先進超高強度鋼奈米碳化物界面析出之穿透式電子顯微鏡分析研究

Abstract

汽車工業為減少石油耗損以及二氧化碳排放量，近幾年來已開發諸多先進超強度鋼。JFE公司與中鋼公司相繼開發抗拉強度780MPa等級之高成形性超高強度熱軋鋼板，其高強度之肥粒鐵約有300MPa的強度由奈米尺寸的碳化物所貢獻，這些微細的碳化物成核在沃斯田鐵至肥粒鐵相變態時的移動界面上，此即為著名的界面析出，而此鋼板被認為是界面析出機制在低碳先進高強度鋼的實現。本研究開發穿透式電子顯微鏡技術來分析研究低碳鋼中奈米尺寸界面析出碳化物，目標對界面析出機制提出完備觀點。 本研究首先以高解析電鏡之Moiré條紋來解析碳化物之晶體結構與方位關係，並發現TiC碳化物於755oC持溫成長後將由單一孿向的Baker-Nutting方位關係轉變為多孿向的Nishiyama-Wessermann方位關係。高解析電鏡影像搭配NanoProbe EDS能證明(Ti, Mo)C為一NaCl結構之MX型碳化物，而其中Mo原子之分佈可利用高角度環場暗視野像來呈現。此外，本研究提出hu+kv+lw = 0 之觀測條件於TEM下量測sheet spacing，同時透過電子能量損失能譜來評估試片厚度並計算interparticle spacing，而界面析出碳化物sheet plane之方位則透過菊池繞射圖樣來分析。 運用本研究建構之穿透式電子顯微鏡分析方法，本研究針對不同合金成分之鋼鐵於不同熱處理條件進行實驗與觀察。其中發現界面析出之sheet plane接近肥粒鐵之{ 2 1 1}、{ 2 1 0}以及{ 1 1 1}平面，而TEM的觀察結果提供了證據說明界面析出碳化物的形成與相變態時階梯成長之非整合型界面有關。界面析出之sheet spacing便直接對應階梯的高度，此高度則可運用Bhadeshia所提出的方程式評估，而interparticle spacing較為複雜，必須同時考慮界面移動的速度以及碳化物成核之速率。以上機制已經由不同合金成分以及不同熱處理的實驗中得到證實。 基於建構之界面析出機制，先進超高強度熱軋鋼板已經於實驗室中開發成功，其抗拉強度可超越700MPa而伸長率可達20%以上。運用電子顯微鏡分析鋼板中碳化物之sheet spacing、interparticle spacing以及碳化物尺寸，接著Orowan方程式被進一步用來分析其中之強化機制，奈米碳化物的強度貢獻量可以達到200 MPa。 此外，本研究以三維原子探針斷層掃瞄解析(Ti, V)C碳化物的熱穩定性。其中發現鋼板中的Ti、V、C原子叢聚可分為兩個群組:（1）含2至30個原子之小叢聚以及（2）含31至350個原子之大叢聚。其中小叢聚的形成能夠延滯碳化物合金元素的擴散並進一步抑制碳化物之粗化速率。並意外發現除了碳化物能夠排成界面析出之分佈外，大原子叢聚亦能形成界面析出之帶狀分佈樣貌，因此本研究認為這些原子叢聚為沃斯田鐵相變態為肥粒鐵時即便形成。自1964年界面析出被發現以後，這些新觀點又將為界面析出理論掀開新的一頁。

To reduce fuel consumption and CO2 emission in automobiles, the development of advanced high-strength steels (AHSS) has been the bull’s eye in recent years. The ultra high-strength hot-rolled steel strips have been developed with tensile strength of ~780MPa and excellent formability by JFE and China Steel Co. The strong ferrite in the steels has been achieved by nanometer-sized carbides which contribute about 300MPa to the total strength. These tiny carbides nucleate on moving γ/α interface during austenite-to-ferrite transformation. It has been well known as interphase precipitation. This steel strip is considered to be avatar of interphase precipitation in low-carbon steels. For the purpose to explore a complete scope for the mechanism of interphase precipitation, TEM techniques have been developed and discussed to characterize the nanometer-sized interphase-precipitated MX carbides in this study. This study initially utilized Moiré fringes in high resolution TEM (HRTEM) to characterize the crystal structure and orientation relationship (OR) of nanometer-sized carbide. It was found that TiC carbides in ferrite will transit from single variant of Baker-Nutting OR to multi variants of Nishiyama-Wessermann OR during isothermal holding at 755oC. HRTEM associated with NanoProbe EDS provided the evidence to suggestthat (Ti, Mo)C carbide is a NaCl structured MX-type carbide. The distribution of Mo has been revealed by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Besides, this study provided the observation condition: hu+kv+lw = 0 to measure the sheet spacing. By estimating the sample thickness from electron energy loss spectrum (ELLS), the interparticle spacing in sheet could be calculated. The orientation of sheet plane has been identified by analyzing the convergent beam Kikuchi diffraction patterns. Using the developed TEM techniques, experiments and investigations were conducted in steels with different chemical compositions under different heat treated conditions. The planar sheets of carbides have been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; transmission electron microscopy results provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing corresponding to the ledge height can be predicted by Bhadeshia’s formula. And the variation of interparticle spacing in sheet is related to both the moving speed of ledges and carbide nucleation rate. Based on the new mechanism of interphase precipitation, the ultra high-strength hot-rolled steel strips have been developed in lab-level. The tensile strength of the strips can exceed 700 MPa and the total elongation can be over 20%. With measured microstructural parameters from TEM, an anisotropy-related Orowan equation was applied to estimate contribution of nanometer-sized carbides to the yield strength; the value is higher than 200 MPa. Furthermore, atom probe tomography (APT) has been used to study the thermal stability of interphase-precipitated (Ti, V)C complex carbides in atomic scale. It is found that the clusters of Ti, V and C can be classified into two groups: (1) tiny clusters with 2 to 30 atoms and (2) coarse clusters with 31 to 350 atoms. It is proposed that the tiny clusters with 2 to 30 atoms in the ferritic matrix retards the diffusion rates of carbide forming elements so that the coarsening rate of carbides could be suppressed. Besides, the density of coarse clusters with above 30 atoms is higher than the density of carbides estimated from TEM by one order. The distribution of clusters is also sheeted distribution and it seems that the clustering occurs during austenite-to-ferrite phase transformation. Since the discovery of interphase precipitation in 1964, the results of this work bring about a whole new perspective to the theory of interphase precipitation.