含鈮鋼筋顯微組織與無降伏現象之探討

Abstract

在2005年間，有鑑於混凝土鋼筋中，所含的合金元素釩之國際價格上揚，故將鈮合金元素用來取代一部分的釩合金元素，預期保有晶粒細化與固溶強化的效果。但是，將釩與鈮以3：1的比例下添加至鋼筋中會在拉伸試驗中發現降伏點消失的情況，因此，將含鈮以及不含鈮鋼筋作一系列的顯微結構分析，以探討降伏點消失的原因。 將含鈮鋼筋以及不含鈮鋼筋兩種鋼材分析金相顯微組織，發現在鋼筋表面、1/2半徑處、中心處的顯微結構並無明顯變化。此外，在含鈮鋼筋中可以發現隨著鋼筋尺寸越大(成形應變量越小)，則波來鐵越增加以及高溫肥粒鐵越減少。值得注意的是：含鈮鋼筋會產生多的費德曼肥粒鐵，若與不含鈮鋼筋組織作比較。再者，將含鈮鋼筋中的費德曼肥粒鐵以SEM的分析方式，可觀察到更細部的費德曼肥粒鐵外貌。再將費德曼肥粒鐵以及高溫肥粒鐵以TEM的分析，可以得到兩者內部差排結構的不同，並且可以明顯發現在費德曼肥粒鐵與高溫肥粒鐵的接觸界面處有偏高的差排密度。由一系列拉伸試驗所獲得的應力應變圖，對照所對應的顯微組織，明白地顯示含鈮的鋼筋大約含有體積百分比5% 費德曼肥粒鐵(其差排密度約2.46±0.91)×1014 m-2 ，此乃造成降伏點消失的主要原因。 本研究，將含鈮鋼筋施以低溫回火的熱處理(300℃，10hrs)，發現原先的連續平滑的應力應變曲線會因為回火處理的關係而轉變成不連續且有降伏點的應力應變曲線。再以TEM分析費德曼肥粒鐵的差排密度，發現降為 (8.97±2.91)×1013 m-2 。此結果更肯定費德曼肥粒鐵之差排密度對降伏點現象之影響。

Because of the rise in international price of alloy element V since 2005, alloy element Nb has partially substituted for V in reinforcing steel bars . It was expected that vanadium still kept the effect of solid-solution strengthening and grain refinement. However, the Nb-containing steel bars do not possess the yield-point phenomena during tensile test. This has puzzled civil engineers. In the present study, the microstructure of Nb-containing and Nb-free steel bars have been investigated. The corresponding the stress-strain curves have also been examined. Optical Metallurgical observation for three sites in each of 14 different steel bars：surface, half radius region and central region have been done separately . More widmanstätten ferrite and more pearlite, but fewer allotriomorphic ferrite can been found in Nb-containing steel bars than that in Nb-free steel bars. It was found that the Nb-containing steels were with about 5 volume (%) widmanstätten ferrite . The microstructural details of widmanstätten ferrite have been investigated using transmission electron microscopy, and the dislocation density has been estimated to be about (2.46±0.91)×1014 m-2 . It is suggested that the formation of Widmanstätten ferrite with a high dislocation density, which is similar to a prestrain state, correlated with no yield-drop phenomenon. A Nb-containing steel bar (steel bar G) has been treated by a tempering at 300℃ for 10 hrs. The corresponding transmission electron microscopy has been studied. The dislocation density of Widmanstätten ferrite is found to decrease to (8.97±2.91)×1013 m-2. The tempering steel has been demonstrated to have a yield-point phenomenon. The result provides strong evidence to confirm that the dislocation density of widmanstätten ferrite should be the main factor to influence the yield-drop.