奈米界面析出物強化雙相鋼之顯微組織及機械性質研究

Abstract

界面析出物為一種成核於沃斯田鐵至肥粒鐵相變過程當中的細微析出物。自從2004年JFE製鋼將界面析出物導入強化肥粒鐵的研究以來，至今奈米級陣列型MC碳化物用於肥粒鐵單相強化的研究已成果豐碩。本研究目的在於利用此種奈米界面析出物強化傳統雙相鋼之肥粒鐵基地，並且透過熱處理、顯微組織觀察，以及機械性質測試得以深入了解界面析出強化雙相鋼之組織及性質間之關聯。 本研究首先單純探討界面析出物添加前後雙相組織以及機械性質的變化。在精準控制晶粒度以及雙相比例之下，吾人發現添加界面析出物後肥粒鐵的強化會伴隨著麻田散鐵的弱化。利用Gleeble熱處理所得試片的拉伸結果顯示雙相鋼強度大幅提升，然延性卻沒有減損，且降伏比提升。後者揭示著相對傳統雙相鋼而言較佳的擴孔性質。透過階段拉伸並且利用(掃描)穿透式電子顯微鏡觀測變形組織的結果呈現，界面析出物能有效抑制差排的蜂巢狀結構的產生，且阻止幾何必須差排層的展延，減少變形過程的應變集中。除此之外，試片傾轉對於差排以及界面析出物的顯像也在本研究被深入探討：若要觀察到界面析出物，析出物所在陣列平面必須要完全與電子束平行；在不同的雙束條件之下，差排的柏格斯向量則可以被決定。 針對開發界面析出雙相鋼當中的合金添加，熱膨脹儀結果顯現鉻及鋁雖同為肥粒鐵穩定元素，卻分別展現為抑制及促進肥粒鐵相變態之能力。肥粒鐵生成速度影響界面析出物之中的顆粒間距甚鉅：鉻貢獻較慢的肥粒鐵生成速度被證實有效減小顆粒間距，增加介面析出物貢獻之強度。 在相同的雙相比例之下，雙相區持溫溫度的降低使得肥粒鐵晶粒度減小、肥粒鐵強化並且伴隨著麻田散鐵的弱化。持溫溫度較低能造就優異的總體機械性質：較高的強度且優良的延伸率。鉬作為硬化能元素，對於抑制肥粒鐵成長以及促進析出這兩方面皆有顯著貢獻，在本研究中證實能提升雙相鋼整體強度。 此外，散亂型析出物以及陣列型界面析出物在本研究中透過選區繞射以及方位關係得以釐清。本研究中觀測到的散亂型析出有二：一為於沃斯田鐵階段形成的散亂型高溫析出物；另一為雙相區肥粒鐵生成時伴隨而生。前者與肥粒鐵基地可能存在有Kurdjumov-Sachs或是Nishiyama-Wessermann之方位關係，亦可能不存在任何方位關係；後者與肥粒鐵基地保持有Baker-Nutting (B-N)的方位關係。後者所提及的散亂型析出物文獻中均無進行方位關係判定，僅靠形貌敘述。本研究證實這類散亂型析出物其實只是沒有平行於入射電子束的陣列型界面析出物，因其與肥粒鐵基地均只保有一組B-N方位關係。 利用熱膨脹儀實施高溫變形對於先前沃斯田鐵及隨後生成的肥粒鐵均有顯著影響。變形溫度決定沃斯田鐵能否發生再結晶行為；變形程度則影響沃斯田鐵的細緻程度。降低變形溫度以及提升變形量均會促進肥粒鐵相變以及細化肥粒鐵晶粒。沃斯田鐵若存在鬆餅狀組織肥粒鐵較易不均勻成核，甚至會導致兩極化晶粒的產生。界面析出物於高溫變形後的所量測到的陣列間距皆有縮小，顆粒間距則無明顯改變，兩者加乘之下的結果將會增加界面析出物的強化貢獻。然而在高溫變形過程當中引發的散亂型析出物將會弱化界面析出物的貢獻，肥粒鐵界面移動快速也將導致界面析出物不易生成。本研究拿兩極化晶粒作為深入研究對象進行進一步探討，並且發現先前生成的細小晶粒內部並無界面析出的發生，導致硬度相較於粗大晶粒為低。此研究成果顯示傳統細晶強化的概念一旦考慮界面析出物則必須做調整。

