雙相鋼內奈米界面析出碳化物對變形顯微組織演變影響之探討

Abstract

為了因應未來汽車的能耗需求，汽車工業開始在不危害安全性的前提下尋求減輕車體重量的方法，也因為如此，具有良好機械性質的先進高強度鋼開始被應用在汽車車體上。而其中雙相鋼因為其相對低廉的製造成本跟優異的強度和延性表現，而被廣泛使用於汽車製造上，但由於其雙相組織的機械性質差異的關係，在變形過程中應力容易不均勻集中而導致成形產生困難。 在本論文利用具有奈米尺度的界面析出物來強化肥粒鐵，藉此降低肥粒鐵與麻田散鐵之間強度的差距以提昇雙相鋼的成形性。由實驗結果證實，奈米界面析出碳化物的確能夠提昇雙相鋼的機械性質，在沒有明顯地影響延伸率的狀況下，降伏強度和抗拉強度各增加了140MPa和161MPa，此外也因為肥粒鐵增強的關係，兩相機械強度的差距也被縮小。 本研究除了對於雙相鋼機械性質的提昇之外，也希望能夠對界面析出物在變形過程中的產生之影響進行探討，因此在研究中利用掃描式穿透電子顯微鏡進行變形組織的觀察，在掃描式穿透電子顯微鏡環形暗場技術的幫助下，差排的組織變化能夠被清楚地解析，而隨著應變而增加的差排密度也能夠被正確的計算。在觀察中發現因為界面析出碳化物的關係，雙相鋼中差排的增生速率以及密度都會比較高; 然而界面析出碳化物也會對於差排組織的形貌演變有所影響，界面析出物會使得差排分布的較為密集且均勻，導致了較強的差排回復現象。肥粒鐵基地中的界面析出物能夠提昇差排增生的速率且同時增強差排的回復效果，使得兩種雙相鋼具有十分類似的加工硬化行為。 在本研究中，界面析出物被認為能夠提昇幾乎在整個塑性變形區的差排增生速率，但由於差排回復效果的增強以及麻田散鐵的開始塑性變形的關係，明顯的加工硬化率的提升只會出現在塑性變形區的一開始。

In order to improve the fuel consumption efficiency for developing the next generation vehicles, automobile manufacturers have made efforts in striking a balance between weight reduction of car bodies and the requirement of the safety and durability. As a result, advanced high strength steels (AHSSs) have been introduced to automotive industries due to the impressive mechanical properties. Dual phase (DP) steels are one of AHSSs commonly used in automotive industries because of the relatively low production cost and a superior strength-ductility combination. However, manufacturing automotive components from DP steel sheets faced difficulties due to the highly localized strain resulted from the mismatch of mechanical properties in dual phases. In the thesis, the concept of interphase precipitation strengthening is introduced in ferrite matrix to reduce the strength mismatch and improve the formability of DP steels. The experimental results showed that the yield and tensile strength of DP steels with interphase precipitation strengthening could be increased by 140MPa and 161MPa without significant drop in elongation. And the strength difference between dual phases was decreased. In addition to the enhancement in performance, the research put much more emphasis on resolving the effects of interphase precipitation on the deformation behaviors of DP steels. As a result, deformation microstructures were observed by STEM in the specimens with and without interphase-precipitated carbides during the deformation process. With imaging techniques of STEM ADF, the dislocation microstructures evolution could be resolved well and dislocation density variation could be measured even in the specimens subjected to deformation. Because of the interaction with interphase-precipitated carbides, the dislocation density and its multiplication rate were higher compared to interphase precipitation-free DP steels in the whole process of deformation. Furthermore, the existence of interphase precipitation contributed to denser and more homogeneous dislocation distribution, so the dislocation cell structure was absent. Despite the higher dislocation multiplication rate, the strain hardening rate of two DP steels are quite similar because interphase precipitation would increase the recovery rate of dislocations at the same time. From the present work, the interphase-precipitated carbides are found to have capability of boosting the dislocation multiplication effectively in the plastic deformation region prior to uniform elongation. However, the increasing effects on strain hardening rate are significant only at the very beginning of plastic deformation. Because the recovery effects strengthened by interphase-precipitated carbides and the commence of deformation of martensite would cover up it’s strain hardening effects.