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Title: | 奈米析出強化肥粒鐵相比例對三相鋼顯微組織開發與其機械性質之研究 The effect of ferrite volume percent with nano-sized interphase precipatation in three phases TRIP steels on the morphology and its mechanical property |
Authors: | Che-Lun Lin 林哲綸 |
Advisor: | 楊哲人(Jer-Ren Yang) |
Keyword: | 介面析出碳化物,相變誘發塑性鋼,殘留沃斯田鐵,多相低碳鋼,變韌鐵,相轉變分率, interphase precipitation,transformation-induced plasticity,retained austenite,multi-phase low carbon steels,bainite,phase transformation., |
Publication Year : | 2018 |
Degree: | 碩士 |
Abstract: | 因應全球環保意識的興起,在各行各業中都要求對於環境的永續發展以及循環再利用,汽車用鋼在近十幾年來致力於提升高強度以及延性,一方面提升乘客的行車安全又可以達到輕量化的目的。
多相鋼是比雙相鋼更為複雜的鋼種,結構較為複雜且比起雙相鋼擁有更好的加工硬化率和更良好的韌性,因此常被用在汽車工業上。在製程上多相鋼多了一道等溫變韌鐵相變化的持溫,進一步使的沃斯田鐵介穩殘留至室溫,若能夠良好的控制沃斯田鐵的穩定性,則能夠在均勻變形區間內有效的相變成麻田散鐵提高加工硬化率,使的整體延性上升。 本研究中首先結合奈米析出碳化物成長在較弱的肥粒鐵基底,使其整體硬度提升至250Hv,大幅提升其機械性質並提升其擴孔性,並藉由熱處理來探討殘留沃斯田鐵含量隨肥粒鐵相分率之變化,並釐清何種條件能獲得最佳的TRIP效應,根據先前的文獻報導中指出目前沒有任何團隊能夠確切的將良好的機械性質和相分率做一個解釋,本研究中藉由拉伸試驗在肥粒鐵相分率65%時可以使整體的延伸率達到30%,大幅提升材料的韌性,然而若要考率強塑積則是在肥粒鐵相分率55%時擁有最好之強塑積。 藉由XRD和EBSD測定沃斯田鐵含量趨勢可以發現有隨肥粒鐵比例上升而有一最好的區間,搭配拉伸試驗測試可以發現擁有較高含量的沃斯田鐵區間恰好和良好機械性質的區間相符合,因此可以使用XRD測定沃斯田鐵含量來預測材料的機械性質。TEM中將沃斯田鐵的形貌依據肥粒鐵相分率和變韌鐵成長動力學的不同分為三類,第一種為低Vα下殘留在變韌鐵單元間的薄膜狀沃斯田鐵,此種沃斯田鐵過於穩定以至於在變形後仍然以變形雙晶的形式存在,第二種則是厚膜狀沃斯田鐵,相較於薄膜狀沃斯田鐵能在 V40α-V70α間貢獻最大的TRIP效應,大幅提升加工硬化率,第三種為塊狀沃斯田鐵,足夠穩定者能在V70α後貢獻TRIP效應,不穩定者則在冷卻至室溫的過程中形成M/A,劣化整體延伸率。 As the progress of the global eco-conscious, all works of life demand the sustainable development and circular economy to the environment. In the past ten years, the automobile steels have been developed to increase the strength and elongation. On top of that, the safety of the driving can be enhanced and the weight of car can be reduced. The structures of multi-phase steels (MP) are more complex than the dual-phase (DP) steels. The work-hardening rate and the toughness of the multiphase steels are better than those of DP steels. Therefore, DP steels are wildly replaced by MP steels in the automobile industry. The differences of heat treatment between DP steels and MP steels are that one more bainitic isothermal transformation process of MP steels have been used to further stabilize the meta-stable austenite to room temperature. Therefore, the work-hardening rate and uniform elongation can be improved by precisely controlling the stability of austenite. In this research, the nano-sized precipitated carbides are used to strengthen the soft ferrite matrix and make the overall hardness increase to 250 Hv, remarkably improving the mechanical property and the hole expansion ratio. Through the heat treatment designs, the best condition of TRIP effect during deformation can be investigated by controlling the volume fraction of ferrite and austenite. Based on the previous research, the relationship between the mechanical property and the volume fraction of ferrite cannot be well understood. In this research, the results indicated that the best elongation up to 30% occurred at the 65 percent of ferrite. However, the 55 percent of ferrite is the best choice for the production of strength and elongation. By XRD and EBSD analysis, austenite volume fraction increases with the increasing ferrite fraction and a maximum value occurs in the certain ferrite fraction region. This region, which is between 40 and 70 percent of ferrite, is consistent with the better mechanical property region which is tested by tensile test; and hence the mechanical property can be predicted before testing. By using TEM , austenite characteristic can be divided into three types depending on the difference of Vα and bainite growth kinetics. The first one is film austenite which is located between the bainite sub-unit in low Vα .The deformation mechanism of thin film austenite is twin-induced plasticity . Therefore, thin film austenite cannot contribute the TRIP effect. The second type is thick film austenite, which can highly enhance the work-hardening rate for Vα between 40% and 70%. The third type is blocky austenite. Depending on its stability, the TRIP effect during deformation can be contributed from retained austenite at room temperature; however, for M/A formed during quenching to room temperature, it will deteriorate the overall elongation and toughness. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/69595 |
DOI: | 10.6342/NTU201801091 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 材料科學與工程學系 |
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