steel, including raising Ar3, solid solution strengthening, accelerating the phase transformation of ferrite. Raising Ar3 makes austenite grain size smaller after materials be heated to austenitizing temperature. This effect also caused smaller grain size of ferrite. In the aspect of solid solution strengthening, 1wt% of silicon can offer 30HV hardness to ferrite based on past experience. As an element which is incompatible with carbon, silicon can accelerate the phase transformation of ferrite and promote the precipitation of carbide. Microhardness test and two stages hot deformation test which used to estimate softening ratio will be used to analyze the comprehensive influence of silicon in this research. It was found that high silicon caused the coarsening of Titanium carbide at the rolling stage which is unfavorable to the nano-precipitation strengthening.

Key words: Transmission Electron Microscopy(TEM), Nano-precipitation, Dual phase steel, Silicon, TMCP

目錄


口試委員審定書 
誌謝 
摘要 
Abstract 
目錄 
圖目錄 ix
表目錄
第1章 研究背景 1
第2章 文獻回顧 2
2-1雙相鋼 2
2-1-1 顯微組織與發展 4
2-1-2 微結構與機械性質 6
2-1-3 熱處理設計 9
2-1-4 微合金元素 11
2-2 材料強化機制 15
2-2-1 固溶強化 16
2-2-2 細晶強化 17
2-2-3 差排強化 18
2-2-4 第二相強化 18
2-2-5 析出強化 22
2-3 TMCP及其冶金反應 27
2-3-1 TMCP簡介 28
2-3-2 熱軋製程之冶金反應 30
2-4相變態反應 35
2-4-1 肥粒鐵相變態 36
2-4-2 TMCP與肥粒鐵相變態 37
2-4-3 界面析出 41
2-4-4 應變誘發析出(strain induced precipitation) 43
2-5 矽的綜合性影響 48
第3章 實驗設計與步驟 50
3-1 實驗材料及試片準備 50
3-2 實驗方法與流程 51
3-2-1熱處理設計 52
3-2-2 金相顯微組織觀察 55
3-2-3 掃描式電子顯微鏡觀察 56
3-2-4 微硬度測試 56
3-2-5穿透式電子顯微鏡觀察 57
第4章 實驗結果與討論 59
4-1沃斯田鐵化溫度對析出強化之影響 59
4-1-1 金相觀察 59
4-1-2 微硬度測試 63
4-1-3 掃描式電子顯微鏡分析結果 64
4-1-4 穿透式電子顯微鏡分析結果 67
4-2 熱軋製程溫度對析出強化之影響 78
4-2-1 金相觀察 78
4-2-2 微硬度測試 91
4-2-3 穿透式電子顯微鏡觀察 95
4-3 矽含量之綜合性影響 100
4-3-1 金相觀察 100
4-3-2 微硬度測試 112
4-3-3 兩階段熱壓(Two Stage Compression) 114
4-3-4 穿透式電子顯微鏡分析結果 120
第5章 結論 123
參考文獻 125

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