矽錳合金鋼之恆溫相變態與變韌鐵結構之研究

Abstract

本研究是針對高矽含量之低碳合金鋼，藉由熱膨脹儀之熱處理程序進行的一系列研究。對於鋼鐵材料而言，不同的相變態恆溫溫度將使得其內部產生不同之微結構相，且這些生成物將會對其機械表現產生很大的影響。在本研究所進行之恆溫過程中，共得到有麻田散鐵、變韌鐵、魏德曼肥粒鐵、高溫肥粒鐵、波來鐵等多種不同結構，這些結構將藉由金相圖形的觀察，以及進行微硬度測試的分析，了解各種不同相變態產物在各方面之性質表現，並藉由熱膨脹曲線，分析出各個不同相變態起始點之溫度。其中，變韌鐵的結構相將被提出做更進一步的分析，因其優良的機械性能通常是一般鋼鐵材料中的極佳選擇。這些更進一步的研究包括SEM、TEM等工具對其在微觀形貌上的分析，還有EBSD在對其結晶成長方位分佈和晶界角度測量上的研究。 研究中主要以兩種不同碳含量之材料搭配不同的恆溫溫度條件，對變韌鐵之結構進行觀察。變韌鐵是由多條呈平行排列之次平板所構成的束狀組織結構，其內部次平板的形貌對於變韌鐵結構之性質有很大的影響，其中一項便是其厚度上的變化趨勢，根據本研究之測量值，次平板之厚度明顯隨碳含量上升而下降，隨溫度上升而增加，且下降之次平板厚度將明顯提升材料在硬度上的表現。此外，變韌鐵之晶體方位亦在EBSD研究中進行分析，具有同一個慣性平面之變韌鐵結構內部通常包含有兩組以上之方位關係，且一個沃斯田鐵晶粒中亦可依其慣性平面之不同區分為數個區域，不同區域內的變韌鐵結構將展現出不一樣的方位關係。隨溫度增加，低角度晶界的比例亦上升，顯示出變韌鐵次平板成長時的方位選擇關係。

A study on silicon steel with different carbon concentration has been made. For steels, different transformation temperatures always lead to a different micro-structure, and have a big influence on its mechanical properties. In this research, some structure like Martensite, Bainite, Widmanstaten ferrite, Allotriomorphic ferrite and pearlite was obtained. Optical metallographs and hardness tests have been examined to explore these microstructure development and their mechanical properties. By the thermal dilatomer curves, we also found many important transformation start temperature of this steel. Besides, we did the advance research on bainite structures, these included SEM, TEM observation, and EBSD techniques. The bainitie sheaf structure is composed by many subunits, and the thickness of these subunits has large influence on bainite properties. In this research, we found that the thickness of subunits is coarsened with the increased isothermal transformation temperature but fined with the increased carbon concentration, and the finer subunit structures also has better hardness. On the other hand, EBSD techniques have been used to study the texture of bainite structure. One grain can be divided to many areas with the same habit plane called as packet, and the packet sturcture usually has at least two variant orientations instead of only one single orientation. In any packet, the variant orientations of the structure are totally different, and because of the variant selection of the subunit lath plates, the difference of variant orientations in a packet reduced by the increased temperature.