本研究主要利用數值模擬方法，針對真空連鑄製程來探討直徑8mm的無氧銅(OFC)棒材微觀組織的演化規律。採用巨觀-微觀耦合的方法，分別建立了連鑄過程的溫度場模型和微組織預測模型。考慮連鑄速度、澆鑄溫度、冷卻強度和鑄件尺寸等連鑄工藝參數，對凝固過程中固液界面的位置、形狀和微觀組織的晶粒成核、成長等影響進行了模擬研究。
巨觀模擬方面，採用有限差分法（FDM）進行巨觀尺度的溫度場計算，並用溫度回升法處理凝固潛熱的釋放。微觀模擬方面，利用三維的Cellular Automaton（3D-CA）模型，並考慮了結晶學的優先生長方向和晶粒生長的動力學理論，模擬了微觀尺度的晶粒競爭生長過程和最終的晶粒形態。
經由模擬結果表明，在真空連鑄過程中，連鑄速度對固液界面的位置、形狀和晶粒的生長形態影響較大，隨著連鑄速度的增大，固液界面的位置明顯往模具出口處接近；固液界面的形狀也由接近平界面的形狀轉為微凹的圓弧狀甚至是深凹的狹長形狀；晶粒的生長形態也由維持軸向生長的晶粒形態轉為軸向與徑向混合生長的晶粒形態及完全徑向生長的晶粒形態。在模擬與實際鑄件實驗相互驗證方面，也得到了相當吻合的結果。進一步針對各連鑄工藝參數對固液界面位置、形狀及微觀組織的影響得到，鑄件尺寸對固液界面位置、形狀和晶粒形態影響較大；澆鑄溫度和冷卻強度則對固液界面的位置產生影響，而對於固液界面的形狀和微觀組織的生長形態影響較小。
綜合以上模擬結果，可以得出一形狀因子(η)的判別式，藉由連鑄速度、澆鑄溫度、鑄件尺寸、冷卻強度係數和鑄件出口表面溫度等影響因素獲得與固液界面形狀的關係式：當η ≥ 4.4時，可以得到接近平界面的固液界面形狀（軸向生長的晶粒形態）；4.4＞η＞1.9時，為圓弧的界面形狀（軸向和徑向混合生長的晶粒形態）；當η ≤ 1.9時，得到深凹狹長的界面形狀（完全徑向的晶粒形態），此形狀因子(η)可以成為判斷連鑄過程固液界面形狀與晶粒形態的一種參考方法。
最後為了進一步測試本微組織模擬預測系統的通用性，將此模擬系統應用於二元合金體系（黃銅合金），搭配不同的連鑄參數條件進行微組織模擬預測，並與實際連鑄實驗進行定性與定量的相互比對，均獲得了相接近的模擬結果，驗證了模擬系統的通用性與準確性。

In this study, the numerical simulation method is used to analyse the microstructure evolution of 8-mm-diameter oxygen-free copper (OFC) rods during the vacuum continuous casting (VCC) process. The macro-microscopic coupling method is adopted to develop a temperature field model and a microstructure prediction model. The effects of various casting parameters including casting speed, pouring temperature, cooling rate and casting dimension on the location and shape of the solid-liquid (S/L) interface, nucleation and grain growth of solidified microstructure are considered in our model.
At the macroscopic scale, the finite difference method (FDM) is utilized to solve the government equation of heat transfer which includes using the temperature recovery method to incorporate the release of latent heat during solidification. At the microscopic scale, a three dimensional cellular automaton (3D-CA) model which considers the preferred crystallographic orientation of nucleation and kinetics of grain growth is used to simulate the competition of grain growth thus defining the resulting grain structure of the cast.
Simulation results show that the casting speed has a large effect on the position and shape of the S/L interface and grain morphology. With an increase of casting speed, the position of the S/L interface moves toward the orifice of the casting mould and the shape changes from a planar shape into a semi-ellipse shape, and eventually a narrower, pear shape. The grain morphology indicates a change from axial growth to axial-radial growth or completely radial growth. The simulation predictions agree well with the microstructure observations of cast specimens. Further analysis of the effects of other casting parameters on the position and shape of the S/L interface reveals that casting dimension has more influence on the position and shape of the S/L interface and grain morphology than do pouring temperature and cooling rate.
The above simulation results can be summarized to obtain a discriminant of shape factor (η), which defines the shape of the S/L interface and grain morphology with respect to casting speed, pouring temperature, casting dimension, cooling rate coefficient and outlet temperature of the oxygen-free copper (OFC) rod. When η ≥ 4.4, the shape of the S/L interface is close to a flat plane (axial growth); the shape is a semi-ellipse (axial-radial growth) when 4.4＞η＞1.9 and narrow pear (radial growth) when η ≤ 1.9.
Finally, the extendibility of microstructure prediction system is further examined. The microstructure of the binary alloy system (brass alloy) is simulated and predicted by the simulation system with different parameters of the continuous casting process. Comparing with the actual casting qualitatively and quantitatively, the experiment results are close to the simulation results. It verifies the extendibility and accuracy of the simulation system.

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