變形對於含鈦析出物強化之高強度雙相鋼的影響

Effect of deformation on precipitation-strengthened Ti bearing high strength dual phase steels

陳致睿(Chih-Ruei Chen)

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/NTU201602840

摘要

為了減少二氧化碳排放，各大廠商正著手研發先進輕量化高效率汽車。輕盈的汽車可以增進效能，所以目前對於製造高強度、輕盈且價格合理的材料有很大的需求。雙相鋼由於優良的成形性和低製造成本，被視為適用於新一代汽車的主要材料。
鈦是鋼鐵中常用的一種合金元素，主要目的在於產生碳化鈦提升鋼的強度；此外鈦的析出物在沃斯田鐵狀態時能限制晶界的移動，達成晶粒細化的結果。在相變態時析出的碳化鈦可藉由介面析出機制來析出，強化肥粒鐵相，而實驗的第一部份便是了解碳化鈦藉由介面析出機制強化雙相鋼中的肥粒鐵基地，以及藉由TTT圖探討鈦元素對肥粒鐵相變態動力學的影響。
實驗的第二部分主要便是討論不同的變形量以及持溫溫度對於雙相鋼中肥粒鐵形貌和機械性質所對應的析出物形式。透過在沃斯田鐵非在結晶溫區進行熱軋可形成煎餅狀的沃斯田鐵晶粒，提供更多缺陷成為肥粒鐵的成核位置，造成肥粒鐵晶粒細化。實驗結果顯示20%的熱軋量會使肥粒鐵中的介面析出物間距變小，硬度稍微上升；而50%的熱軋量會產生bimodal的大小晶粒，且造成硬度下降。50%熱軋條件下的小晶粒是在相邊界移動較快速的狀況下產生，因此析出物的種類為過飽和析出的形式，而非介面析出的形式。維氏硬度測量的結果顯示不同大小分佈的肥粒鐵具有不同的硬度值。
藉由TEM的觀察，界面析出和過飽和析出之兩種形貌析出物在肥粒鐵的[100]正晶帶軸下，分別有一組和三組的B-N方位關係。此外，在軋延的過程中，往往會伴隨著應變誘發析出物的產生，對後續肥粒鐵相的析出有所影響。由SEM的BSE影像統計100nm~1um 的大顆析出物發現在不同熱軋程度下直接淬火的試片中，析出物所佔的面積百分率幾乎相同，因此認為熱軋對於析出物的影響可能在更小的尺度。此外，由HRTEM的觀測，發現在5分鐘的短時間持溫下，介面析出物和過飽和析出物的尺寸都不會超過3nm，因此3nm可以作為一個劃分析出物是在肥粒鐵或是沃斯田鐵相時所析出的依據。通過對析出物尺寸的統計，可以發現在50%熱軋下，尺寸介於3~50nm的析出物數量大幅增加。接著藉由選區繞射發現有尺寸介於3~50nm區間且不屬B-N方位關係的析出物，證明在軋延過程產生的析出物，確實有落在3~50nm的統計區間。因此50%熱軋的硬度下降可以解釋為熱軋時應變誘發析出物的影響。

關鍵字

雙相鋼 ； 熱軋 ； 碳化鈦 ； 界面析出

並列摘要

To reduce fuel consumption, CO2 emissions and ensure passengers’ safety, there is an attempt to develop advanced high strength, lightweight, and affordable steels used for automobiles.
Titanium is one of the most commonly used alloy addition in steels. The major effect of titanium is to form titanium carbide in steels. Titanium carbides precipitation formed in austenite state can hinder austenite grain boundary movement and thus refines prior austenite grain size. On the other hand, precipitation occurring during phase transformation was found to strengthen ferrite matrix by interphase precipitation mechanism, which will be discussed in part I. Also, the role of Ti element on transformation kinetics during austenite decomposition will be studied using time-temperature-transformation (TTT) diagram.
The second part of the experiment discusses the effect of deformation on ferrite phase in titanium strengthened dual phase steel. Pancaked austenite can be obtained by hot deformation in non-crystalizing region, providing much more nucleation site for ferrite in two phase region, and result in grain refinement. 20% hot deformation can slightly increase ferrite hardness due to smaller sheet spacing of interphase precipitation, while 50% heavy hot deformation generating bimodal distributed ferrite grain with lower hardness. In the 50% deformation condition, when austenite/ferrite interface moves rapidly, which usually happens in small ferrite grains, interphase precipitation could not be able to happen. Instead, randomly distributed titanium carbides formed in a form of supersaturated carbide. Vickers hardness test shows that different morphology of ferrite would result in different hardness. By TEM observation, supersaturated random precipitation and interphase precipitation show different pairs of B-N OR under [100] zone of ferrite, 3 and 1 for each. Furthermore, strain induced precipitation occurs during hot deformation, consuming alloy element (titanium or carbon) for later precipitation. Statistic taken in SEM BSE images shows that precipitates containing titanium ranged from 100nm to 1µm don't show much difference in area fraction between each hot deformation condition. By HRTEM observation, size of interphase precipitated and randomly precipitated titanium carbides are smaller than 3nm, so 3nm is a dividing line for statistic to differentiate the phase state in which titanium carbides precipitate. By statistic, size of precipitates ranged from 3~50nm drastically increase under 50% hot deformation. Using selected area diffraction, precipitates smaller than 50nm and larger than 3nm is observed without B-N orientation, which can match the statistic result. So lower Vickers hardness in 50% deformed samples can somewhat attribute to strain induced precipitates formed during hot deformation.

並列關鍵字

dual phase steel ； hot deformation ； TiC ； interphase precipitation

參考文獻

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