先前沃斯田鐵晶粒尺寸對於矽錳合金鋼之變韌鐵束狀組織之研究

Abstract

在鋼鐵中添加約2wt%的矽(Si)，並在適當相變態溫度環境下可以生成無碳化物析出之變韌鐵(carbide free bainite)，這種組織與一般的變韌鐵組織所不同的地方在於原本變韌鐵中存在的碳化物由富含碳的殘留沃斯田鐵取代，因此比起一般的變韌鐵更具有更優益的韌性，但是對此矽合金鋼的顯微組織發展仍不甚了解，有待進一步研究。 本研究是針對不同含碳量之矽合金鋼，經由熱膨脹儀設計熱處理程序所進行的一系列研究，透過不同的沃斯田鐵化溫度來改變先前沃斯田鐵之晶粒尺寸，再進一步進行不同相變溫度之相變態，探討其組織之演化。由於變韌鐵相變態過程為成核控制，主要成核位置是在先前沃斯田鐵晶界上，在先前沃斯田鐵晶粒尺寸較小的試樣中，由於先前沃斯田鐵晶界所佔比例較高，因此其進行變韌鐵相變態的速度愈快；而先前沃斯田鐵晶粒尺寸愈大，其相變態速度愈低。以SEM、TEM與電子背向散射繞射(EBSD)進一步分析其變韌鐵之束狀組織，發現不同的先前沃斯田鐵晶粒尺寸，對於變韌鐵束狀組織之形狀、長度(sheaf length)與束狀組織之數目(sheaf variant)有所影響，隨著先前沃斯田鐵晶粒尺寸的增加，變韌鐵束狀組織會由原本不規則的外型逐漸變成平整；長度則會隨著晶粒尺寸增加而增加；束狀組織之數目則是隨著晶粒尺寸增加而減少。而改變相變態溫度與碳含量則是會對於次平板寬度產生變化，當提高相變態溫度或降低碳含量時均會使次平板寬度提高。此外，由電子背向散射繞射分析中亦獲得變韌鐵之晶體方位關係，從中顯示出變韌鐵次平板成長時的方位選擇關係。

Adding about 2wt% silicon to the carbon steel can produce carbide free bainite under the proper heat treatment. This kind of structure is different from normal bainite which consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. Therefore, carbide free bainite has more toughness and strength compared with the normal bainite. However, the development of this microstructure in silicon steel is still not well understood and need further study. The present study attempts to control the austenitizing temperature to explore the microstructure development with the different transformation temperature and the effect of prior austenite grain size on bainite sheaf via dilatometer in different carbon concentration silicon steel. Because bainite transformation rate is nucleation control. The nucleation sites are mainly at the prior austenite grain boundary so that the finer prior austenite grain will increase the transformation rate due to the much more grain boundary area; Conversely, the greater prior austenite grain will decrease the transformation rate. The SEM, TEM and EBSD results show that different prior austenite grain size will influence the morphology, sheaf length and sheaves variant of bainite. With the increase of the prior austenite grain size, the sheaf length will longer, the sheaf variant will decrease and the shape of bainite sheaf become smoother. Besides, changing the transformation temperature or carbon concentration influences the bainite sub-unit width. The sub-unit width is increased when the transformation temperature increased or carbon concentration decreased. From the EBSD analysis, the texture of bainite structure have been studied. The result shows the variant selection of bainite sub-unit during growth.