低碳鋼在銲接時，於銲處周圍的粗晶熱影響區會產生沃斯田鐵晶粒成長的現象，為了解決這個問題，添加鎂於低碳鋼中，可有助於阻止晶粒成長。本研究探討添加不同鎂含量的低碳鋼SS400在高溫下晶粒成長的影響，使用晶粒成長公式預測不同熱處理溫度及時間下沃斯田鐵晶粒尺寸。
本研究將試片使用高溫共軛焦雷射掃描顯微鏡(HT-CLSM)做熱處理實驗，並使用光學顯微鏡分析軟體分析高溫沃斯田鐵晶粒，取樣至少300個晶粒。接著運用統計學的概念，求得出平均數、中位數、眾數3個不同意義的特徵值。本研究將3個晶粒成長公式的模型以及3個平均值逐一比較，最終選擇眾數作為晶粒尺寸，並採用新發展的晶粒成長公式。接著，本研究使用晶粒尺寸、晶粒成長公式求得出晶粒成長速率及晶界遷移速率。
實驗結果顯示，晶粒成長公式中的活化能Q值，以0 ppm Mg最大，1.6 ppm Mg最小，隨著鎂含量的增加，Q值也會隨之提升。在相同溫度下，晶粒尺寸由大到小依序為0ppm、7.7ppm、1.6ppm、25ppm Mg SS400。在1200°C下，不含鎂的SS400與含鎂的SS400的晶粒成長速率(持溫20分鐘)與晶界遷移速率差異不大；隨著溫度升高，兩者的差異漸趨明顯，晶粒成長速率(持20分鐘)及晶界遷移速率由快到慢依序為0ppm、7.7ppm、1.6ppm、25ppm Mg SS400。

For the large heat input welding technique using heat input over 50kJ/cm, as increasing the heat input energy, coarsening in austenite grains will be enhanced in the heat-affected zone (HAZ) during welding and coarse grains have detrimental effect on impact toughness of weldments. Thus, Mizoguchi et al. firstly proposed oxide inclusions for improving HAZ impact toughness in the case of large heat input welding. Titan and magnesium oxides can be called the “first” and “second” generation of oxide inclusions for acicular ferrite (AF) formation in terms of development time, respectively.
In this study we mainly investigated on the effect of Mg addition on the austenite grain growth in low carbon steel of SS400 using thermal etching. Furthermore, in-situ observations of austenite grain growth were also performed with a high-temperature confocal scanning laser microscope (HT-CSLM). The results show that the austenite grain size and grain growth rate decrease with the increase of magnesium content.

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