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標題: | 鉬對含鈮低碳變韌鐵系熱軋鋼板的微結構發展之影響 Effect of Mo addition on the development of microstructure in Nb Containing Low Carbon Bainitic Hot Rolled Steel Strips |
作者: | Bo-Ming Huang 黃柏銘 |
指導教授: | 楊哲人(Jer-Ren Yang) |
關鍵字: | 鉬合金,含鈮低碳變韌鐵系熱軋鋼板,背向散射電子繞射圖譜,高解析電子顯微鏡影像,二次硬化效應, Mo,Nb containing low carbon bainitic hot rolled strips,high resolution transmission electron microscopy,Electron backscattering diffraction,secondary hardening effect, |
出版年 : | 2014 |
學位: | 博士 |
摘要: | 鉬合金添加至含鈮的變韌鐵系耐火鋼板,可在變韌鐵基地內細化其奈米級碳化鈮與延遲差排結構的回復,進而提高其高溫強度。本研究將此概念應用至熱軋鋼板的製造上,以期望在模擬的高溫盤捲過程中,鋼板有良好的二次硬化效果。此外,鉬合金也對含界面析出物之低碳肥粒鐵系熱軋鋼板有強化效果。因此,探討鉬合金的添加對此兩種結構的汽車鋼板的影響,以決定何者結構適合做為低碳鈮鉬熱軋鋼板。
本研究鋼板基本成分為0.05C-1.7Mn-0.08Nb, wt% 初步加入0與 0.1wt%鉬合金至編號為Nb,Nb-Mo鋼板。在鋼板熱軋過程中,因鋼板內大量殘留應變量易引發肥粒鐵相變態反應,而導致最終組織為完全肥粒鐵。因此,本試驗將盤捲溫度分別降低450度與550度以達到加快冷卻速度的目的,希望產生變韌鐵組織。在微結構的觀察下,發現盤捲溫度為450度與0.1wt% Mo的添加可有效抑制肥粒鐵並產生粒狀變韌鐵在鋼板內。接著在模擬盤捲過程的回火處理(600度)中,擁有粒狀變韌鐵的鋼板產生明顯的二次硬化效果。將盤捲溫度升高至650度,使熱軋鋼板成為具有界面析出碳化鈮的完全肥粒鐵組織。將此鋼板與有二次硬化效果之Nb鋼板做比較,兩者鋼板在不添加鉬的條件下均有相同的強度。然而,添加0.1wt% Mo至此肥粒鐵系鋼板後,發現Mo合金非但不增強界面析出的強化效果,還使得在盤捲過程中不穩定的二次相快速增生。因此,鉬合金的添加對含鈮低碳變韌鐵系熱軋鋼板有較正面的效果。 為了提高Nb-Mo鋼板內的變韌鐵以及增強變韌鐵組織的二次硬化效果,將其鉬含量提高至0.3wt%並編號為Nb-3Mo鋼板。在Nb-3Mo鋼板微結構觀察中,其變韌鐵組織維持粒狀變韌鐵形貌,且其整體組織有因變韌鐵增加而被細化的現象。利用背向散射繞射電子圖譜技術(EBSD),可解析出粒狀變韌鐵具有較高的累積方位梯度(Misoreintation gradient)。利用此特性,分析出有與肥粒鐵相似外貌的粒狀變韌鐵組織,並且統計出三個鋼板內的粒狀變韌鐵含量。然而,在此量化數據中,卻發現僅Nb-3Mo鋼板有較多粒狀變韌鐵生成。在熱膨脹儀比對分析中,得知肥粒鐵相變態須被抑制到使未相變態之沃斯田鐵的體積夠多,使利於變韌鐵成核的晶界面積擴大,方能促使更多粒狀變韌鐵生成。 經過回火處理後,發現0.3wt% Mo竟難以加強鋼板內變韌鐵組織的二次硬化效果。為了釐清鉬在二次硬化的角色,利用電子能量損失能譜(EELS)、高解析電鏡影像(HR TEM)、能量散射光譜儀(EDX)以及三維原子探針技術(APT)儀器,觀察鉬原子對碳化物尺寸與差排結構的影響。在量測差排密度中,鉬原子對於差排的回復作用並無顯著的作用。推測在回火過程中,在無外加應力的條件下,在變韌鐵晶粒內之差排線本來就難以移動,導致鉬原子無法產生明顯的延遲作用,產生與耐火鋼不同的結果。在奈米碳化物的觀察中,鉬原子不會偏析在碳化鈮與基地的界面,卻是與鈮原子均勻地形成鈮鉬碳化物。隨著鉬的增加,鉬原子逐漸成為碳化物的形成元素,使得鈮鉬碳化物吸收過量的鉬原子而有些微粗化的現象。 以往耐火鋼的研究中,使用碳複製膜方式易將粗大的應變析出碳化鈮混入統計,造成鉬添加能細化碳化物的誤導。在回火過程中析出的碳化物與基地具有獨特的B-N方位關係,本研究使用高解析影像可精確地分辨它並統計其尺寸。此外,在回火處理中,因鉬原子訊號顯示在應變析出碳化鈮的成分分析內,推論鈮與鉬原子同時在應變析出碳化鈮表面生成鈮鉬碳化物。因以往研究無法有效區別應變析出碳化鈮與在回火變韌鐵內的碳化物,導致有鉬原子偏析至後者碳化物的誤解。因此,本研究證實在含鈮變韌鐵系熱軋鋼板中,鉬合金應是以增強硬化能以生成更多變韌鐵組織達到提升整體二次硬化效果,強化而非以細化碳化物之方式。 In related research, Mo element was shown to effectively enhance precipitation strengthening via nanometer-sized carbides and retard the recovery of dislocations in Nb-containing low-carbon bainitic fire-resistant steel. It is of some interest to apply the concept to another steel product, hot-rolled strips. In addition, the present study investigated the effects of adding Mo on interphase precipitation in ferritic hot-rolled strips to promote precipitation strengthening. After the comparison of the effects of Mo on the two types of hot-rolled strips, it was possible to determine whether the addition of Mo is beneficial to bainitic hot-rolled strips. The composition was primarily designed to be 0.05C-1.7Mn-0.08Nb (wt% based composition) with 0 and 0.1 wt% Mo, separately labeled Nb and Nb-Mo strips. In the practical hot-rolling process, the residual strain in prior austenite grains inevitably promotes greater transformation of ferrite, so in the present study, it was necessary to reduce the coiling temperature and accelerate the cooling rate so as to prevent the formation of a fully ferrite structure in present study. The coiling temperatures (C.T.) in the hot rolling process in this study were 450oC and 550oC. Observations of the microstructure revealed that C.T. 450oC and Mo both efficiently suppressed the ferrite transformation and promoted the granular bainite structure in bainitic hot-rolled strips. Then during the simulated coiling procedure, tempering treatment at 600oC, the strips with granular bainite demonstrated a significant secondary hardening effect. Elevating the coiling temperature to 650oC produced a fully ferrite structure with interphase precipitation in the hot-rolled strips. The strips achieved the same strength as the strips with the secondary hardening effect. However, the addition of 0.1 wt% Mo did not improve the precipitation