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摘要: 采用電解相分析方法, 結合X射線衍射分析和電感耦合等離子體原子發射光譜儀(ICP)、掃描電鏡(SEM)、透射電鏡(TEM)等對高鋁鐵素體基體中的析出相顆粒粉末和電解液進行定性定量分析. 試驗結果表明, 試驗鋼中固態析出相主要為NbC以及少量的Al2O3和AlN夾雜. 通過掃描電鏡觀察不同再加熱溫度下NbC分布狀態, 發現隨著固溶溫度的升高, 鑄態組織中存在的NbC析出逐漸回溶, 數量隨之減少且發生明顯的粗化行為. 當溫度升高到1100℃, 大部分NbC已經回溶到高溫鐵素體基體中. 在利用Thermo-Calc熱力學計算軟件分析Nb及其碳化物的熱力學性質基礎上, 計算得到Al與Nb的相互作用系數, 表明Al能夠降低Nb在鐵素體基體中的活度, 提高其在基體中的固溶度, 進一步得到了NbC在高鋁鐵素體鋼中的固溶度積公式, 發展了高溫鐵素體中的Nb微合金化理論, 為進一步的應用提供了理論基礎.Abstract: With the rapid development of the global economy, problems in energy production and environmental protection are becoming severe. To reduce fuel consumption and CO2 emissions, it is essential to reduce the weight of automobiles and other huge construction structures. Recently, a number of studies have been conducted on the use of low-density steels for automobile applications by incorporating aluminum in steel. The light elements can increase the lattice constant of steel while reducing the density of steel to achieve a lower atomic weight. Aluminum as a light element replaces the iron atoms in the unit cell, increasing the volume while reducing the weight, thereby reducing the density of steels. In this regard, ferritic Fe-8%Al steels indicated a 10% reduction in density compared with the conventional steels. To clarify the solid solution and precipitation behavior of Nb in Al-bearing ferritic steels, heat treatment tests were carried out under a series of temperature. The precipitates of NbC and the dissolved Nb solute in ferrite matrix with high Al content were studied using electrolytic dissolution technique, X-ray diffraction technique, and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also applied. The experimental results show that the precipitates are NbC and also some Al2O3 and AlN inclusions. It is also found that with increase in the solution temperatures, the NbC in the as-cast samples becomes fewer and the coarsening behavior occurs. Moreover, when the temperature was over 1100℃, almost all the precipitates were dissolved. Furthermore, using Thermo-Calc software, the thermodynamic properties of Nb and relevant compounds were studied, and the interaction coefficient between Al an Nb was calculated. The results indicate that Al decreases the activity of Nb, and the solubility of NbC increases. Finally, the solid solubility formula of NbC was deduced, which can provide a basis for further application of ferritic steels with a high Al content.
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圖 1 電解化學相分析中過濾剩余產物的X射線衍射測定結果
Figure 1. XRD patterns of particles in experimental steels as determined by physical and chemical phase analysis
圖 2 Thermo-Calc軟件得到Fe-Al-Nb相圖
Figure 2. Fe-Al-Nb phase diagram calculated by Thermo-Calc
圖 4 試驗鋼透射電鏡和能譜分析
Figure 4. TEM images and corresponding EDS results of carbide precipitates in experimental steels
圖 5 試驗鋼在不同溫度下的掃描電鏡照片. (a)600 ℃; (b)700 ℃; (c)800 ℃; (d)900 ℃; (e)1000 ℃; (f)1100 ℃
Figure 5. SEM images of carbide precipitates in experimental steels held at different temperatures: (a) 600 ℃; (b) 700 ℃; (c) 800 ℃; (d) 900 ℃; (e) 1000 ℃; (f) 1100 ℃
圖 6 Thermo-Calc軟件得到Nb和C在鐵素體中的擴散系數
Figure 6. Diffusion coefficient of Nb and C in ferrite calculated by Thermo-Calc
圖 7 試驗鋼不同溫度下的掃描電鏡照片. (a)700 ℃; (b)800 ℃; (c)900 ℃
Figure 7. SEM images of carbide precipitates in experimental steels held at different temperatures: (a) 700 ℃; (b) 800 ℃; (c) 900 ℃
表 1 Nb在鐵素體中的固溶度
Table 1. Solubility of Nb in ferrite
溫度/℃ wNb/% 900 0.0012 950 0.0130 1000 0.0341 1050 0.0452 1100 0.0535 
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表 2 Al與Nb相互作用系數計算中使用的參數
Table 2. Parameters in present model for calculating interaction coefficients of Al and Nb
元素 nws1/3 ? V2/3 p q/p Fe 1.77 5.10 3.69 12.3 9.4 Nb 1.64 4.05 4.90 12.3 9.4 Al 1.39 4.20 4.4 12.3 9.4 其中，nws1/3為電子密度，?為電中性，V2/3為摩爾體積，p和q/p均為常數.
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表 3 與Laves相平衡的Nb在鐵素體中的固溶度
Table 3. Solubility of Nb equilibrium with Laves in ferrite
T/℃ wNb/% 500 0.067 600 0.149 700 0.322 800 0.669 900 1.137 1000 1.747 1100 2.513 1200 3.452
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