鈮複合碳化物對低碳變韌鐵與雙晶誘發塑性變形高錳鋼之影響的研究

Investigation on the Synergistic Effect of Niobium Composite Carbide in Low Carbon Bainitic Steels and High Manganese Twinning-Induced Plasticity Steels

陳昱文(Yu-Wen 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/博士(2018年)

https://doi.org/10.6342/NTU201800905

摘要

低碳鋼板因為其優良的抗拉強度、成型性及焊接性能至今已被廣泛地使用在管線用鋼、汽車工業和海底用鋼中，在西元兩千年，由於對鋼板性能的要求日漸提高，微合金元素開始被添加至低碳鋼中藉以進一步提其性能，在本研究中，我們利用添加至少兩種以上之易析出碳化物元素及適當的熱處理來控制顯微組織，進一步地提升低碳鋼板和雙晶誘發塑性變形鋼板的性能。本研究的目的在於釐清鈮與其他易析出碳化物元素在體心立方晶體和面心立方晶體之間的共同效應，透過光學顯微鏡、掃描式電子顯微鏡及穿透式電子顯微鏡進行顯微結構的觀察，並搭配機械性質的測試藉以深入了解低碳鋼板以及雙晶誘發塑性變形鋼板之組織與機械性質之間的關聯。
在第四章中，吾人開發了一種高效率以及高正確率的方法來對於複相鈮鉬低碳鋼板進行相比例的鑑定以及定量分析，而此種鋼中含有粒狀變韌鐵、少量的肥粒鐵及退化波來鐵。在我們先前的工作中，我們團隊利用電子背向散射繞射儀來偵測粒狀變韌鐵內束狀組織的次結構，由於變韌鐵次結構間有著微小角度差異而肥粒鐵內並無此種結構，故利用此關鍵的差異來分辨在光學顯微鏡下幾乎沒有差異的粒狀變韌鐵與肥粒鐵便可以準確地將此兩者區分，然而此種方法必須人工的將每一顆晶粒進行微小角度的分析，故其缺點為會耗費大量的時間，此研究利用電子背向散射繞射儀以及所附加之核內平均取向差(KAM)功能來進行微結構表徵分析藉以克服曠日費時的相比例鑑定，此研究探討了不同的步長(step size)以及不同的核尺寸(kernel size)對於相比例定量結果的影響，發現粒狀變韌鐵的定量相比例隨著核尺寸的的增加而有被過度高估的情形，而當核尺寸與粒狀變韌鐵中的次結構大小(500 nm)相近時，其定量的結果最為準確，故利用核內平均取向差的方法不僅可以有效的辨別粒狀變韌鐵及肥粒鐵，使用此方法還能有效地節省許多時間。
第五章所探討的主題為高溫回火對於含鈮及含鈮鉬低碳變韌鐵鋼板二次硬化的影響，添加0.3 wt.%的鉬元素後，當回火溫度控制在700 ºC且持溫10分鐘時，其硬度顯著地提升了40 Hv，相反地，只有含鈮的低碳變韌鐵鋼板其硬度僅僅上升了15 Hv，在穿透式電子顯微鏡對於微觀組織的觀察下，我們發現添加0.3 wt.%的鉬元素後，其變韌肥粒鐵內的差排組織不僅較僅含鈮的低碳變韌鐵鋼板更為穩定且其差排密度亦較高，高解析穿透式顯微鏡提供了強烈的證據指出當添加鉬元素到低碳變韌鐵鋼板後，在高溫回火狀態下差排的回復被有效地抑制並且鉬元素進入鈮的碳化物後可以有效地防止鈮鉬複合奈米析出物的粗化，進一步地達到防止低碳變韌鐵鋼板過時效後的軟化效果。
第六章則是在探討在高溫回火下鈮與其它易析出碳化物元素在雙晶誘發塑性變形鋼板中的效用，雙晶誘發塑性變形鋼板經過高溫時效後其降伏強度以及最大抗拉強度分別顯著地提升了46 MPa以及146 MPa，但是其增加量卻有著明顯的不同，本研究利用穿透式電子顯微鏡來進行雙晶誘發塑性變形鋼板時效前以及時效後其差排組織的變化，發現時效前沃斯田鐵呈現了輥軋後沃斯田鐵結構且其內部分佈著均勻且密度極高的差排，而時效過後的差排組織不僅沒有明顯的回復現象且還自己重新排列並形成了條狀結構且碳化物的分布密度在時效後明顯地升高，除此之外，破斷面的分析顯示兩者在不同熱處理下皆為延性破壞，除此之外，利用穿透式電子顯微鏡進行變形過後的顯微結構分析，發現在時效前的試片中其變形的機制主要由單一變體的機械雙晶所主導，另一方面，大量多重變體的機械雙晶在時效後的沃斯田鐵中形成並且在兩個不同變體的機械雙晶交會處形成了α’麻田散鐵，其原因為在沃斯田鐵變形時，大量的奈米析出物阻擋了機械雙晶的形成造成了單一變體的機械雙晶提早達到飽和進而造成更多二次機械雙晶的啟動並且在兩個機械雙晶的交會處形成α’麻田散鐵，此現象進一步的增加了加工硬化率使得最大抗拉強度在最後階段獲得了顯著的提升。

