無間隙原子鋼之塊型相變顯微分析及機械性質

Abstract

無間隙型原子鋼因其良好的成形性故被廣泛應用於工業中，在相關的研究中，在不犧牲延展性的情形下增強它的強度為相當重要的課題。傳統上以細晶強化作為主要的強化方法，然而此方法必須要花費相當大的能源及時間，因此在本篇研究中，塊型相變提供了另一種強化無間隙型原子鋼的方法，並對塊型肥粒鐵的型貌及機制上做深入的探討。 在本研究的第一個部分中，以熱膨脹儀來對材料進行精確溫度的熱處理，總共的沃斯田鐵化溫度有三個(1000°C、1100°C、1200°C)，接著以每秒500°C快速冷卻，在精準的溫度控制下，一旦試片內部有相變產生，放出來的潛熱即可得知相變的起始溫度，因此可以比較不同沃斯田鐵化溫度對塊型相變起始溫度的影響，隨著沃斯田鐵化溫度降低，肥粒鐵晶粒因成核點增加而細化。除此之外，其組織在光學顯微鏡及穿透式電子顯微鏡下可以被清晰的觀察，因試片的熱處理在快速冷卻的環境中，故可以避免緩慢成長的高溫肥粒鐵，只剩下具典型鋸齒狀外觀的塊型肥粒鐵及些許剪切型組織。 在第二個部分中，主要為比較同屬重構型相變的高溫及塊型肥粒鐵，相變機制的差異導致兩者不同的外貌及性質，前者具平滑的晶界，後者晶界不規則呈鋸齒狀，且內部含有次結構。本章節實驗將無間隙型原子鋼以雷射焊接做處理，目的為模擬真實的工業焊接情形，在焊道的部分，組織為典型的塊型肥粒鐵，與原金屬基材中的高溫肥粒鐵在外貌上有顯著差異，硬度也由於塊型相變的出現而顯著提升。在顯微結構的分析中，掃描式電子顯微鏡及穿透式電子顯微鏡皆有針對塊型肥粒鐵及高溫肥粒鐵進行觀察分析，光學顯微鏡中晶粒內部的次結構，經過掃描式電子顯微鏡背向散射電子繞射分析後可以發現是由小角度晶界所構成，經過穿透式電子顯微鏡分析後，發現為差排在高溫回火聚集而成的結果，塊型肥粒鐵中的差排密度也較高溫肥粒鐵高出許多。 最後的部分在探討塊型相變的機械性質，由前章節可知塊型相變導入大量差排使硬度上升，其機械性質因從未有研究說明及文獻提及，故本章節將進行初步探討分析。三試片分別在1200°C沃斯田鐵化後進行爐冷、空冷及水淬，分別代表不同的冷卻速率，隨著冷越速率的提升，高溫肥粒鐵逐漸被抑制，取而代之的是塊型肥粒鐵的組織，由於晶粒中高密度且均勻散佈的差排，在拉伸結果有較佳的降伏強度及抗拉強度，此外，因為塊型肥粒鐵仍然保有大晶粒的肥粒鐵組織，其延展性上仍然有不錯的表現。因此，塊型相變在無間隙型原子鋼中是備受期待的改善機械性質的方法，無論是在節省時間或能源的表現上，都比傳統的方法更好，同時也能維持工業上最需要的延展性。

Interstitial-free (IF) steels are used in industry because of its good formability. Therefore, enhancing the strength without the sacrifice of ductility becomes an important topic. Traditionally, grain refinement was the main method. However, it took a lot of energy and time to complete the strengthening process. In this study, massive transformation was investigated, offering another choice for the strength enhancement in IF steels. In the first part, an IF steel was heat treated precisely by a dilatometer. With three different austenitization temperatures (1000, 1100 and 1200°C), massive ferrite formed with 500°C/s cooling rate. By the measurement of releasing latent heat owing to phase transformation, the start temperatures of massive ferrite transformation were analyzed and compared. As the austenitization temperature decreased, the ferrite grains became finer owing to more nucleation sites. Besides, the microstructures under three austenitization temperatures were observed. The irregular boundaries and substructures within the grain appeared, which was the classic morphology of massive ferrite. Under transmission electron microscope (TEM), massive ferrite along with the displacive products, such as granular bainite and martensite, were clearly resolved. In the second part, two IF steels were treated by laserweld in order to simulate the practical industry application. To compare the morphology and transformation mechanism, samples in the base metal and the weld part were observed and analyzed. Massive ferrite with irregular boundaries was produced in the weld part, and allotriomorphic ferrite with coarse grains and smooth boundaries was found in the base metal. The hardness of ferrite increased as the massive transformation occurred in the weld part. Afterwards, scanning electron microscope-electron backscattered diffraction (SEM-EBSD) analysis and TEM analysis were executed. The substructures within the ferrite grain were resolved to be low-angled boundaries under SEM-EBSD and dislocations accumulation under TEM. The dislocation density was carefully estimated so that the microstructures between allotriomorphic and massive ferrite could be distinguished easily. According to the second part, the hardness of ferrite was enhanced due to massive transformation, so the mechanical properties of steels with massive transformation was expected to be good. In the final part, three heat treatments (furnace cooling, air cooling and water quench) were conducted on samples respectively. As massive ferrite replaced allotriomorphic ferrite in the structures with the increasing cooling rates, the strength was gradually enhanced. After elucidating the microstructures by SEM-EBSD and TEM, the deformation mechanism was deduced that the higher amount and more homogeneous dislocations in massive ferrite led to better strength performance. Furthermore, with ferrite morphology, the ductility was maintained. Since the process experienced only heat treatment with a short time period, it is expected to be applied into industry for the consideration of saving energy and time.