Interphase precipitation forms during austenite-to-ferrite transformation. Since the development of interphase precipitation strengthened ferritic steels by JFE company in 2004, there have been enormous studies regarding the nanometer-sized MC-type carbide strengthened ferrite. The present study makes use of this kind of precipitation to strengthen conventional dual-phase steels. Through heat treatments, microstructural observations, and mechanical property testing, the relationship between microstructures and properties of interphase precipitation strengthened dual-phase steels can be built up. The present study in the first place simply investigated the addition of interphase precipitation to the dual-phase structures. Under the precise control over structural parameters such as grain size and volume fraction, interphase precipitation was found to strengthen ferrite but weaken martensite. Tensile specimens heat treated by Gleeble showed higher strength with unsacrificed elongation. Yield ratio could also be increased, which implies better hole expansion properties in the present steel. (Scanning) transmission electron microscopy ((S)TEM) was used to ex-situ observe the deformed structures in the dual-phase steel, and we found out that the formation of cell structures and the development of geometrically necessary dislocation (GNDs) can be effectively hindered by interphase precipitation. The strain localization and better stress-strain balance can be achieved by the simple addition of interphase precipitation. Furthermore, effect of the specimen tilting on both interphase precipitation and dislocation imaging was also comprehensively addressed: where for the observation of interphase precipitation, a rigorous edge-on condition is needed (i.e., sheet plane of carbides should be parallel to the incident beam); Burgers vector of dislocations can, on the other hand, be determined by using different two-beam conditions. The alloy addition of Cr or Al showed different characteristics of either retarding or accelerating ferrite transformation, which was thoroughly studied by a dilatometer. Different ferrite transformation rates gave rise to different morphologies of interphase precipitation. Addition of Cr was proved to bring about smaller intercarbide spacing than Al, which increased the strength contribution from interphase precipitation. Under the same volume fraction of martensite, decreasing the dual-phase holding temperatures reduced the ferrite grain size, increasing ferrite hardness, yet weakening martensite hardness. These factors made the overall mechanical properties excellent, which bore higher strength together with good elongation. Mo, as an element to raise hardenability, retarded ferrite transformation and contributes to denser precipitation, which gave rise to increasing the strength of dual-phase structures. Moreover, the present study clarified the difference between random precipitates and interphase precipitates by using selected area diffraction pattern (SADP) as a tool. Random precipitates can further be divided into two classes: one is random strain-induced precipitation formed in the austenite state; the other is formed in the two-phase holding period. The former holds either Kurdjumov-Sachs, Nishiyama-Wessermann, or none of the above-mentioned orientation relationship with the ferrite matrix, while the latter holds Baker-Nutting orientation relationship instead. The present work initiatively proved that the latter random precipitation holds only single variant Baker-Nutting orientation relationship with the ferrite matrix. Therefore, the seemingly “random” precipitation is regarded as interphase precipitation that is simply not in an edge-on condition by the present author. Moreover, through dilatometer, hot deformation had great impact on both austenite and the subsequent ferrite transformation. Deformation temperatures determined the ability for recrystallization of austenite; increasing deformation strain, on the other hand, refined austenite structures. It was shown that lower deformation temperature or increased deformation strain lead to faster ferrite transformation and finer ferrite grains. Pancaked austenite structures brought about inhomogeneous nucleation of ferrite grains, even making grain distribution bimodal. Hot deformation decreased the sheet spacings of interphase precipitation, but had nearly no influence on intercarbide spacing, which leads to higher strengthening contribution. However, if high-temperature strain-induced precipitates are also considered, the precipitation hardening will be reduced. High temperature precipitates together with very fast transformation movement are thought to obstruct interphase precipitation from forming. Bimodal grain distribution was taken as a case study in the present research, where earlier-formed tiny-grained ferrite was discovered to be devoid of interphase precipitation but distributed with strain-induced larger precipitation, resulting in a lower hardness than larger-grained counterpart. This result reveals that when interphase precipitation is considered, the concept of strengthening through grain refinement should be modified.