摘要
銲接構造用鑄鋼SCW 450為澆鑄量相當大的鑄造用材料，由於經常需要以銲接方式與其它材料進行接合，因此對其機械性質的規定在SC 450之外，額外要求其低溫衝擊韌性於0 ℃衝擊時，其衝擊值最小須達27J。
　　研究旨在探討在肥粒鐵-沃斯田鐵雙相區沃斯田鐵化後以空冷及爐冷的熱處理製程對於SCW 450低溫衝擊韌性的影響，並尋找較佳的製程及參數。研究方法包括，以OM及SEM觀察鑄鋼的顯微組織，及於0 ℃的衝擊試驗及破斷面型態觀察。另外觀察鑄鋼於920 ℃沃斯田鐵化時，其原沃斯田鐵晶粒尺寸與持溫時間的關係。
研究結果顯示熱處理後的波來鐵分佈型態、肥粒鐵晶粒尺寸皆是影響鑄鋼低溫韌性的重要因素;原沃斯田鐵的晶粒尺寸隨持溫時間增加而粗大化。

關鍵字：鑄鋼、低溫韌性、雙相區熱處理

Abstract
Cast steel SCW 450 for welded construction has been cast in a considerable volume, in most cases designed bonding together with other materials by welding, so other than the mechanical properties of SC 450 , it requirements of a low temperature impact toughness at 0 ℃with a impact value at least up to 27 J.
The study was designed to explore the effect of intercritical heat treatment on the low temperature impact toughness of cast steel SCW 450. Experimental methods include the furnace cooling and air cooling after austenitizing treatment, the OM and SEM examination of the microstructure of cast steel, impact testing at 0 ℃ and the fracture surface of impact specimens. The relationship of austenite grain size of SCW450 at 920 ℃ with holding times was studied also.
The results show that the pearlite distribution pattern after heat treatment and the ferrite grain size are the most important factors which affect the low temperature impact toughness of SCW450; the original austenite grain size is coarsening with the increase of holding times.

Keywords：Cast steel, Low temperature toughness, Intercritical heat treatment

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