strengthening by interphase precipitation and induced more unstable second phases in the coiling procedure in Nb-Mo strips with C.T. 650o C. From the above, it is concluded that Mo alloy has a positive effect on Nb-containing low-carbon bainitic strips. In order to increase the volume fraction of bainite and enhance the secondary hardening effect, the concentration of Mo in Nb-Mo strip with C.T. 450oC was increased to 0.3wt%, labeled the Nb-3Mo strip. In the microstructure, the morphology of bainite was the same as that of granular bainite, and the amount seems to increase with the greater addition of Mo. In further analysis to quantitate granular bainite, the electron backscattering diffraction technique was used to feature the unique misorientation gradient in granular bainite because scanning electron microscopy was unable to distinguish ferrite from granular bainite due to their similar morphologies. In the phase qualification of three hot-rolled strips with C.T. 450oC, it was surprising to find that only the high addition of Mo, 0.3wt%, effectively increased the volume fraction of granular bainite. Further dilatometry experiments led to the conclusion that the formation of ferrite must have been prohibited, allowing more prior austenite regions and the grain boundaries in the promotion of granular bainite. During tempering treatment, the high addition of Mo rarely promoted the secondary hardening effect of tempered granular bainite in Nb-3Mo strip. To determine the reason, Electron Energy Loss Spectroscopy (EELS), high resolution TEM (HR TEM), Energy dispersive X-ray spectroscopy (EDX), and atom probe tomography (APT) were used to investigate the effect of Mo solute on nanometer carbides and the dislocation structures in tempered bainite. Measurements of dislocation density revealed that no obvious recovery occurred in the three strips. It is probable that, unlike in the high temperature tensile test, the slipping of dislocation lines was slight in the current tempering treatment lacking external stress. Thus, the Mo solute could not retard the movement of static dislocation lines in granular bainite during tempering. In the evolution of nanometer carbides in tempered bainite with increasing Mo, Mo solute was homogeneously dissolved in (Nb,Mo) carbides in tempered bainite. Furthermore, the Mo solute became a significant carbide-forming element, and the carbides were slightly coarsened by absorption of excess Mo solute. In the reports on fire-resistant steel, the carbon replica method allowed confusion of the nanometer carbides in tempered bainite with strain-induced Nb carbides in prior austenite. This confusion is likely to have caused misunderstanding of the refinement of nanometer carbides by the addition of Mo in tempered bainite . In the present study, HR TEM images were used to focus on the nanometer carbides and statistically analyze their sizes during tempering because the featured Baker-Nutting orientation relationship between carbides and matrix could be recognized accurately with the technique. In addition, the signal of Mo occurred in strain-induced Nb carbides after tempering. It was concluded that Nb and Mo solute segregated toward the strain-induced Nb carbides and then formed a layer of carbides on the surface. It is likely that in previous research, segregation of Mo solute was believed to occur in the nanometer carbides in tempered bainite because the two types of carbides could not be resolved. Therefore, in the present study, it was necessary to dispel the fallacy that the addition of Mo can promote precipitation strengthening via nanometer carbides in tempered bainite in steels. Mo alloy strengthens the Nb-containing low-carbon bainitic hot-rolled strips by increasing the hardenability, rather than by a synergistic effect of Nb and Mo. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/56901 |
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