關鍵字

奈米析出物 ； 變韌鐵 ； （掃瞄）穿透式電子顯微鏡 ； 機械雙晶 ； 二次硬化

並列摘要

Low carbon steels have been wildly used in line pipes, the automotive industry, and submarine environments due to their remarkable mechanical properties, weldability and formability. In the 2000s, micro-alloying elements were added into low carbon steels to further enhance the performance and meet increasing demand. In the present study, the performance of low carbon steels and TWIP steels is enhanced by the addition at least two strong carbide forming elements, appropriate heat treatment and microstructure control. The effects of Nb accompanied with other strong carbide forming elements in BCC and FCC matrix are investigated through microstructural observations with optical microscopy, scanning electron microscopy, transmission electron microscopy, and mechanical property tests. The relationships between mechanical properties and microstructure are clearly illustrated in this study.
Chapter 4 presents an efficient and accurate method for microstructural quantification of complex phases in a low carbon Nb-Mo bearing steel. Optical micrographs showed that this steel consisted of granular bainite with a small amount of ferrite and degenerated pearlite. In our previous work, we proposed a method to measure the misorientation angles via electron backscatter diffraction (EBSD) so as to differentiate granular bainite and ferrite. That method is accurate for phase quantification but laborious for the characterization process. To resolve this difficulty, in this study, EBSD was used in combination with a kernel average misorientation (KAM) map for phase characterization. Comparisons were made among KAM maps with different kernel sizes (300 to 600 nm) and various step sizes (100 to 600 nm). It is found that a kernel size close to the sub-structure size of granular bainite (500 nm) is optimal for phase identification, while varied step sizes produce relatively invariant results. Therefore, KAM maps can be used for fast and reliable phase quantification, provided that an appropriate kernel size and a large step size are used.
Chapter 5 describes an investigation of the effects of tempering at three high temperatures (660, 680 and 700 ºC) for different holding times (5 min to 16 h) on the secondary hardening in Nb-bearing and Nb-Mo-bearing bainitic steel strips. It is found that with 0.3 wt.% Mo addition in Nb-Mo-bearing bainitic steel strip, the peak hardening occurs with a significant increment in hardness (up to 40 Hv) after approximately 10 min of tempering at all temperatures (660, 680 and 700 ºC). In contrast, for the Nb-bearing steel strip, the corresponding increment in hardness is small (about 15 Hv). Investigations of the microstructural evolutions of Nb-bearing and Nb-Mo-bearing steel strips during tempering at 700 ºC are also presented. It is found that during tempering, bainitic ferrite platelets in Nb-Mo-bearing steel strips remain much more stable and have a higher dislocation density than those in Nb-bearing steel strips. High-resolution transmission electron microscopy provides strong evidence that the Mo addition has significant effects on suppressing the annihilation rate of dislocations and retarding the coarsening rate of nanometer-size carbide particles at higher temperatures, leading to a remarkable resistance to softening after secondary hardening.
In chapter 6, the microstructure evolution and mechanical properties of twinning-induced plasticity (TWIP) steel subjected to high temperature aging are elucidated. The yield stress and ultimate tensile stress of 18Mn-0.4C TWIP steel respectively increase by 46 MPa and 146 MPa after 700 ºC aging. Microstructural characterizations conducted to clarify the increments in strength are also presented. The as-received TWIP steel contained dense and homogeneously distributed dislocations, which, after aging, rearranged themselves into a lath-like sub-structure without obvious annihilation. In addition, the number density of precipitates was higher in the aging than in the as-received TWIP steel sheet. Ductile fracture surfaces were found both in the aging and as-received TWIP steel, and the deformation structures were investigated using transmission electron microscopy. In the as-received TWIP steel, austenite grains were dominated by single-variant primary mechanical twins, and few secondary twins were found. On the other hand, in the aging TWIP steels, a high amount of multiple-variant nano-sized mechanical twins with some strain-induced α’ martensite in the intersections was found. It is appropriate to conclude that after aging, the presence of a high amount of precipitates retards the formation of deformation twins, causing premature saturation of single-variant mechanical twins. Therefore, more secondary mechanical twins are activated, and they in turn interact with the primary twins as strain increases, inducing α’ martensite formation in the intersection. This phenomenon increases the strain hardening rate at a late stage (εe = 0.41) and thus the ultimate tensile stress in the aging TWIP steel.

並列關鍵字

nanometer-sized carbide ； bainite ； (scanning) transmission electron microscopy ((S)TEM) ； secondary hardening ； mechanical twin

參考文獻

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