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標題: | 界面析出強化多相相變誘發塑性鋼內沃斯回火溫度對顯微組織與機械性質之影響 Influence of Austempering Temperature on Microstructure and Mechanical Properties in Interphase Precipitation Strengthened Multi-phase TRIP Steels |
作者: | Shih-Ning Tsai 蔡世寧 |
指導教授: | 楊哲人(Jer-Ren Yang) |
關鍵字: | 相變誘發塑性鋼,界面析出物,沃斯田鐵形貌,殘留沃斯田鐵含量,殘留沃斯田鐵碳含量,聚束電子繞射,殘留沃斯田鐵演變, transformation induced plasticity (TRIP) steels,interphase precipitation,austenite morphology,retained austenite content,carbon content in retained austenite,convergent beam electron diffraction (CBED),retained austenite evolution, |
出版年 : | 2017 |
學位: | 碩士 |
摘要: | 為了因應節能減碳,汽車工業欲改善汽車之燃油效率,開始在兼顧安全性的前提下,尋求減輕車體重量的方法。因此,具有良好機械性質的先進強度鋼大量被應用在汽車車體上。而相變誘發塑性鋼因為其相對低廉的製造成本以及優異的強度與延性之表現,在汽車工業上有良好的應用潛力。但傳統相變誘發塑性鋼之強度仍有待提升,且其中殘留沃斯田鐵之含量與穩定性需要被更加良好地控制,才得以貢獻相變誘發塑性效應提升延性。
在本論文中,將具有奈米尺度的界面析出碳化物導入多相相變誘發塑性鋼當中的肥粒鐵,以提升強度。藉由穿透式電子顯微鏡觀察,肥粒鐵中確實存在奈米界面析出碳化物,且能夠提升肥粒鐵相硬度至300Hv以上,而整體抗拉強度可達到880MPa等級。同時含有殘留沃斯田鐵以在變形時貢獻相變誘發塑性效應以提升延性,總延伸率最高可達26.8%。 本研究除了在沒有先前文獻報導過的情況下,首度成功結合了奈米界面析出碳化物及多相相變誘發塑性鋼。更在殘留沃斯田鐵的特性上有深入的探討。藉由碳原子在熱處理過程中各種相變態的分配狀態之計算,發現沃斯回火溫度會顯著影響殘留沃斯田鐵含量與其中之碳含量。實驗中利用電子背向散射繞射、X光繞射、穿透式電子顯微鏡、聚束電子繞射等技術,證實沃斯回火溫度會影響變韌鐵之成長量,導致第二相島狀組織形貌不同,具有不同的殘留沃斯田鐵含量與其中碳含量。其中殘留沃斯田鐵主要分為三種形貌: Type I:分佈在變韌鐵次單元之間的薄膜狀沃斯田鐵;Type II:分佈在變韌鐵束狀組織之間的厚膜狀沃斯田鐵;Type III:位在島狀第二相中靠近肥粒鐵晶界的塊狀沃斯田鐵。沃斯田鐵尺寸:Type III > Type II > Type I;沃斯田鐵碳含量與穩定性:Type I > Type II > Type III。隨著沃斯回火溫度上升,變韌鐵成長量減少,尺寸較大的殘留沃斯田鐵比例逐漸上升,同時大塊麻田散鐵/沃斯田鐵混和相也隨之增加。 根據拉伸試驗,在390°C沃斯回火獲得最佳的延性,而不是擁有最多殘留沃斯田鐵的在420°C沃斯回火之試片。由於在390°C沃斯回火得到的殘留沃斯田鐵擁有較高的碳含量與穩定性,能夠在變形過程中緩慢逐漸相變態為麻田散鐵。在高應變量時仍維持高加工硬化率,有效貢獻相變誘發塑性效應,得以提升延性。在420°C沃斯回火得到的殘留沃斯田鐵雖然最多,但是碳含量低而穩定性差,在變形初期即大量地相變態為麻田散鐵。到變形後期時無法再維持高加工硬化率,並無有效貢獻相變誘發塑性效應,因此延性較差。 In order to reduce energy consumption, automobile manufacturers have made efforts in weight reduction of car bodies without losing safety and durability to improve fuel consumption efficiency. As a result, advanced high strength steels (AHSSs) have been introduced to automotive industries due to the good mechanical properties. Transformation Induced Plasticity (TRIP) steels have favorable application potential in automotive industries because of the relatively low production cost and impressive combination of strength and ductility. However, the strength of conventional TRIP steels is not outstanding enough. Also, the quantity and carbon content of retained austenite in TRIP steels need to be controlled more brilliantly to elevate ductility owing to TRIP effect. In the thesis, the concept of nanometer-sized interphase-precipitated carbides is introduced in ferrite phase in multi-phase TRIP steels to enhance the strength. Through TEM observation, nanometer-sized interphase-precipitated carbides indeed disperse densely in ferrite phase and elevate the hardness more than 300Hv. The tensile strength of steels sheets is up to 880MPa grade. In the meantime, the retained austenite in microstructure contribute TRIP effect during deformation process to improve the total elongation up to 26.8%. In this research, despite no previous related paper publication, the combination of nanometer-sized interphase-precipitated carbides and multi-phase TRIP steels is successfully achieved. Furthermore, the characteristic of retained austenite is investigated comprehensively. By calculation of carbon partition in each phase transformation during heat treatment process, it is found that the austempering temperature has significant influence on the quantity and carbon content of retained austenite. Utilizing EBSD, XRD, TEM and CBED, experimental results show that austempering temperature influence the amount of bainite formation, resulting in different microstructure morphology in second phase island with different quantity and carbon content of retained austenite. It is found that there are three different types of morphology of retained austenite: Type I: Thin film austenite between bainite sub-units;Type II: Large film austenite between bainite sheaves;Type III: Blocky austenite in second phase island adjacent to ferrite grain boundary. Austenite dimension: Type III > Type II > Type I. Carbon content and stability of austenite: Type I > Type II > Type III. The amount of bainite formation decrease as the austempering temperature increase. Therefore, the fraction of large-sized retained austenite gradually increase. Also, the amount of large block M/A phase increase with increasing austempering temperature. By tensile test, the specimen austempered at 390°C has the best ductility, instead of the specimen austempered at 420°C which has the highest retained austenite content. During deformation process, the austenite retained after austempering at 390°C gradually transform to martensite thanks to high carbon content and stability. The work hardening rate is kept high at large strain and contribute TRIP effect to improve elongation. Although the specimen austempered at 420°C has the highest retained austenite content, the carbon content and stability of retained austenite is low. Consequently, most retained austenite rapidly transform to martensite at early stage of deformation process. At large strain, the work hardening rate is unable to be kept high and no more TRIP effect can be generated, leading to lower elongation. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/77975 |
DOI: | 10.6342/NTU201701585 |
全文授權: | 有償授權 |
顯示於系所單位: | 材料科學與工程學系 |
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